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© International Telecommunication Union INTERNATIONAL TELEPHONE CONSULTATIVE COMMITTEE (C. C. I. F.)

XVIIth PLENARY ASSEMBLY

GENEVA, 4-12 OCTOBER, 1954

VOLUME III

LINE TRANSMISSION MAINTENANCE

Published by the International Telecommunication Union Geneva, 1956 INTERNATIONAL TELEPHONE CONSULTATIVE COMMITTEE (C. C. I. F.)

XVIIth PLENARY ASSEMBLY

GENEVA, 4-12 OCTOBER, 1954

VOLUME III

LINE TRANSMISSION MAINTENANCE PAGE INTENTIONALLY LEFT BLANK

PAGE LAISSEE EN BLANC INTENTIONNELLEMENT TABLE OF CONTENTS OF VOLUME III OF THE GREEN BOOK OF THE C.C.I.F.

Line Transmission Maintenance

Page Tables summarising the characteristics of circuits: Telephony (and telegraphy)...... 16 Programme transm issions...... 18 T elevision...... , ...... 19

Part 1 — Telephony

Section 1

General characteristics of international telephone connection within a continent and of intercontinental connections 1.1. General characteristics of the complete international telephone connection or of the . continental section of an international intercontinental telephone connection . . . 21 1.1.1. Reference equivalents...... 21 1.1.2. Indices of transmission q u a lit y ...... 23 1.1.3. Impairments of transmission q u a li t y ...... 27 1.1.4. Group propagation time ...... 28 1.1.5. Phase distortion ...... 29 1.1. General characteristics of the chain consisting of the international telephone circuit and the national trunk extension circuits...... 30 1.2.1. Admissible equivalent in service of a trunk circuit as regards stability and echo 30 1.2.2. Echo effects ...... 30 1.2.3. Transmission stability...... 34 1.2.4. Linear c r o s s ta lk ...... 35 1.2.5. Circuit n o ise ...... 36 1.2.6. Interconnection of international and national c ir c u its...... 37 1.2.7. Impedance of international and trunk circuits ...... 38 1.2.8. Characteristics of loaded cable trunk circuits liable to carry international conversations...... 38 1.3. General characteristics of an international circuit within one continent...... 38 1.3.1. General ...... 38 1.3.2. Nominal equivalent...... 39 1.3.3. Frequency band effectively transmitted and attenuation distortion...... 40 1.3.4. Variation, as a function of time, of the equivalent of a complete circuit . . 41 1.3.5. Group propagation time...... 41 1.3.6. Phase d isto r tio n ...... 41 4 TABLE OF CONTENTS

Page 1.3.7. Linear c r o s s ta lk ...... 42 1.3.8. Circuit noise including non-linear c r o s s ta lk ...... 42

Section 2

General characteristics of intercontinental circuits 2.1. General characteristics of a very long intercontinental circuit on open wire lines . . 43 2.2. General characteristics of a very long intercontinental circuit with possibly one or more short submarine se c tio n s...... 43 2.3. Provisional general characteristics of an intercontinental circuit comprising a long deep sea submarine section ...... 44

Section 3

Detailed characteristics of international carrier telephone systems on metallic lines 3.1. Characteristics common to all modern carrier systems providing 12 long-distance telephony circuits ...... 45 3.1.1. General recommendations...... 45 Note. — Definition of psophometric power and general definition of “ nominal maximum circuits ” ...... 47 3.1.2. Terminal equipm ents...... 50 . 3.1.3. Transfer of a group or supergroup ...... 52 3.1.4. Group and supergroup pilots ...... 54 Note. — Definitions concerning international carrier sy ste m s...... 57 3.2. Systems providing 12 carrier telephone circuits on an open wire p a i r ...... 59 3.2.1. General characteristics...... 59 3.2.2. Terminal equipm ents...... 65 3.2.3. Intermediate repeaters...... 66 3.2.4. L in e s ...... 67 3.3. Carrier telephone systems on non-loaded symmetrical pair cables providing 1, 2, 3, 4 or 5 g r o u p s ...... 68 3.3.1. G eneral...... 68 3.3.2. Systems providing 1, 2 or 3 g r o u p s...... 73 3.3.3. Systems providing 4 groups . ' ...... 75 3.3.4. Systems providing 5 g r o u p s ...... 75 3.3.5. Intermediate and terminal repeaters...... 77 3.3.6. L in e s ...... 79 A. Cable specification...... 79 B. Specification of a repeater section...... 82 3.4. Carrier telephony systems on coaxial c a b le s ...... 85 3.4.1. General characteristics...... 85 3.4.2. Terminal E q u ip m en t...... 95 3.4.5. R ep eaters...... 95 3.4.4. L in e s ...... 96 A. Cable specification...... 96 B. Specification of a repeater section ...... 98 TABLE OF CONTENTS 5

Page 3.4.5. Constitution, supervision and supply of power to a coaxial system .... 98 3.4.6. Interconnexion of coaxial systems of different ty p e s...... 102 3.5. Other modern carrier s y s te m s ...... 104 3.5.1. Systems providing 3 carrier telephone channels on open wire lines .... 104 3.5.2. 12 + 12 systems in c a b l e • . 107

Section 4

General characteristics of carrier telephone systems on radio relay systems 4.1. Use of radio relay systems to carry international telephone circuits...... 110 4.2. Terminal equipments of radio relay systems forming part of a telecommunication netw ork...... 110 4.3. Radio relay systems employing pulse m odulation...... I ll

Section 5

Audio circuits 5.1. General characteristics -. . 113 5.1.1. Frequency band effectively tra n sm itted ...... 113 5.1.2. Non-linear distortion ...... 113 5.1.3. Interconnection of international circu its...... 115 5.1.4. Other characteristics...... 115 5.2. L in e s ...... 115 5.2.1. Open wire lines and mixed lines ...... 115 5.2.2.' C a b le s ...... 118 5.3. Repeaters ...... 119 5.3.1. Four-wire repeaters...... 119 5.3.2. Repeaters at the junction of two cables with different characteristics . . . . 120 5.4. Specifications recommended by the C.C.I.F. for audio circuits in ca b le...... 122 5.4.1. Repeater section of a line and its constituent p a r t s ...... 123 Specification A I. — Factory lengths of load telecommunication cable . . . 123 Specification A II. — Loading coils for telecommunication cab les...... 132 Specification A III. — Repeater sections of loaded ca b le ...... 135 Note. — Co-operation for the construction of the frontier section of loaded c a b le ...... 138 5.4.2. Terminal equipment and intermediate repeaters...... 140 Specification B I. — Line tran sform ers...... 140 Specification B II. — Audio repeaters (2 and 4-w i r e ) ...... 142 Specification B III. — Terminating u n its...... 146 Specification B IV. — Echo suppressors...... 147 Specification B V. — Repeater power supply ...... 149 Specification B VI. — Valves for repeaters...... 149 Specification B VII. — Repeater station c a b lin g ...... 150 6 TABLE OF CONTENTS

Part 2 — Utilization of international telephone circuits for telegraphy or photo-telegraphy Co-existence of telegraphy and telephony

Section 1 Use of carrier telephone channels for voice-frequency telegraphy page Essential characteristics for a carrier telephone channel intended to carry 24 voice-frequency telephony channels each at 50 bauds ...... 151 Conditions for the setting up and switching of a voice-frequency telegraphy circuit .... 154 Reserve circuits for voice-frequency telegraphy...... 155

Section 2 Alternate speech and voice-frequency telegraphy on a rented international c ir c u i t ...... 157

Section 3 Photo-telegraphy transmissions 1. with amplitude m od u lation ...... 159 2. with frequency modulation ...... 161

Section 4 Co-existence of telegraphy and audio frequency telephony Telegraphy and telephony on the same conductors sim u ltan eou sly...... 166 A. Telegraphy infra-acoustic...... 166 B. Telegraphy ultra-acoustic...... 167 C Telegraphy on phantom and double phantom circuits...... 167 Co-existant telegraphy and telephony on separate conductors ...... 168 Voice frequency te le g r a p h y ...... 168 Note. — Conditions to be fulfilled for circuits used for voice frequency telegraphy . 169

Part 3 — International programme transmissions Use of international cables for relaying programme transmissions...... 171

Section 1 Old type programmecir c u its...... 172 Line up and supervision of the international programme lin k ...... 177 L in e s ...... 180 Am plifiers...... 180

Section 2 Normal type programme circuits Provisional general characteristics of normal programme c ir c u its...... 183 Line up and supervision of the international programme lin k ...... 186 L in e s ...... 190 TABLE OF CONTENTS 7

Part 4 — International television transmissions on metallic lines Page I. — General characteristics of an international television circuit on metallic lines (provi­ sional recommendation of the C.C.I.F. applicable to all standards of television transmission used in Europe) ...... 193 II. — Characteristics recommended by the C.C.I.F. for the transmission on line of television s ig n a ls ...... 210

Part 5 — Co-ordination of radio telephony and telephony

S e c t io n 1 International radio telephony links using radio relay systems for international circuits .... 215

S e c t io n 2 Arrangements used to improve transmission 2.1. Measurement and regulation of speech volume. Instrument enabling the technical operator, at the junction between the radio and metallic circuit, to measure the volume 218 Adjustment of volume ...... 219 Note. — Conditions which should be satisfied by an automatic regulator at the junc­ tion of the metallic telephone network and the radio lin k ...... 219 2.2. Fading correctors ...... 220 2.3. Reaction and echo suppressors ...... 221 Classification of various types of reaction suppressors...... 221 Protection of reaction suppressors on radio telephony c ir c u it s ...... 225 False operation (by noise) of reaction suppressors or echo suppressors in an inter­ national telephone communication on radio and metallic circuits...... 225 2.4. Interconnection of two radio-telephony circuits by a 4-wire metallic circuit...... 226

S e c t io n 3 Principle of arrangements employed to ensure p r iv a c y ...... 227

S e c t io n 4 Requisite conditions for connections between mobile radio telephony stations and international telephone l i n e s ...... 229 Essential characteristics of arrangements controlled by speech currents in stations on ships and by the carrier on shore stations...... 231 8 TABLE OF CONTENTS

Part 6 — Maintenance

Page General recommendation: maintenance of international circuits ...... '. 233

Maintenance Instructions

Section 1

General Introduction. — Role of the 9th Study Group responsible for maintenance questions .... 236

Chapter I: Control and subcontrol s t a t io n s 236 • 1. Control sta tio n ...... 236 2. Subcontrol stations ...... 237 3. Role of control and subcontrol stations in localizing and clearing of faults . . . 238

Chapter II: Designation of international circuits or of assemblies of international circuits 239 1. Telephone circuits...... 239 2. Circuits used for voice-frequency teleg ra p h y...... 239 3. Circuits designated specially for phototelegraphy or facsim ile...... 240 4. Circuits intended for programme or television transm issions...... 240 5. Groups of telephone circuits: 12-circuit group, supergroup...... 240

Chapter III: Periodic maintenance program m e...... 241 1. Preparation ...... 241 2. Modifications to the p rogram m e...... 241

Chapter IV: General precautions to be taken to improve the transmission stability of interna­ tional circuits in E u rop e...... 242 Note. — Method of making vibration tests...... 243

Section 2

Establishment and maintenance of international carrier systems providing at least one primary (12-circuit) group

Preliminary note. — Designation of groups or supergroups and numbering of telephone channels of a carrier s y s te m : . . . 261

Chapter I: G en eral...... 267 1. Definitions concerning international carrier sy ste m s...... 267 2. Types of measurements to be m a d e ...... 269

Chapter II: Bringing into service of an international carrier s y s te m ...... 270 1. Preliminary exchange of in fo r m a tio n ...... i ...... 270 2. Establishment of high-frequency line for the bringing into service of a carrier system 270 3. Establishment of international group and supergroup lin k s...... 272 4. Establishment of telephone channels...... 278 TABLE OF CONTENTS 9

Page Chapter III: Periodic maintenance of an international carrier s y s t e m ...... 279 1. Coaxial line regulating s e c t io n ...... 279 2. Regulating section on a symmetrical pair...... 279 3. Supergroup link ...... 280 4. Group lin k ...... ' ...... 280 5. Apparatus use for maintenance m easurem ents...... 281 6. Measurements of non-linear distortion and tests on v a lv e s ...... 281 7. Check of master oscillators ...... 282

Section 3

Establishment and maintenance of international circuits used for telephony and for voice-frequency telegraphy Chapter I: Bringing an international circuit into service ...... 284 1. Preliminary exchange of in fo r m a tio n ...... 284 2. Measurements before bringing the circuit into s e r v ic e ...... 284 3. Conditions for the establishment and changeover arrangements for a voice-fre­ quency telegraph c ir c u it...... 285 Chapter II: Periodic maintenance of an international c ir c u it...... 286 1. Organization of periodic maintenance measurements . 286 2. Periodicity of maintenance m easurem ents...... 287 3. Method of making periodic m easurements...... 290 a) measurements of equivalent ...... - 290 b) tests of sig n a llin g ...... 291 c) determination of stability of circuits (only for circuits or sections of circuits containing 2- wire rep ea ters)...... 291 Chapter III: Maintenance of circuits used for voice-frequency te le g r a p h y ...... 291 ' 1. Recommendations for the organization of periodic maintenance measurements on circuits used for voice-frequency telegrap h y...... 291 2. Measurements to be made ...... 292

Section 4

Periodic measurements to be made on the line for circuits, or sections of circuits entirely on audio-frequency or low-frequency carrier systems 1. Periodicity...... 293 2. Method of carrying o u t ...... 294 a) measurements of repeater g a i n ...... *...... 294 b) tests for rejection of v a lv e s ...... 295 c) determination of singing point or of the stability margin of 2-wire repeaters 295

'S ection 5 Line-up and maintenance of international programme transmissions Chapter I: General organization of international programme transmissions...... 299 1. Technical responsibilities during an international programme transmission. Definition of the constituent parts of an international programme lin k ...... 299 2. Various types of circuits used for programme transmissions...... 300 3. Various classes of programme transmissions. Use of control c ir c u its...... 300 4. Definition and duration of line-up period and preparatory p e r io d ...... 302 10 TABLE OF CONTENTS

Page Chapter II: Establishment and maintenance of permanent circuits for programme transmiss­ ions ...... 303 1. Control and subcontrol stations...... 303 2. Establishment of the circuit ...... 303 3. Reference measurements...... 304 4. Periodic maintenance measurements ...... 305 Chapter III: Constitution of line-up, supervision and clearing-down the international pro­ gramme l i n k ...... 305 1. Measurements to be made before the line-up period which precedes a programme transmission...... 305 2. Measurements to be made during the line-up period which precedes a programme transmission ...... 306 3. Measurements made by the Broadcast Authority during the preparatory period 306 4. Maximum power to be transmitted during the programme transmission .... 307 5. Identification sig n a l...... 307 6. Supervision of the transmission...... 309 7. Procedure at the end of the programme tran sm ission ...... 309

S e c t io n 6 Maintenance of circuits used for television

Appendices Appendix I: Line-up record of coaxial lin e s ...... 311

Appendix II: Line-up record of symmetrical pair lin es...... 312

Appendix III: Routing form of a supergroup l i n k ...... 313

Appendix IV: Routing form of a group l i n k ...... 314

Appendix V: Line-up record of a supergroup link ...... 315

Appendix VI: Line-up record of a group lin k ...... 316

Appendix VII: Routing form of a circuit (hypsogramm)...... 317

Appendix VIII: Line-up record for a programme circ u it...... 318

Guiding principles for the maintenance of semi-automatic circuits

Chapter I: Definitions...... 321 Chapter II General rules for the organization of the maintenance of semi-automatic circuits 323 1. Principles...... 323 2. International maintenance centre (I.M.C.) ...... 323 3. Control repeater station ...... 324 Chapter III: Preventative m a in ten a n ce...... 325 1. Functional tests...... 325 2. Limit te stin g ...... '...... 326 3. Installation of testing apparatus provided for in the sp ecification ...... 326 t a b l e ' o f c o n t e n t s 11

Page Chapter IV: Corrective maintenance—Localisation and clearance of fa u lts...... 327 1. G e n e r a l...... 327 2. Reporting of faults to the I.M.C...... 327 3. B locking...... 328 4. Broad localisation of faults...... 328 5. Priority of localisation tests ...... 328 6. Clearance of the fault ...... 328 7. Record of the nature of faults when cleared ...... 329

Alphabetical Index 331 TABLE OF CONTENTS OF THE BOOK OF ANNEXES TO VOLUME III OF THE GREEN BOOK

Part 1 — Annexes to the recommendations of the C.C.I.F. contained in Volume HI of the Green Book

Annex 1. Calculation of the effects of echo and stability for a trunk circuit...... Annex 2. Cabling of carrier system bays used by the Cuban Telephone Company .... Annex 3. Note concerning the cabling of type 44 bays by the French Administration . .

Annex 4. Arrangement of cabling to reduce crosstalk in carrier system bays used by the British Adm inistration......

Annex 5. Method used by the British Administration for the balancing of new symmetrical pair non-loaded cables for use with 24-circuit carrier system s......

Annex 6. Methods used by the Netherlands for the balancing of new symmetrical pair non-loaded cables for 48-circuit carrier sy s te m s......

Annex 7. Method used by the French Administration for the balancing of repeater sections of symmetrical pair, non-loaded cables, intended to carry 60-circuit carrier systems ......

Annex 8. Methods used in Mexico by the Societe Telefonos de Mexico for the balancing of symmetrical pair, non-loaded cables for use with carrier systems ......

Annex 9. Specification of far-end crosstalk balancing frames used by the British Adminis­ tration ......

Annex JO. Specification of far-end crosstalk balancing frames used in Mexico by the Societe Telefonos de M exico......

Annex 11. Method of synchronisation used by the Bell System of the United States of America on coaxial systems of the types L 1 and L 3 ......

Annex 12. Method used in France for the coordination of master oscillators in a network of carrier and coaxial systems . . . , ......

Annex 13. Synchronisation of carrier frequencies in the carrier system network of the United Kingdom of Great Britain and Northern Ireland...... Annex 14. Principal characteristics of frequency standards and time signals (details fur­ nished by the C.C.I.R.)...... Annex 15. Frequency standard used by the Italian Administration......

Annex 16. Pilot waves with multiple functions used in the United States of America by the American Telephone and Telegraph Company...... Annex 17. Pilot waves with multiple functions used by the French Administration .... Annex 18. Pilot waves with multiple functions used by the British Administration .... TABLE OF CONTENTS OF THE BOOK OF ANNEXES

. Annex 19. Practices used or envisaged in different countries to improve the transmission on old-type trunk circuits ...... Annex 20. Tests of dielectric str e n g th ......

Annex 21. Note by the French Administration on the definition of the cut-off frequency of a loaded c a b l e ......

Annex 22. Study of the telegraph distortion caused by the phase distortion on a voice- frequency telegraph c ir c u it ...... Annex 23. Note by the French Administration on the use of v. u. meters for the supervi­ sion of programme tran sm ission s......

Annex 24. Note by the Administration of the Federal German Republic on the use of maximum amplitude indicators for the supervision of programme transmissions ......

Annex 25. Note by the British Administration concerning the peak programme meter used in agreement with the British Broadcasting Corporation for the supervision of programme transmissions......

Annex 26. Note by the British and French Administrations on the method of calculation of transient s t a t e ......

Annex 27. Description of the volume regulator used by the British Administration for use on radio telephone c ir c u its...... ■ . . . . Annex 28. Arrangement used in Switzerland to avoid pulse operation of echo suppressors .

Part 2 — Methods and apparatus for measurement on lines

Section 1. — Steady state alternating current measurements ...... - ...... 1.1. Composite attenuation and insertion loss measurements; measurements on line transformers ...... ,...... 1.1.1. Measurement of composite loss or gain of any quadripole ...... 1.1.2. Measurement of insertion loss or gain of any quadripole...... 1.1.3. Measurements on line transform ers...... 1.2. Measurement of equivalent and level at all frequencies ...... 1.2.1. Measurement of equivalent of telephone circuits...... 1.2.2. Measurement of level, absolute or r e la t iv e ...... 1.2.3. Apparatus for the measurement of equivalent or l e v e l ...... 1.3. Measurement of repeater g a in ...... 1.4. Measurement of impedances, and of capacity or inductance unbalances...... 1.4.1. Measurement of impedance — steady s t a t e ...... 1.4.2. Measurement of impedance — transient s t a t e ...... 1.4.3. Measurement of capacity u n b alan ce...... , . 1.4.4. Measurement of inductance unbalance......

1.5. Measurement of a tte n u a tio n ...... 1.5.1. Measurement of balance return l o s s ...... 1.5.2. Measurement of regularity return l o s s ...... 1.5.3. Measurement of return lo s s ...... 1.5.4. Measurement of return loss (echo currents)...... 1.6. Measurement of transmission stability ...... 14 TABLE OF CONTENTS OF THE BOOK* OF ANNEXES

1.6.1. Determination of singing p o i n t ...... 1.6.2. Measurement of stability of a telephone c ir c u it ...... 1.6.3. Measurement of the oscillation margin of a telephone c ir c u it ...... 1.7. Measurement of phase d is to r tio n ...... 1.8. Measurement of non-linear distortion ...... 1.8.1. Measurement of harmonic distortion...... 1.8.2. Measurement of intermodulation distortion...... 1.9. Crosstalk measurement...... 1.9.1. At audio-frequencies...... 1.9.2. Between symmetrical pairs for carrier system s...... 1.9.3. Between coaxial p a i r s ......

Section 2. — Measurements in the transient state on factory lengths or on repeater sections of coaxial pairs (used for telephony or for television) and on repeater sections of open wire lin e s......

Part 3 — Annexes concerning methods and apparatus for line measurement

Annex 30. Method of equalization during the setting up of coaxial systems studied in Great B rita in ......

Annex 31. Measuring apparatus used by the Swiss Administration for the maintenance of symmetrical pair carrier system s......

Annex 32. Description of a method of selective measurements on 12 circuit carrier systems used in Mexico by the Societe Telefonos de M exico......

Annex 33. Note by the French Administration on the measuring apparatus for the main­ tenance of carrier systems ......

Annex 34. Measuring apparatus used by the Cuban Telephone Company for the main­ tenance of carrier systems ......

Annex 35. Measuring equipment used by the Netherlands Administration for the main­ tenance of carrier sy stem s......

Annex 36. Methods used by the Administration of the Federal Republic of Germany for the measurement of attenuation distortion on symmetrical pairs intended for carrier systems......

Annex 37. Method of measurement of the characteristic impedance of a coaxial pair, terminated at the end by a specified impedance (memorandum by the Standard Telephone and Calls C om pany)......

Annex 38. Measurement of phase distortion on a looped circuit (Note by the British Broadcasting Corporation)......

Annex 39. Method proposed for the measurement of group delay time in television terminal equipments and in television transmission sy stem s......

Annex 40. Method of measurement of group delay time between the extremities of a link (Note by the French Administration)......

Annex 41. Measuring arrangements used by the Swiss Administration for the measurement of non-linear distortion o f line repeaters and of systems of 12, 24, 36 and 48 tele­ phone carrier channels on symmetrical pairs in ca b les......

Annex 42. Methods employed by various Administrations and Private Operating ‘Com­ panies for the measurement of non-linear distortion of the whole of a carrier system on non-loaded symmetrical p a ir s...... TABLE OF CONTENTS OF THE BOOK OF ANNEXES 15

Annex 43. Methods and apparatus used by the British Administration to measure non­ linear distortion of a coaxial l i n e ......

Annex 44. Methods and apparatus used by the Cuban Telephone Company for frequency checking on a coaxial system ......

Annex 45. Methods used by the Danish Administration to measure the difference between the frequency of a wave at the input of a carrier channel and at the output .

Annex 46. Method used by the French Administration to measure the ratio between the frequency of a wave at the input of channel routed over several carrier systems and at the channel o u tp u t......

Annex 47. Apparatus for the localization of impedance irregularities on lines by means of impulses, used by the Australian Administration...... Annex 48. Apparatus for the localization of impedance irregularities on lines by means of impulses, used by the Cuban Telephone Company . . • ...... Annex 49. Apparatus for the measurement and localization of impedance irregularities on lines by means of impulses, used by the French Administration......

Annex 50. Apparatus used by the British Administration for testing coaxial cables by means of impulses...... Annex 51. Apparatus for the localization of impedance irregularities on coaxial pairs by . means of impulses, used by the Swedish Administration...... Annex 52. Description of a television signal generator constructed by the Cuban Tele­ phone Company ...... Annex 53. Television waveform generator used in Great Britain...... Annex 54. Television waveform generator used by Radiodiffusion Fran?aise...... Annex 55. Television monitoring apparatus used by Radiodiffusion Frangaise ..... Annex 56. Essential characteristics of monitoring arrangements (to be placed at junction points between international television circuits and national extension circuits) proposed by the British Administration. 16 17

Summary of the principal characteristics specified by the C.C.I.F. for the various types of international circuits and of continental communications (the page numbers in brackets are those of the present work) (the page numbers in brackets are those of the present work)

I I I. Circuits and telephone communications For a i For the international circuit complete communication or its parts For the For a complete communication international circuit or for its parts {!) (2)

1 2 Near end crosstalk ration between the 4-wire circuit: 4 N for In an intematio- two directions of transmission voice frequency telegraphy nal centre 5.8 N 1 (p. 36, 154) ) (p. 36) 1) Reference equivalent and in- index of trans. qual...;;::; 0.8 N *) For the communications and 1 I dex of transmission quality (p. 25) for the national systems · at the end of the Not exceeding 2 m V for (See p. 36) 1) 'i:' v I equivalent < 0.8 N (p. 37, ref. equiv. (seep. 21-22) I u~ ~ circuit 99 % of the two (p. 42) *) 39) *) ind. of trans. qual. (seep. 25-27) .g ..9 I echos and stability (p. 30) -~~~ relative level I -0.SN gg~ Q) ~ I impairments (seep. 27-28) E·- referred to zero _g 0 Band of Limits in c/s 300-3 400 (p. 40) from international trunk ex- ·s(.) c~~ 0 °'0 e0 relative level e.m.f. 4.4 mV frequencies change to trunk exchange: ·-~ ·-"""'"O (.) rn *'- effectively 300-3 400 (p. 24) u .2 voltage (P.D.) 2.2 mV C.> ~i transmitted g absolute level of power additional ~ - 50 db (10 OOO pW) I attenuation 1 N (p. 40) Circuit impedance A single value for an ex.- For trunk cir- at limits of 1 N (p. 40) change: frequency in cuits: 600 or 800 ohms (p. 38) as ( 1) (p. 38) nepers I Attenuation distortion graph. No. 1 (p. 40) I .... Telephony not specified I see also graph No. 5 (p. 153) I t:l Group delay (t) I t ..;;::; 150 ms (p. 29, 41) *) For the communication: ·s I Total maximum I 5 mW (p. 160) or for each I 0. I preferred to 100 ms *) t < 250 ms (p. 28) ~ power of telegraph telegraph channel: (p. 29, 41) For a national system: .... currents rarely 24 channels: - 9µW ~~ I t < 50 ms (p. 29) ""> attained in voice 18 channels: - 15 µW 1 >. frequency telegra- 12 channels and less : Phase distortion (from the tm - /~in < 10 ms ) *) For the communication: ~~ .s:: 2 1 0 ~ 0. phy .:..._ 35 µW group delay t) ) fM- lmin 5 ms ) "') Im - lmin 5Q ms o.._ I < < ~ IM - fmin < 25 ms (p. 29) ~(;; Iii> 1 I ,.c 'i) Private telegraphy 0.3 mW (p. 157) (p. 29): Im - fmin ± 20 ms ) For each national section: ~ .... 1 ~ 0 I IM - fmin < 10 ms ) ~ I -u0 .... Photo-telegraphy Amplitude modulation Stability of transmission For the chain of national and ~N I 1 mW (p. 159) 1) international circuits 0.2 N e 'o 1 ::l Frequency modulation (p. 34) ) for telephone 2 N 1 0.1 mW (p. 161) ) I (p. 161) and phototelegraphy .§ I ~ Maximum variation of equi- ± 0.2 N (p. 41) For voice-frequency See volume V 3 ~ ISi.snail- valent as a funtion of time ) I I mg signalling Linear crosstalk between diffe- 4-wire circuits in cable: Trunk circuits in cable (2 or rent circuits (near or far-end a > 6.7 N for 90% (p. 42) 4-wires): as (1) (p. 35) 1) Duration of interruption in changeover around 150 ms (p.101) 0 ) crosstalk ration6.) a > 6.0 N for 100% Trunk circuits on open wire to standby power supply open wire circuits; some corn- as (1) (p. 35) 1) I I combinations In an international centre: 1) Provisional recommendation. 1 a > 5.4 N (p. 42) ) a> 8 N (p. 35) •) For "the nominal maximum circuit" on coaxial pairs: the limit for other types of circuits is being studied. for others ..) Unless the mains supply is guaranteed. 1 a > 6. 7 N {p. 42) )

1) Provisional recommendation. 2) m = nominal minimum frequency eft'ecfrrely transmitted. M = nominal maximum frequency effectively transmitted. min = frequency corresponding to minimum group delay time. 2) Desirable value; the values met in practi~ are the object of a question for study by the C.C.I.F. •) These values apply to the chain of international circuits when there is more than one.

2 18

Summary of the principal characteristics specified by the C.C.I.F. for various types of international circuits (the page numbers in brackets are those of the present work)

II. Circuits for programme transmissions

Old type Normal type (3) (4)

Frequency band Limit in c/s at least 50-6 400 at least 50-10 000 effectively transmitted (p. 174) (p. 184)

Additional 0.5 N 0.5 N attenuation at these frequency (p. 174) (p. 186) limits in nepers

Attenuation distortion Groups Nos. 8 and 9 Group No. 10 (p. 175) (P. 184)

Phase distortion *50 — *800 < 70 ms *10000—*min < 8 m s 1) (from the group delay time *) 2) *6400 — *800 < 10 ms *100— *min < 20 ms 1) (p. 174) *50 — *min < 80 ms *) p. 184)

Variation of relative level as a function of . it 0.2 N in the course time of a transmission (P. 186) i)

maximum allowable power at zero level 8 mW (average power as (3) i) point of a sine wave of the same peak voltage (p. 183) (P- 174)

definition of zero relative level point (see p. 174) (see p. 183)

linear crosstalk (near or far-end crosstalk A > 8.5 N (cables) x) as (3) i) ratio A) *) 7 N (open wire lines) (P. 176) (p. 185) **)

circuit psopho- At the end of the cir­ in cable on open noise metric cuit wire (including voltage non-linear 6.2 mV 15.6 mV crosstalk) (P- 176). (p. 176) as (3) i)

relative level + 0.7 N + 0.7 N (p. 184)

referred to 2.2 V = 2.2 V = zero relative level 710 = 283 = 3 mV 7.75 mV

1) Provisional recommendation. 2) See page 16. *) Between circuits for programme transmission. Also on the same page are the limits for crosstalk with telephone circuits. **) Special precautions for crosstalk between the two directions of transmission (p. 185). 19

Summary of the principal characteristics specified by the C.C.I.F. for international long-distance metallic lines for television transmission (the page numbers in brackets are those of the present work)

Table 1 Characteristics independent of the television standard, for all standards used in Europe

Desiderata of a Provisional Recommendation Committee of the C.C.I.R. for the of the C.C.I.F. Characteristics television AD *) for the long distance line BC *)

(5) (6)

A. Input and output im­ Nominal value 75 ohms **) Return loss > 20 db pedance at the video (p. 196) terminals

B. Polarity of signal Positive Positive (p. 197)

Non useful d.c. Should not dissipate more component than 0.5 watt in a 75 ohm impedance: should not ex­ as (5) (p. 197) ceed 60 volts when this im­ pedance is disconnected.

C. Amplitude of si­ The C.C.I.F. asks the gnal C.C.I.R. to standardise 1 volt peak to peak (p. 197) at video at video junction points

Characteristics Characteristics of video signal D. Ratio of vision The C.C.I.F. asks the signal to syn­ * C.C.I.R. to standardise chronising signal a value (p. 197)

E. Non-linear distortion For the vision signal: Being studied (p. 198) annexes 1 to 4 (p. 204). For the synchronising signal: slow Variations + 10% to — 30 % of the nominal am­ plitude

F. Variations of attenua­ + 0.3 db in 1 second for Being studied (p. 198) tion as a function of example; ± 1 db in*l hour time for example or in 1 month for a circuit specially super­ vised; ± 2 db in 1 month for example.

Difference in group delay 0.1 second (p. 200) between image and sound

*) See the definitions p. 193. **) The C.C.I.F. has recommended the following provisional tolerances: return loss equal or greater than 20 db cr 75 ohm ± 2.5 (p. 195). 2 0

Table 2 Characteristics of long distance line *) for which the provisional specifications of the C.C.I.F. fixed different values according to the television standards

Television Standard Characteristics 405 line 625 line 819 line

Value o f / c (upper limit of video fre­ quency) ...... 3 ‘ Mc/s 5 or 6 Mc/s 4) 10 Mc/s

G. Signal-to-noise ratio for various types of disturbance (p. 214-215): a) Continuous random noise of uniform spectrum *).... ■ 50 db 48 db ***) a ') continuous random noise rising with frequency * ) ...... 42 db 41 db ***) 40 db b) pattern noise **) Frequency 50 c / s ...... 300 db 30 db 45 db 100 c / s ...... 45 db 45 db 45 db 1 k c /s...... 55 db 50 db 45 db 1 M c / s ...... 55 db 50 db 45 db

Decreasing Decreasing Decreasing linearity linearity linearity 6 M c / s ...... 20 db fc ...... 25 db 30 db 20 db c) erratic noise (for very short im­ pulses of low repetition fre-» quency ...... 25 db 25 db 25 db

C r o s s ta lk ...... As for pattern noise in G.b (p. 200) x)

H. Amplitude and phase response . Fig. 50 Fig. 51 Fig. 52 (p. 202) (p. 203) 2) (p. 204)

*) Logarithmic ratio between the peak amplitude of the vision signal and the r.m.s. value o f noise in the band 0 to /c (p. 199). **) Logarithmic ratio between the peak to peak amplitude of the signal to noise (p. 199). ***) Also applicable to the Belgian 819 line system. 1) See in note at bottom of this page other values proposed for 405 line standard. 2) If the HF channel is 8 Mc/s take 6 Mc/s for / c . 8) The C.C.I.R. intend that these values apply to the television circuit if the junction lines are short. 4) fc = 5 'Mc/s if the HF channel is 7 Mc/s. fc = 6 Mc/s if the HF channel is 8 Mc/s.

/ FIRST PART

TELEPHONY

SECTION 1

General characteristics of international and intercontinental connections and of international and intercontinental circuits

1.1. General characteristics of the complete international telephone connection or of the continental section of an international intercontinental telephone connec­ tion. 1.1.1. Reference equivalents

Practical limits of the reference equivalent between two subscribers, of the reference equivalent o f the national sending system and the reference equivalent of the national receiving system.

In all international telephone connections between two subscribers within the same continent, the reference equivalent between the two subscribers should not exceed 4.6 N (40 db). The reference equivalent of the national sending system (from the ends of the international circuit) should not exceed 2.1 N (18.2 db). The reference equivalent of the national receiving system (from the ends of the international circuit) should not exceed 1.5 N (13 db). If gain is introduced at the international exchange (for example by adding a repeater to compensate for the attenuation of the circuit between the international exchange and the end local exchange), this gain will be included in the above-mentioned reference equivalents of the national systems. If, in certain connections, the nominal equivalent of the international circuit is reduced by a certain amount at the international exchange concerned, this reduction, will be considered as equal to a corresponding gain introduced into the national systems.

Note 1. — Efforts should be made to ensure that the maximum limit of 4.6 N for the reference equivalent between the two subsribers, is met for all international connections. All types of variation should be taken into account including variations 22 REFERENCE EQUIVALENTS with time and tolerances with respect to nominal values of reference equivalents of lines and equipments. Administrations or Private Operating Companies should allow for the fact that it is possible to have variations of about 0.35 N (3 db) in the values of the reference equivalents measured in the laboratory of the C.C.I.F., but it is not thought for the present any tolerances can be specified for possible variations due to those causes in the preparation of plans for national telephone networks.

Note 2. — The limiting conditions for transmission shewn above, concern only reference equivalent (4.6 N for the whole of the transmission system) and do not take into account reductions in quality of transmission due to the effects of noise and limitation of the band of frequencies effectively transmitted (see section 1.1.3 below).

Practical limits for the reference equivalent between two operators or between one operator and a subscriber

In an international telephone communication, the reference equivalent between two operators or between an operator and a subscriber should not exceed the values given in the following table: —

Communications Communications between one operator and a subscriber between two operators

Reference equivalent Reference equivalent Reference equivalent of the connection between of the connection between of the connection between an operator and a subscriber an operator and a subscriber two operators at the same end of the at either end of the international line international line

Subscribers’ Subscribers’ International International Subscribers’ Subscribers’ lines . lines circuit circuit line line disconnected connected disconnected connected disconnected connected

2 5 N 3 3 N ■ 2 55 N ' 2 95 N 3 55 N 3 95 N

(21 8 db) (28 7 db) (22 2 db) . (25 7 db) (30 9 db) (34 5 db) i

Note — To ensure that the limits indicated for reference equivalents are not exceeded, Administrations and Private Operating Companies may use various methods. For example, models could be made representing the principal combina­ tions of commercial subscribers’ instruments, subscribers’ lines, auxiliary lines and units of the local and trunk/toll exchanges, each of these models representing a com­ plete national sending system or a complete national receiving system which may be compared by an articulation test with the master Telephone Transmission Reference System (S.F.E.R.T.) without distortion or with a working standard which has already been compared with S.F.E.R.T. One could also limit oneself to measuring the reference equivalent of the subscribers’ apparatus under certain specified condi­ tions: to this reference equivalent should be added the factory tolerances of the subscribers’ apparatus considered, the image attenuation (calculated or measured at 800 c/s) of the subscribers’ lines, auxiliary lines and circuits connecting this apparatus to the international exchange, and the composite attenuations (measured or calculated TRANSMISSION PERFORMANCE RATING 2 3 at 800 c/s and terminated with a non-reactive resistance of 600 ohms) of the units of the telephone exchanges used in the connection, between the subscribers’ apparatus and the international exchange (including the units of the exchange serving the subscriber and the units of the international exchange). But in all cases it is necessary to verify the results of the calculations by means of an articulation test made on models representing the typical complete national sending and receiving systems.

1.1.2. Transmission performance rating a) Definitions of transmission performance rating and of the articulation reference equivalent (A.E.N.) Articulation reference equivalent (A.E.N.) G.B. — Equivalent articulation loss (Am.) Affaiblissement equivalent pour la nettete (A.E.N.) (F.) Supposing that articulation tests are made alternately on a telephone system and on the Reference System for the determination of A.E.N. (S.R. A.E.N.) with different values of line attenuation, up to values such that the articulation on the two systems is very considerably reduced; the results of these tests are traced in the form of curves representing the variation of the sound articulation as a function of attenuation and from it is determined the value A3 of the attenuation for the system considered and the value A2 for the attenuation for the « S.R.A.E.N. » at a fixed value of 80 % of the sound articulation. (A2 — Ax) is by definition equal to the articulation reference equivalent (A.E.N.). Transmission performance rating relative to S.R.A.E.N. (Indice de qualite de transmission) It is the value (in decibels or nepers) of the additional attenuation which has to be inserted in, or removed from, the Reference System for the determination o f the articulation reference equivalents (S.R.A.E.N.) to obtain an equal transmission qua­ lity when the apparatus under consideration is either substituted for the complete reference system, or added to one of the suitably selected sections. This equality of transmission quality in the national plan should be based on articulation tests. b) Calculation o f the nominal transmission performance rating of a national sending or receiving system •The nominal transmission performance rating of a national sending or receiving system is the sum of the following quantities: — 1) The transmission performance rating (average value in service) of the local system, 2) The transmission performance rating of the connection between the local exchange and the international exchange (average value in service).

1) When it is only needed to check if a national sending or receiving system meets the limits fixed for the international service, in paragraph d) below, the transmission performance rating of the local system can be take to be the same as the A.E.N. of this system (see note 1 below). The nominal transmission performance rating of the local system considered is thus equal to the A.E.N. (articulation reference equivalent) of the system compri­ 2 4 TRANSMISSION PERFORMANCE RATING sing a typical subscribers’ set (that is to say the average value in service for the type of set considered) connected by an artificial line (representing the fine of the subscriber actually used) to a typical feeding bridge of the local exchange. This value is equal to the A.E.N. determined at the Laboratory of the C.C.I.F., decreased by a correc­ tion if the subscriber’s line actually used is less unfavourable than that which has been given to the Laboratory of the C.C.I.F.

2) The average transmission performance rating (relative to S.R.A.E.N.) of of the connection between the local exchange and the international exchange is equal to the sum of the following numbers: — . — the equivalent of the trunk/toll circuits between the last trunk/toll exchange and the international exchange, measured at 800 c/s, increased by the reduc­ tion of quality of transmission, due to limitation of the band of frequencies effectively transmitted (see section 1.1.3 below) when these circuits have an attenuation distortion greater than that which is allowed in the recommen­ dations of the C.C.I.F. (see section 1.3.3. below). — the nominal transmission performance rating of the intermediate lines given by the following expression: — i = K X L where i = average transmission performance rating in nepers or decibels, L = length of the intermediate line in kilometers, K = coefficient which depends on the type of intermediate line con­ sidered, in nepers per kilometer or decibels per kilometer (see note 2 below) — the transmission performance rating of each intermediate exchange. The rating resulting from the insertion of a unit which, in accordance with the recommendations of the C.C.I.F., effectively transmits frequencies from 300 to 3 400 c/s, can be calculated by taking the arithmetic mean of the four values of insertion loss (or gain) of the unit considered measured at 500, 1000, 2 000, 3 000 c/s and expressed in decibels or nepers. Until more accurate values of this rating are available, after measurements have been made by each Administration, a provisional value of 1 db. for each exchange in the connection will be used.

Note 1. — Circuit noises which are within the Emits fixed by the recommen­ dations of the C.C.I.F. are not taken into account.

Note 2. — The composite attenuation of the lines connecting the international exchanges to the local exchanges should be such, that the reference equivalent of the national sending system, and the reference equivalent of the national receiving sys­ tem remain within the limits considered compatible with good telephone trans­ mission.

Note 3. — For future network projects, the lines connecting international exchanges to local exchanges should be capable of transmitting effectively the band of frequencies from 300 to 3 400 c/s. TRANSMISSION PERFORMANCE RATING 2 5

The attenuation distortion of these lines connecting the international exchanges to the local exchanges should not appreciably increase the attenuation distortion of the international connection; therefore, if loaded lines are used it is necessary to select a sufficiently high cut-off frequency.

Note 4. — It is necessary, on the international level, to evaluate the transmission performance rating by measuring the transmission quality by articulation tests and the following note indicates the relation between the transmission performance rating and the A.E.N. (for sending and receiving) for a commercial telephone system. c) Determination of A.E.N. The reference system for the determination of the A.E.N. (S.R.A.E.N.) and the method of determination of the A.E.N. of commercial telephone systems at the Laboratory of the C.C.I.F. are described in sections 3.1.3. and 3.1.4. of Vol. IV of the Green Book. d) Limits o f the transmission performance rating between two subscribers, o f the transmission performance rating of the national sending system and the trans­ mission performance rating of the national receiving system. It is very desirable that for 90 % of the international communications, the nominal transmission performance rating of the complete connection, from subscri­ ber to subscriber, should not exceed 5.65N (49 db.). Provisionally this limit can be sub-divided as follows:— For 90% of the international calls: — the nominal transmission performance rating of the national sending system should not exceed 2.77 N. (24 db). — the nominal transmission performance . rating of the national receiving system should not exceed 2.07 N. (18 db). It is assumed in the above that the nominal transmission performance rating of the international circuit does not exceed 0.8 N. (7 db). This nominal trans­ mission performance rating is equal to the nominal equivalent of the circuit at 800 c/s (defined as in sections 1.1.1. above and 1.3.2. below), increased if necessary on account of reduction in transmission of quality (see section 1.1.3. below): This limit does not take account of the variations of equivalent as a function of time of the international circuit, with respect to its nominal value.

Note 1. — The limits (24 and 18 db.) for the national sending and'receiving systems, do not include the probable variations as a function of time, of the equiva­ lents of the trunk/toll circuits which form part of the national system.

Note 2. — These limits apply to the values of A.E.N. deduced from the values measured for a local system, at the Laboratory of the C.C.I.F., as shewn in section 3.1.4. of Vol. IV of the Green Book with, in particular, room noise at the receiving end of 60 db. for commercial systems and background noise (characterised by a psophometric e.m.f. of 2 millivolts) injected into the input of the receiving system of the S.R.A.E.N. 2 6 TRANSMISSION PERFORMANCE RATING

R EM A R K 1

Relation between the transmission performance rating and the A.E.N. (for sending and receiving) for a commercial telephone system

The variation of the reference equivalent of the side-tone path of a telephone set affects at the same time the sending efficiency and the receiving efficiency of the instrument. The resulting effect on the “ transmission performance rating ” of a symmetrical telephone system, having the same conditions of subscriber’s local line and room noise at the two ends, is approximately equal to the sum of the separate effects for sending and receiving. The technique for the measurement of A.E.N. includes the direct measurement of the effect on reception of the level of room noise used in the tests, because this effect at the receiving end comes from the masking effect produced on the vocal sounds received, by the room noise arriving at the telephone receiver by the side-tone path. The effect at the sending end is due to the fact that a variation of the reference equivalent of the side-tone path causes the talking subscriber to alter his voice level. Measurement of the A.E.N. implies the use of a “ constant volume ” (of vocal sounds), and in consequence, does not take, account of this effect. In principle, it would be expedient to make corrections to the measured values of the A.E.N., to allow for effects which are produced in service because of differences with respect to the conditions specified for the determinations of these values of A.E.N. Never­ theless, when it is only a question of evaluating the transmission quality of commercial telephone connections, having characteristics corresponding to a transmission quality near to the admissible limit in service, the small differences which result because of the different side-tone conditions at the sending end, can, in practice, be ignored. It is therefore con­ sidered that, provisionally, the correction on account of the effect of side-tone at the sending end is practically nil. Administrations or Private Operating Companies wishing to prepare transmission plans for their national network, on the basis of transmission performance rating, will find in Annexe 2 of the Book of Annexes to Vol. IV of the Green Book, information on the correc­ tions to be made to the values of A.E.N. to allow for this side-tone effect at the sending end.

R EM A R K 2

Average transmission performance rating of intermediate links An intermediate line may be considered as a quadripole inserted between the im­ pedance of the first trunk/toll circuit, seen through the switchboard (or switches), and the impedance of the local system (feeding bridge + subscriber’s line -(- subscriber’s apparatus). For a given frequency, the loss introduced by such a line is represented by its “ composite attenuation ” *) which is the sum of the image attenuation of the line itself and of other terms representing all the effects due to reflections introduced by mismatch between the image impedance of the line and the impedances of the terminations defined above. Tests made by the British Administration indicate that the impairment due to the reflections can be taken to be equal to the arithmetic mean of the reflection losses measured at frequencies of 500, 1 000, 2 000 and 3 000 c/s. The transmission performance rating of an unloaded line, is measured by its image attenuation at 1 500 c/s and this is approximately equal to the arithmetic mean of the image attenuations at the four frequencies quoted above. **)

*) In practice, instead of using the composite attenuation, insertion loss may be used **) The attenuation of a non-loaded cable circuit is proportional to the square root of the frequency. The frequencies 500, 1 000, 2 000, 3 000 c/s are in the ratio 1,2, 4,6 and their square roots in the ratio 1, 1 41, 2, 2 45 o f which the arithmetic mean is 1 72, i e almost the square root of 3; therefore this mean corresponds to a frequency of 3 x 500 = 1 500 c/s. REDUCTIONS IN QUALITY OF TRANSMISSION 2 7

Therefore, the transmission performance rating of the intermediate line may be obtained directly, taking account not only of the effect due to the image attenuation but also of the effect' of reflections, by taking the arithmetic mean of the composite attenuations measured at the four frequencies above. As the impedance of the local systems varies widely, it is not possible to define a single value for the average transmission performance rating for an intermediate line, but only an average value obtained by taking the arithmetic mean of many values of the transmission performance rating, measured under many terminal conditions (see “ C.C.I.F. — 1952/1954 — 4th SG — Document No. 32 ” Annexe). For each type of intermediate line (defined by the electrical characteristics of the line), the average transmission performance rating is proportional to the length of the line, the ratio being easily determined when three or four values of the transmission performance rating are known. It is given by the formula i = K X L . (1) where i = average transmission performance rating in nepers or decibels. L = length of intermediate line in kilometers. K = coefficient, which depends on the type of intermediate line considered, in nepers per kilometer or in decibels per kilometer. To determine, once for all, the different values of the coefficient K, it is possible to measure the composite attenuation of three or four different lengths of each type of inter­ mediate lines used in a particular network (if necessary using artificial lines); to do this it is possible to use the technique described in document 32 referred to above (see also Annexe 2 to question N o. 10 in Vol. I ter o f the Yellow Book of the C.C.I.F. page 400), and one of the methods of measuring of the composite attenuation described in Book of Annexes to Vol. Ill of the Green Book, 2nd part, section 1.1.1. , From equation (1) the value of the -average transmission performance rating may be calculated for any length and any type of intermediate line in the national network con­ sidered.

1.1.3. Reductions in quality of transmission a) Reduction in quality of transmission due to limitation of the frequency band effec­ tively transmitted by the trunkltoll circuits. In the United States of America repetition rate tests have been made, and arti­ culation tests have been made in various national laboratories, as well as in the Laboratory of the C.C.I.F. From the results obtained the curve of figure 1 has been prepared shewing the reduction in quality of transmission due to limitation of the band of frequencies effectively transmitted by a trunk/toll circuit. The equation of this curve is y = 2 (3.7 — f)e, where y is the reduction in the quality of transmission (in db) due to the limitation of the frequency band effectively transmitted, and f is the frequency (expressed in kc/s) for which the equivalent of the circuit exceeds by 10 decibels the equivalent at 1000 c/s. N o te—■ The reduction in quality of transmission due to the limitation of the band of frequencies effectively transmitted, for a chain of national trunk/toll circuits, or for a connection between two international exchanges using several international circuits, is not determined by adding individual reductions in quality of transmission. The reduction in quality of transmission of the circuit which transmits effectively the narrowest band of frequencies is the determining factor. b) Reduction in quality of transmission due the circuit noise This question is being studied by the C.C.I.F. 2 8 PROPAGATION TIME

Annexe 3 of the Book of Annexes to Vol. IV of the Green Book gives information regarding a method of evaluating the “ reduction in quality of transmission due to circuit noise ” which can be used until the result of this study is available.

c) Reduction in quality of transmission due to room noise The method of measuring A.E.N. takes account of room noise (x) of 60 db (Hoth spectrum) at the receiving end; also Annex 4 of the Book of Annexes to Vol. IV of the Green Book gives information regarding the method of evaluating the “ reduction in quality of transmission due to room noise ” used in the United Slates of America.

A,8 3.0 2,8 2.H 8.6 8,8 3,0 3.8 3M 3,6 3,8 Ke/t>

The frequencies shewn as abcissae are the “ maximum frequencies effectively transmitted ” as used by the United States of America, i.e. those having attenuation 10 db greater than the attenua­ tion at 1 000 c/s.

F ig u r e 1. — Reduction in quality of transmission due to limitation of the band o f frequencies effectively transmitted

1.1.4. Group propagation time

It is necessary, in a continental communication, to limit the propagation time between two subscribers to a value fixed provisionally at 250 ms (group propagation time measured at a frequency around 800 c/s).

*) Details on room noise are given in paragraph c of section 1.3 in Vol. IV of the Green Book. PHASE DISTORSION 29

In order to ensure that this limit is met, the group propagation time on each of the national sending or receiving systems, should not exceed 50 ms, and the group propa­ gation time on the international circuit or on the chain of international circuits should not exceed 150 ms. These figures represent the maximum values: but for the establishment of the General Interconnection Plan values of 50 ms for each of the national sending and receiving systems and a total of 100 ms for the two international circuits entering into a transit type international connection, will be taken as a basis. It is desirable for an intercontinental connection, wherever economically possible, to limit the group propagation time on the connection within the European continent, between the subscriber and the ends of the intercontinental circuit, to a value fixed provisionally at 100 ms.. In future, it is desirable to use on main arteries of the international network, circuits with a high velocity of transmission. Note. — The “ group propagation time ” referred to above is the differential of the phase shift of the circuit (or chain of circuits), measured in radians, for the frequency f considered, with respect to the frequency measured in radians per second. This group propagation time is the • time taken for the peak of the envelope formed by two close frequencies to and (co+dw) to pass through the circuit (or chain of circuits).

1.1.5. Phase distorsion

Generally, the phase distortion of the international circuits and the national trunk/toll circuits (including equipment) should be such that the differences between group propagation times do not exceed the following values *:

Admissible difference between the minimum value of the group propagation time throughout the band of frequencies to be transmitted and the group propagation time at the

Lower nominal limit Upper nominal limit of the frequency band of the frequency band to be transmitted to be transmitted ■

1. Continental connection On the international section of the connection . 10 ms 5 ms On each of the national sections ...... 20 ms 10 ms Or on the chain of c ir c u its ...... 50 ms 25 ms

2. Intercontinental connection On the section included between the subscriber and the origin of the intercontinental circuit . 30 ms 15 ms

*) Provisional Recommendation. — It is possible that these limits cannot be achieved in the case of very long calls or calls with many modulations or demodulations. 3 0 CHAIN OF INTERNATIONAL AND TRUNK/TOLL CIRCUITS — ECHOES

1.2. General characteristics of the chain formed by the international telephone circuits and by the trunk/toll extension circuits

1.2.1. Minimum allowable attenuation of a trunk/toll circuit having regard to echo and stability

It is necessary to make sure by calculations or measurements that an existing circuit having a particular attenuation (allowing for variations with time of the transmission characteristics of the circuit), between the switchboard jacks at the two ends of the circuit, will give satisfactory results as regards echo and stability, before deciding if this trunk/toll circuit can be used to form part of a communication in the General Interconnection Plan. On the other hand, for new circuits, it is necessary to ascertain by calculations the type of circuit to be used, so that with a given attenuation value between the jacks of the trunk/toll switchboards, the results will be satisfactory as regards echo and stability (allowing for variations with time of the transmission characteristics of the circuit). The nominal minimum allowable attenuation for a trunk/toll circuit is obtained by calculating: 1) the minimum allowable attenuation as regards echo (for the spea­ ker); 2) the minimum allowable attenuation as regards stability (near singing). The higher of the two minimum attenuation values determined in considering respectively echo and stability is the attenuation value below which the circuit should not fall at any moment. Thus, the nominalvalue in service is obtained by adding to the above value, the variations of attenuation with time which may occur; Annexe 1 of the Book of Annexes to Vol. Ill of the Green Book, entitled “ Calculation of the effects of echo and stability for a trunk/toll circuit ” indicates the methods to be applied to determine the nominal allowable attenuation as regards echo and stabi­ lity for a trunk/toll circuit of a given type.

1.2.2. Effects of echo

The backbone circuits (high velocity circuits) of a modern international tele­ phone network are carrier circuits on symmetrical or coaxial cable pairs: echo sup­ pressors are not used. Nevertheless, loaded extension circuits with a low velocity are still used in many international telephone calls, and it is necessary to examine, for the chain of circuits in such a communication, if it is necessary to insert echo suppressors in accordance with the annex entitled “ Calculation of the effects of echo and stability for a trunk/ toll circuit ” (Annex 1 of the Book of Annexes to Vol. Ill of the Green Book). If, following a calculation made in accordance with instructions in Annex N° 1 mentioned above, echo suppressors are considered necessary on the subsidiary manually-operated circuits of the European network, it is recommended to asso­ ciate echo suppressors with the 4-wire terminating sets and to fix their operating time and hangover time, as indicated in the following note. CHAIN OF INTERNATIONAL AND TRUNK/TOLL CIRCUITS — ECHOES 31

In principle, with semi-automatic or automatic working the use of circuits having terminally fitted echo suppressors should be avoided because these interfere with international signalling. If an echo suppressor is normally fitted on a circuit at a transit centre, it should be cut out when this circuit is the second (or third) circuit of an international connect­ ion. In the semi-automatic or automatic service, the method for inserting echo suppressors at the out-going international exchange is left to the Administration con­ cerned. It seems that one practical solution is to arrange for the out-going register to cause the insertion of an echo suppressor according to the international destination and route taken by the call.

REMARK

Definitions and recommendations relating to the operation of echo suppressors (The following definitions relating to the operation of an echo suppressor are provisional)

— A relay type echo suppressor (Suppresseur d'echo a action discontinue) is a suppressor which intruduces instantaneously into the return channel a fixed loss called “ blocking attenuation ”; an example of this is an electro-magnet type echo suppressor.

— Valve or rectifier type echo suppressor (Suppresseur d ’echo a action continue) is a suppressor which introduces into the return channel an attenuation which increases from zero to a maximum value which may be equal to or greater than the attenuation considered adequate to block the return path and called “ blocking attenuation ” ; an example of this is a triode lamp type echo suppressor which functions by modifying the grid bias of a repeater lamp, or a metal rectifier type echo suppressor.

— Intermediate echo suppressor is one located at an intermediate point on a circuit, the two halves of the echo suppressor being preferably in the same repeater station.

— Terminal echo suppressor (half) is half of an echo suppressor placed in a terminal repeater station (case where the two halves of an echo suppressor are placed at the two ends of the circuit).

— Terminal echo suppressor (full) is a complete echo suppressor located at one of the terminal repeater stations (case where the halves of the echo suppressor are placed at the same end of the circuit).

— Local sensitivity of a valve or rectifier type echo suppressor. This is the value of the attenuation to be inserted between a “ normal ” generator *) and a pure resistance of 600 ohms, the echo suppressor being connected across the terminals of the resistance, which only just allows the echo suppressor to operate i.e. to introduce the attenuation of 0.7 N (6.1 db) into the return channel.

Note. — The attenuation introduced by the echo suppressor into the return channel, when the voltage applied at the input of the echo suppressor is double the voltage applied at the input of the same echo suppressor in the above test, should be very much higher [about 4.6 N (40 db)].

— Local sensitivity of a relay type echo suppressor. — This is the maximum value (nepers or decibels) of the attenuation which can be inserted between a “ normal ” gene­ rator and a pure resistance of 600 ohms, the echo suppressor being connected across

*) A “ Normal generator ” is a source of sinusoidal current (with internal resistance equal to 600 ohms and of negligible reactance) with an output power of 1 mW into 600 ohms non-reactive. 3 2 CHAIN OF INTERNATIONAL AND TRUNK/TOLL CIRCUITS — ECHOES

the terminals of the resistance, which allows the echo suppressor to operate, i.e. so that the relay contacts blocking the return path are just operated.

— Sensitivity referred to zero relative level of a valve or rectifier type echo suppressor. — This is the attenuation which, when introduced between a “ normal ” generator and the sending end of a circuit (point of zero relative level) causes the echo suppressor, connected to the circuit in normal conditions of use, to introduce an attenuation of 0.7 N (6.1 db) into the return channel.

— Sensitivity referred to zero relative level of a relay type echo suppressor. — This is the attenuation which, when introduced between a “ normal ” generator and the sending end of a circuit (point of zero relative level) causes the echo suppressor, connected to the •circuit in normal conditions of use, to be “just operated ”.

Note. — The'four following values can be defined in the same way: — Local operate level o f a valve or rectifier type echo suppressor. — Local operate level o f a relay type echo suppressor. — Operate level {referred to zero relative level) o f a valve or rectifier type echo suppressor. — Operate level {referred to zero relative level) of a relay type echo suppressor. These four values are the same as the corresponding sensitivity but with opposite sign. ■ To determine the “ time characteristics ” of an echo suppressor as defined below (operate time, hang-over time, partial hang-over time), a sinusoidal voltage, having a frequency equal to that for which the echo suppressor is most sensitive, is suddenly applied to or disconnected from the input to the echo suppressor. The difference between the absolute level of the input voltage and the “ local operate level ” or the “ local sensitivity of the echo suppressor ” must be shewn; in general this difference is either 0.7 N (6.1 db) or 3 N. (26.1 db) (corresponding relatively to double and twenty times the operate voltage). .

a) Operate time of valve or rectifier type echo suppressor. — This is the time interval between the instant when the signal defined above is applied at the input of the echo suppressor and the instant when the additional attenuation of 0.7 N. (6.1 db) is introduced into the return channel.

b) Hang-over time of a valve or rectifier type echo suppressor. — This is the time interval between the instant where the signal defined above ceases to be applied at the input of the echo suppressor and the instant when the additional attenuation on the return channel has fallen to 0.7 N. (6.1 db.)

c) Partial hang-over time of a valve or rectifier type echo suppressor. — This is the time interval between the instant when the signal ciefined above ceases to be applied at the input of the echo suppressor and the instant when the additional attenu­ ation in the blocked channel has fallen to 2.3 N. (20 db).

d) Operate time of a relay type echo suppressor. — This is the time interval between the instant when the signal is applied at the input of the echo suppressor and the instant when the relay contacts introducing loss into the return channel are operated. CHAIN OF INTERNATIONAL AND TRUNK/TOLL CIRCUITS — ECHOES 33

e) Hang-over time of a relay type echo suppressor. — This is the time interval between the instant when a signal ceases to be applied to the input of an echo suppres­ sor and that when loss is removed from the return channel.

f) Partial hang-over time of a relay type echo suppressor. — In the case of relay type echo suppressors where the slope of the operating characteristic can be considered as infinite, and also in the case of rapid operating valve or rectifier type echo sup­ pressors in which the slope of the falling part of the characteristic is also very steep, the period of partial opening is practically nil and the hang-over time and the partial hang-over time are practically equal.

Echo suppressors used on international circuits must satisfy the conditions shewn in Specification B. IV (see section 5 below). It is provisionally recommended that the operate time and the optimum hang­ over time of an echo suppressor be determined as defined below (see figure 2).

1°) Provisionally, for intermediate echo suppressors, the operate time ta2o must be less than twice.the shortest propagation time between the echo suppressor and the point of reflecion, and the operate time ta2 must be less than four times the short­ est propagation time between the echo suppressor and the point of reflection.

Attenuation introduced into _ return channel \ Partial . hang-over

o Hang-over time tb2 or tb20 ac­ Operate time ta2 or ta20 accor- cording to whether the voltage OG -ding to whether the voltage ap­ applied to the suppressor is plied to the suppressor is twice twice or twenty times the or twenty times the voltage used voltage used to define local to define local sensitivity. ' sensitivity.

Total hang-over time

F ig u r e 2. — Valve or rectifier type echo suppressors

For terminal echo suppressors the operate time ta20 should be 1.5 ms (-f 1 ms and —0.5 ms), and the operate time ta2 should be less than 15 ms. 34 CHAIN OF INTERNATIONAL AND TRUNK/TOLL CIRCUITS — STABILITY

Note. — If differential echo suppressors were used exclusively on a circuit or part of a telephone network an operate time t&2o > 1-5 ms (e.g. in the region of 5 ms) could be allowed; but, because it is always possible in the international tele­ phone service, for a circuit fitted with differential terminal echo suppressors to be connected to a circuit fitted with echo suppressors of another type, it is recom­ mended that the above limits be used.

2°) The hang-over time tb2o of an echo suppressor should be equal to the sum of the two following terms: a) 2.25 times the propagation time at 800 c/s, between the echo suppressor and the longer of the two 4-wire cable extensions. The additional tolerance of 0.25 is included to allow for the propagation time of the repeaters and other equipments, and also for the fact that at other frequencies the propagation time of the reflected current may be somewhat higher at 800 c/s. b) A constant term of 50 ms to allow for four-wire circuits without echo suppressors and two-wire circuits of the national transmitting and receiving systems forming part of the international communication. Having made the above calculation the result is rounded to one of the three values 50, 150, or 150 ms. It is recommended that in no case should 250 ms be exceeded because of the requirements of voice-frequency signalling. The hang-over time tb 2 should be greater than a quarter of tb 20.

Note 1. — It will be possible to give a limit for the partial hang-over time, after the study of the operation of echo suppressors by the different Administrations and Private Operating Companies.

Note 2. — It is recommended to give to international and trunk/toll circuits forming the national sending and receiving systems, equivalents such that the effects of echo should not be objectionable (see Annex 1 of the Book of Annexes to Vol. Ill of the Green Book entitled “ Calculation of the effects of echo and stability for a trunk/toll circuit ”).

Note 3. — In order to avoid false operation of echo suppressors the operate level (related to zero relative level) of a terminal echo suppressor, for the frequency at which the echo suppressor is most sensitive, should be not greater than —2.5 N. (—22 db) and not less than —3.5 N. (—30 db).

1.2.3. Stability of telephone transmission

The stability of the chain of national and international circuits between two terminal trunk/toll exchanges, when the terminals of this chain are open circuited should be provisionally at least 0.2 N. (1.74 db) (see Annex 1 of the Book of Annexes to Vol. Ill .of the Green Book entitled “ Calculation of the effects of echo and stab­ ility for a trunk/toll circuit ”). CHAIN OF INTERNATIONAL AND TRUNK/TOLL CIRCUITS — NOISE 35

To enable this condition to be met, the balance return loss (with respect to the balance used in the terminating set) of the national system (sending or receiving), measured or calculated at the international exchange, should be at least equal to 0.6 N. (5.2 db) throughout the band of frequencies effectively transmitted by the international circuit. During these measurements the terminals of the circuit should be “ open ” at the terminal trunk/toll exchange to which the subscriber is connected.

1.2.4. Linear Cross-talk

A. — Cross-talk between different connections

The cross-talk which may exist between two connections consisting of inter­ national circuits and national trunk/toll circuits is determined by the following recommendations: a) International cable circuits The near-end cross-talk ratio between two complete international cable circuits (4-wire circuits) terminating at the same point, in terminal service, should not be less than 6.7 N. (58.2 db) for 90% of the combinations of two circuits, at 6 N. (52.1db) for all combinations of such circuits. b) Cable circuits between an international exchange and a trunk)toll exchange (pro­ visional recommendation) The near or far-end cross-talk ratio between two complete cable circuits, ter­ minating at the same point and connecting the international exchanges to the ter­ minal trunk/toll exchanges, should not be less than 6.7 N. (58.2 db) for 90% of the combinations of two circuits and 6 N. (52.1 db) for all combinations of two circuits. c) Circuits on open-wire lines and connecting either two international trunk exchanges, or international exchanges to terminal trunk)toll exchanges (provisional recom­ mendation) The near or far-end cross-talk at audio-frequency at the trunk/toll exchanges (intell­ igible cross-talk only) between two audio-frequency channels, or between a high frequency and an audio-frequency channel on the same pair of open wires, or between any two high-frequency channels transmitted to line in different bands, or between any high-frequency channel and an audio-frequency channel on wires of the same route, should be greater than 6.7 N. (58.2 db): the near or far-end cross-talk ratio, under the same conditions, between any two high-frequency channels on lines of the same route and transmitted to line in the same frequency band, should be greater than 5.4 N. (46.9 db). d) International exchanges The cross-talk ratio measured at the test jack frame between any two 2-wire connections through the international exchange should not be less than 8 N. (70 db). A “ two-wire connection ” consists of the two wires from the test jack frame, through the switching equipment (consisting of an answering equipment, a connect­ ing circuit and a calling equipment) connected back to the test jack frame. 3 6 CHAIN OF INTERNATIONAL AND TRUNK/TOLL CIRCUITS — NOISE

With this limit of 8 N. (70 db) the overall cross-talk should be tolerable on a connection consisting of international circuits and national trunk/toll circuits, allowing for at least two international exchanges in the connection and for this number to be four in the future. The limit given above should normally apply to the most unfavourable case, i.e. when two connections have parallel paths throughout the international exchange. It should he noted that this case does not occur in practice, because the normal arrangement of the switching units is that when two connections make use, at one stage of the switching, of two adjacent units, the two connections will use, in the following stage, units which are widely separated.

B. — Cross-talk between go and return channels OF A 4-WIRE CONNECTION

The near-end cross talk ratio between the two directions of transmission of a telephone circuit used for voice-frequency telegraphy should not be not less than 4 N. (35 db). It appears moreover, that this value can probably be obtained in practice for any telephone circuit on modern carrier-cable systems. This value (measured or calculated when there is no terminating set at the other end of the circuit) seems also to meet requirements for echo and singing margins. The C.C.I.F. is studying the requirements to be included in specifications for voice-frequency signal receivers, so that with the above value the signal receiver at one end of the chain of international circuits is not likely to be operated by signals sent from the same terminal. It should be understood that when checking that a communication consisting of international circuits and national toll/trunk circuits meets this limit of 4 n (35 db), account should be taken of the cross-talk in the exchanges. On this question the C.C.I.F. has provisionally taken as a basis for the study mentioned above, a limit for the cross-talk ratio of 5.8 N. (50 db), between the go and return channels of a 4-wire circuit in an international exchange.

1.2.5. Circuit Noise

The C.C.I.F. is studying the production of a curve of general application giving the reduction of transmission quality due to circuit noise as a function of the reading of the psophometer standardized by the C.C.I.F. and connected across the ends of the connection consisting of international circuits and trunk-toll circuits. Until the results of this study are available, the following recommendations are made: 1) As regards noise at the end of an international circuit, see section 1,3,8 below. 2) As regards noise induced by electrical distribution or traction lines, see the Directives concerning the protection .of telecommunication lines against harmful effects of industrial power lines (brought up to date at Geneva in 1952), paragraph 93 for open wire circuits and paragraph 100 for cable circuits. INTERCONNECTION OF INTERNATIONAL AND TRUNK/TOLL CIRCUITS 37

3) The noises induced on lines connecting international exchanges to trunk/ toll exchanges should not, during an international call, increase appreciably the induced noises considered admissible on international circuits. (It will be necess­ ary to fix the upper limit for the psophometric voltage produced by noises induced on the lines connecting the international trunk exchanges to the trunk/toll exchanges; similarly, when definitive limits have been fixed for the admissible value of the out- of-balance of the international telephone circuits, it will be necessary to lay down the admissible limits for the out-of-balance of the lines connecting the international exchanges to the trunk/toll exchanges).

1.2.6. Interconnection of international and trunk/toll circuits a) Method o f interconnection The interconnection of two four-wire circuits should be made in such a way that the overall equivalent and stability are practically the same as if there were a single direct four-wire circuit, having its terminals at the two end international exchanges. To avoid reflections at the point of interconnection of the two international 4-wire circuits (which reflections could interfere with signalling), it is recomm­ ended that the interconnection of these circuits should always be by joining directly the line wires. Also any low-pass filters, which may be fitted for reasons of stab­ ility when one of the international circuits is used on its own, should be cut out. This recommendation does not relate to the interconnection of two national trunk/toll circuits in international calls. Administrations and Private Operating Companies concerned may make suitable arrangements to connect these circuits. Similarly the method of connection between a national and international circuit may be chosen by national Administrations or Private Operating Companies, it being understood that the signal receiver is connected to the four-wire end of the international circuit and that the connection with the national network does not cause reflections during the time when signals are liable to be transmitted between two registers. b) Use of terminal repeaters associated with pads.

T he I nternational T elephone C onsultative Committee, Considering, that the use of terminal repeaters and of automatically switched pads is markedly superior to the use of cord circuit repeaters as regards transmission, and has some advantage in respect of ease of operation, carried unanimously the motion, that the use of terminal repeaters and pads is to be recommended in future transit trunk/toll circuits, whenever this is economic. Note. — Cord circuit repeaters are still in use in certain countries. 38 INTERNATIONAL CIRCUITS — GENERAL CHARACTERISTICS

1.2.7. Impedance of international and trunk/toll circuits All circuits, whether international circuits or national two-wire or four-wire trunk/toll circuits, terminating at the same trunk/toll exchange, should have the same nominal value of impedance as seen from the switchboard (or from the switches), For any particular exchange, this should be either 800 ohms or 600 ohms.

1.2.8. Characteristics of loaded cable trunk/toll circuits liable to carry international calls

T he C onsultative I nternational T elephone Committee, Considering, that the establishment of long international cable circuits would be facilitated if the cables in the internal networks were to have similar characteristics to those of the specification for international cables; that these characteristics are moreover, those of the usual type of cables in the different countries; that the national trunk/toll circuits liable to carry international calls can be either four-wire or sometimes 2-wire circuits: carried unanimously the motion, that it should be recommended, to Administrations and private Telephone Operating Companies of the different countries, when planning their network of loaded trunk/toll cables for internal service, to follow preferably the instructions laid down in Specification A. Ill entitled “ Essential Clauses of a Model Specifi­ cation for Repeater Sections of Loaded Telecommunication Cables ” (This speci­ fication is shewn in section 5.4.1. below).

1.3. General characteristics of an international continental telephone circuit

1.3.1. General considerations

Make-up of long-distance circuit.

The International Telephone Consultative Committee Considering that the length of long-distance continental circuits is limited by the recomm­ endation that the transmission time should be less than 100 milliseconds (see section 1.1.4 below); carried unanimously the motion, that it is desirable in future to use in the main arteries of the international network, high velocity circuits. In future, all international telephone circuits (with the exception of audio circuits on open-wire lines) should be four-wire circuits, conforming to the recomm­ endations of the C.C.I.F. contained in the present section 1.3 and applicable to modern circuits (see particularly section 1.1.3 the recommendation concerning the band of frequencies effectively transmitted by such circuits). INTERNATIONAL CIRCUITS — EQUIVALENT 39

Remark. — A carrier circuit having go and return channels in different frequency bands on the same pair of wires, is considered as a four-wire circuit.

Location of repeater stations

T he International T elephone C onsultative C ommittee carried unanimously the motion that the location of repeater stations should be determined by technical con­ siderations and not by political considerations.

Limitation o f the number o f test points on international cable circuits

T he International T elephone C onsultative C ommittee, carried unanimously the motion that it is desirable that the only test points on international cable circuits should be those at repeater stations. With the agreement of Administrations and Private Operating Companies concerned, an exception could be made for frontier crossings.

1.3.2. Nominal equivalent

For all international circuits, the nominal equivalent should be the same for the two directions of transmission. For manually operated international circuits, the nominal equivalent (insertion loss between non-reactive resistances of 600 ohms) between the switchboard jacks at the end international exchanges, including the line transformers, measured at 800 c/s should not exceed 0.8 N. (7 db). This limit includes the insertion loss of the connecting circuit between the two international circuits at an international transit exchange. For semi-automatic international circuits, it is necessary to standardize the nominal equivalent, and the value recommended by the C.C.I.F. in the present state of knowledge is 0.8 N. (7 db) in each direction of transmission. This value includes the insertion loss of the incoming and out-going switching equipments and also of pads included in the circuit in terminal service. As in the future it may be considered desirable to reduce the nominal value of 0.8 N. or 7 db, it is necessary to arrange the equipments so that it is possible readily to change this value. The interconnection of two semi-automatic international circuits on a four- wire basis in a transit centre should be effected in such a way that the overall equi­ valent has the same nominal value as the equivalent of a single circuit. This equivalent is measured under the same conditions as for a single circuit; it includes the insertion loss of the transit switching equipments, as well as any pads included in the connection. 40 INTERNATIONAL CIRCUITS — ATTENUATION/FREQUENCY DISTORTION

1.3.3. Frequency band effectively transmitted and attenuation frequency distortion

For modern type international circuits, the only ones dealt with in this volume, each telephone circuit should effectively transmit a frequency band of at least 300 to 3 400 c/s. A frequency is said to be effectively transmitted if the loss for this frequency does not exceed by more than l.ON (8.7 db) the loss at 800 c/s. The variation (as a function of frequency) of the equivalent in terminal service of an international circuit transmitting effectively the frequency band from 300 to 3 400 c/s should not exceed the limits shewn in diagram N° 1. (fig. 3)

Remark 1. — The limiting conditions in this diagram should not be reached in the case of a carrier circuit with no intermediate modulators and demodulators and no spike filters*. Also the limits shewn in diagram 1 might be made more stringent when Administrations have more experience in using circuits on coaxial pairs.

Remark 2. — There are still in service old-type international circuits which do not meet these limits. Their prescription is given in the following pages of Vol. Ill bis of the Yellow Book (Florence 1951):

F ig u r e 3. Diagram No. 1. — Admissible limits for the variation, as a function o f frequency, o f the equivalent in terminal service with respect to its value measured at 800 c/s (International circuit transmitting effectively the band of frequencies from 300 to 3 400 c/s) Note. — The curve of the variations of the equivalent as a function of frequency should be within the hatched surface.

*) See paragraph b of section 1 of the 2nd part below, for the diagram with more stringent limits applicable to selected circuits which can be used for example to provide 24 telegraph channels. INTERNATIONAL CIRCUITS — ATTENUATION/FREQUENCY DISTORTION 41 pages 26,to 33: attenuation/frequency distortion for old-type voice frequency circuits; page 41: noise on old-type voice frequency circuits. pages 49 to 52: construction of open-wire lines — B. Electrical characteristics **; pages 54 to 55: directives for the construction and loading of cables inserted in open-wire lines **; pages 67 to 74: multiplex telephone systems providing a small number of carrier telephone channels on open-wire lines **; pages 85 to 89: (1 + 1).carrier cable systems; pages 89 to 93: (1 + 3) carrier cable systems; pages 137 to 142: Twin-band telephony. Attention is drawn to Annex 19 of the Book of Annexes to Vol. Ill of the Green Book, which gives some information on the possibility of improving, in certain cases, the quality of telephone transmission on these old-type circuits.

1.3.4. Variation, as a function of time of the equivalent of a complete circuit

All possible efforts should be made to ensure that the maximum variation, as a function of time, of the equivalent of the complete circuit, with respect to its nominal value, does not exceed 0.2 N (1.7 db). (See the Maintenance Instructions of the 6th section of the present publication).

1.3.5. Group transmission time

The group transmission time of a chain of international circuits should not exceed 150 milliseconds. Instead of this maximum value of 150 milliseconds, for the establishment of the General Plan of Interconnection, a value of 100 milliseconds has been taken as a target for the chain of two international circuits in a transit-type international communication. 1.3.6. Phase Distortion

The phase distortion of international circuits should be such that the differ­ ences between the transmission time of the whole of the international section of a continental connection does not exceed the following values (provisional recomm­ endation) : For all audio and carrier-frequency telephone circuits, if: tm represents the group transmission time for the nominal lowest frequency transmitted, tM represents the group transmission time for the nominal highest frequency transmitted, and represents the minimum value of the group transmission time through­ out the band of frequencies to be transmitted,

**) A new question is being studied, with the object of replacing this recommendation with a new text, applicable to circuits effectively transmitting the frequency band 300 to 3 400 c/s. 4 2 INTERNATIONAL CIRCUITS — NOISE

then tm — tmin < 1 0 milliseconds tM — rmin < 5 milliseconds

1.3.7. Linear Cross-talk a) Cable circuits The far-end or near-end cross-talk ratio between two complete cable circuits (four-wire circuits) terminating at the same points, in terminal service, should not be less than 6.7 N (58.2 db) for 90% of the combinations of the two circuits and 6 N (52.1 db) for all combinations of the two circuits. b) Open-wire lines The near or far-end cross talk ratio at audio frequencies at the trunk/toll exchange (taking account only of intelligible cross-talk) either between two audio-frequency circuits, or between one carrier circuit and the audio-frequency circuit established on the same pair of open wires, or between any two carrier circuits, transmitted to line in different frequency bands, or between one carrier circuit and any audio circuit established on lines of the same route, should be greater than 6.7 N (58.2 db); the near or far-end cross-talk ratio, under the same conditions, between any two carrier circuits, established on the wires of the same route and transmitted to line in the same frequency band, should be greater than 5.4. N (46.9 db).

1.3.8. Circuit noise (including non-linear cross-talk) It is desirable that the total circuit noise (including non-linear cross-talk) meas­ ured at the end of a circuit *) should not exceed, during more than 1 % of the time, the value corresponding to a psophometric e.m.f. of two millivolts, at a point of relative level of — 0.8 N (— 7 db), which corresponds approximately to 10 000 picowatts at a point of zero relative power level.

*) It should be possible to meet this limit in the case of the nominal maximum circuit on coaxial pairs; for other types of circuits, the question is being studied by the C.C.I.F. SECTION 2

General characteristics of international intercontinental telephone circuits

2.1. General characteristics of a long intercontinental circuit on open-wire lines

In the present circumstantes, modern carrier systems on open wire lines meet all practical requirements for long-distance land lines, provided special precautions are taken and, in particular: — regularity of construction of the line, — accurate operation of line regulators, — the possibility of modifying, if necessary, the level diagram of the telephone circuits, to take account of special climatic conditions (ice, etc.). Further, it is necessary to consider ‘noise’ carefully in each particular case and to fix repeater spacing so as to have an acceptable signal/noise ratio during most of the time. Provisionally, for new systems, the target should be for the psophometric e.m.f. at the end of the circuit, taking account of all noise which exists, with the exception of noise due to radio transmitters, not to exceed 5 millivolts for at least 90 % of the time, it being understood that this target cannot always be achieved economically when there are very unfavourable climatic conditions.

2.2. General characteristics of a long-distance intercontinental land cable circuit, with, if needed, one or more short submarine sections

In the case of intercontinental circuits of the type considered, the recommend­ ations of the C.C.I.F. relating to continental circuits (see Section 1 above) apply if the length of the circuit is not greater than 2,500 kilometers. The C.C.I.F. is studying the limits applicable to circuits of this type having a length greater than 2 500 kilometers. 4 4 INTERCONTINENTAL CIRCUITS

2.3. General provisional characteristics of an intercontinental circuit having a long deep-sea submarine section

T he International T elephone Consultative C ommittee, Considering, that it is desirable to ensure a satisfactory transmission quality in the inter­ national telephone service by making use of technical possibilities in so far as it is economically reasonable to do so. carried unanimously the motion, that, in each particular case, Administrations or Private Operating Companies concerned should agree on the extent to which the technique generally used for international land lines, or some other appropriate technique should be used. If the general technique used for land lines can be applied, the target should be to meet, as far as possible, the limits recommended by the C.C.I.F. for inter­ national telephone land lines. (See above under 2.2) If not, Administrations or Private Operating Companies concerned should study the best solution from the technical and economic point of view, and if necess­ ary communicate to the C.C.I.F. the limits which they have been led to select. SECTION 3

Specific characteristics of international telephone carrier fsystems

3.1. Characteristics common to all modern carrier systems having groups of 12 long­ distance telephone circuits

In the international telephone network, the interconnection of the various carrier systems on symmetrical cable pairs, on open-wire lines, on coaxial pairs, or on radio relay systems, should be envisaged. It is therefore recommended to use, for a basic group, terminal equipments 'which allow general conditions recommended for an international telephone circuit, transmitting effectively a frequency band of 300 to 3 400 c/s, to be met, when used on any type of transmission system (see section 1 above). Cable circuits should be based on a nominal maximum circuit 2 500 kilometers long (see note at the end of section 3.1.1 below).

3.1.1. General recommendations

a) Frequency stability of virtual carrier frequencies As the channels of any international telephone circuit should be suitable for voice-frequency telegraphy, the stability of the virtual carrier frequencies should be such that the difference between an audio frequency applied to one end of the circuit and the frequency received at the other end should not exceed 2 c/s, even when there are intermediate modulating and demodulating processes.

b) Linear Cross-talk In addition to the limits prescribed for cross-talk between carrier circuits on open-wire lines, symmetrical-cable pairs, or coaxial pairs in the same cable, the near end cross-talk ratio between the two directions of transmission at frequencies used for regulating and measuring pilots on carrier systems, should be not less than 4.6 N (40 db).

c) Circuit noise (including non-linear cross-talk) The fixing of admissible limits for circuit noise, on various types of carrier systems, is based on the definition of the “ psophometric power ” which appears 4 6 CARRIER SYSTEM — GENERAL in the following note, and on ,the consideration of the “ nominal maximum circuit ” relating to the type of line in question. Note. — The other characteristics of the complete circuits, measured between audio-frequency terminals, (equivalents in terminal service and in transit service, frequency bands effectively transmitted and attenuation distortion, variation of the equivalent as a function of time, phase distortion, stability etc.) should meet the general conditions indicated in Section 1 of the present section for four-wire tele­ phone circuits.

• * d) Standardization in Europe of carrier-system racks

T he International T elephone C onsultative C ommittee, Considering that countries not having a national industry for the construction of carrier systems must obtain them from different factories, and that the variations of the dimensions of the racks between different sources of supply does not allow of a simple and economic lay-out of the cables and efficient use of accomodation. Carried unanimously the motion, that, in future, the dimensions of the carrier-system racks made in the various European countries, should satisfy the following conditions: Space between bays. — The minimum space between the bays should be such that it is possible to move test trolleys from place to place (between two bays), and also for maintenance staff to be able to work comfortably between two bays. A spacing of 75 centimeters (29!/2 in.) at least, seems reasonable. Overall height. — The overall height of a rack above the floor (not including the space provided for cable runs) should not exceed 320 centimeters (126 in.). In principle, 30 centimeters (11.8 in.) should be allowed for overhead cable runs, and also about 30 centimeters (11.8 in.) for access to these cables, which makes at the most 60 centimeters (23.6 in.) between the top of the rack and the ceiling: nevertheless some Administrations consider that a total height of 40 centimeters (15.8 in.) between the top of the rack and the ceiling is sufficient in certain cases. In main repeater stations (or terminal equipment stations), where, in addition to cables connecting one rack to another, general distribution cables have to be allowed for, it is recommended that the height of the building between the floor and the ceiling should be at least 4 metres (13 ft. 2 in.) to facilitate access to the various cables. Depth. — The depth of a double-sided rack should not be greater than 45 centi­ meters (17.7 in.). Note 1. — It does not seem necessary to fix a maximum depth for single-sided racks because the present tendency is to use double-sided racks or back-to-back single-sided racks. Note 2. — Precautions taken to reduce cross-talk. — Annexes 2, 3 and 4 of the Book of Annexes to Vol. Ill of the Green Book contain some suggestions from the Cuban Telephone Company and the French and the British Administrations, con­ cerning the arrangements to be recommended for the rack cabling, to limit the risks of cross-talk. CARRIER SYSTEMS — GENERAL 4 7

REMARK

Definition of psophometric power and general definition of nominal maximum circuits

Psophometric power. — Where square law addition (power addition) of noises can be assumed, it has been found convenient for calculations and plans for international circuits to use the idea of “ psophometric power ” as defined below: (psophometric voltage 2 psophometric power = ------600 or (psophometric e.m.f.)2 psophometric power = ------F v v 4 x 600 A convenient unit is the micro-microwatt or picowatt (pW), and this equation can then be given as follows: „ . ^ (psophometric e.m.f. in mV)2 Psophometric power (in picowatts) = ------0024------

General definition of a nominal maximum circuit. — A hypothetical circuit of defined length and with a specified number of terminal and intermediate equipments, this number being sufficient but not excessive. It forms a basis for the study of certain characteristics of long-distance circuits (noise, for example).

Nominal maximum circuit for telephony in cables *)

A complete telephone circuit (between audio-frequency terminals) established on a hypothetical international telephone carrier system with a specified length and a specified number of modulations and demodulations of the groups and super-groups, these numbers being reasonably great but not having their maximum possible values. This “ nominal maximum circuit for telephony on cables ” has been defined to allow the co-ordination of the different specifications concerning the constituent parts df the multi-channel carrier telephone systems, so that the complete telephone circuits set up on these systems can meet the C.C.I.F. standards. . This nominal maximum circuit has two variants (see fig. 4 opposite): the nominal maximum circuit on symmetrical pairs (described in detail in paragraph 3.3.1 below) and the nominal maximum circuit on coaxial pairs (described in detail in paragraph 3.4.1 below). Note. — These two nominal maximum circuits have the same total length and are used in the same way. They are only a guide for planning carrier systems. For the nominal maximum circuit on coaxial pairs a psophometric power limit has been fixed at 3 picowatts per kilometer of high-frequency line. The corresponding limit for the nominal maximum circuit on symmetrical pairs is being studied by the C.C.I.F. In addition, because.of the use of three pairs of channel modulators and demodulators, the “ nominal maximum circuit for telephony on cables ” can be used to study not only the case of a circuit of 2,500 kilometers, set up on a carrier system or systems* but also that of an international connection having the same total length and made up of three circuits set up on channels of different carrier systems, and interconnected at two international transit exchanges.

*) The C.C.I.F. is studying the definition of a “ nominal maximum circuit for telephony on open-wire lines ”; the C.C.I.R. is studying the definition of a “ normal maximum circuit on radio­ relay systems 48 CARRIER SYSTEMS — GENERAL

(1) on co-axial pairs Basic Line Audio Basic Group Super-group frequency frequency frequency frequency

2500Kw(^1600 h

- n W -M DopHM——[NHM-

Line Audio Basic Group frequency frequency frequency (b) 'on symmetrical cable pairs

_ Q_ Channel translating equipment (translation of the audio-frequency band into the basic group and vice versa /

_ n _ Group translating equipment (translation of the basic group into the basic supergroup and vice versa)

- t Supergroup translating equipment (translation of the basic super-group into the line frequency and vice versa)

F ig u r e 4. — Diagram of the principle of the nominal maximum circuits defined by the C.C.I.F. for telephony on cable circuits Variation of the relative power level ARE SSES GENERAL — SYSTEMSCARRIER

Corresponding audio frequency in c/s

F ig u r e 5. Diagram No. 2. — Admissible limits for the variation as a function o f frequency, of the relative power level at the output from terminal equipments

VO4^ 5 0 CARRIER SYSTEMS — TERMINAL EQUIPMENT

3.1.2. Terminal equipments a) Frequency band effectively transmitted The terminal equipments should be such that the requirements of the con­ dition shewn in 1.1.3 can be met, even in the case of the nominal maximum circuit corresponding to the type of line considered. b) Stability of virtual carrier frequencies See 3.1.1 a) above. c) Carrier leak transmitted to line In the case of a line entirely in cable, at a point of zero relative level, the absolute power level of the carrier leak transmitted to line should never be greater than the values given below:

Measured carrier leak on a channel...... ’ — 2.0 N. (— 17 db.) on a group ...... — 1.7 N. (— 14.5 db.). If a group is transferred from a cable to an open-wire line, the absolute power level of the carrier leak transmitted to line should never, at a point of zero the rela­ tive level, be greater than the values shewn in paragraph 3.2.2 below, for 12-channel open-wire carrier systems. The place and method to be used for the supplementary reduction of the carrier leak, when a group on a cable is transferred to an open-wire line, should be agreed by the Administrations concerned. d) Variations (as a function of frequencyJ of the carrier output from the terminal equipment. It might be desirable, to avoid argument between the Administrations respon­ sible respectively for the terminal equipments at the two ends of a carrier circuit, to have a diagram giving the admissible limits for the variation (as a function of frequ­ ency) of the relative power level at the output of the terminal equipments. For this purpose, diagram N° 2 (fig. 5) will be used, applicable to each channel of a carrier system at the output of the terminal equipments, where: — N is the value of the relative power level measured at the audio frequency 800 c/s translated into the channel considered; — the frequencies shewn as abcissae are audio frequencies applied to this channel and not the corresponding frequencies after translation. The preparation of such a diagram may present the difficulty of necessitating numerous measurements; but Administrations which find the diagram N° 1 above (fig. 2) relating to the attenuation distortion of the complete carrier circuit, is not complied with, can first measure the “ attenuation frequency ” characteristics of their respective terminal sending equipments, and exchange the results of these measurements. In many cases this would dispense with the need for making level measurements at frontier repeater stations. CARRIER SYSTEMS — TERMINAL EQUIPMENT 51

Variation of the equivalent (in nepers)

Absolute power level (in nepers, relative to zero relative level) applied to the audio input terminals of the combina­ tion of the sending and receiving ter­ + o ,o 3 5 minal equipments

0 0,1b - o ,o d 5

F ig u r e 6. — Diagram No. 3 e) Non-linear distortion of the combination of the sending and receiving terminal equipments.

The curve representing the variation (as a function of power) of the equivalent of the combination of the sending and receiving terminal equipments, should be within the limits of diagram N° 3 of Figure 6 above, the measurement of the output power being made by means of a square law device. , . f) Cross-talk

The cross-talk ratio (intelligible cross-talk only) measured between two carrier channels.of the same group, should be greater than 7.5 N. (65 db.). To check that this limit is met, it is only necessary to make measurements with a sinusoidal wave, with, a power of 1 milliwatt at a point which would be at zero relative power level under normal working conditions. The measurement can also be made by means of a wave analyser. g) Impedance seen from the switchboard jacks

The nominal value of the impedance of the trunk/foil circuits (seen from the manual switchboard jack or from the automatic switch) should be the same for all circuits connected to the same trunk/toll exchange. In order to obtain in the future a greater uniformity in the European telephone network, it is recommended that, if possible, future terminal equipments of carrier systems should be designed to have a value of 600 ohms for the impedance of trunk/toll international circuits. 5 2 GROUPS AND SUPERGROUPS

3.1.3. Transfer of a group or supergroup *)

a) General considerations When an Administration or Private Operating Company has a network with carrier systems on unloaded symmetrical cable pairs, or systems having 12 carrier tele­ phone circuits on open-wire lines, or a larger number of circuits on coaxial pairs, using standardized “ basic groups ”, it may desirable (on economic and technical grounds) to transfer groups or supergroups from one system to another, without demod­ ulator to audio frequency. This can be done, for example on a group distribution frame, at a point in the carrier systems where “ basic groups ” (for example the group 60 to 108 kc/s) appear, or in the case of the transfer of supergroups, at the point where the basic supergroups (312 to 552 kc/s) appear. When this is done it is always necessary to ensure that the derived group is transmitted “ clean ”, that is to say after having suppressed as far as possible the groups on either side of the group to be transmitted. It is also possible, in certain cases, to extract groups or supergroups from the carrier line by simple filtering, without demodulation and transfer to basic group or supergroup. The facilities offered in this respect by carrier systems on symme­ trical cable pairs, or on coaxial pairs, are shewn in paragraphs 3.3.4 b) and 3.4.1a) below. To fix the degree of suppression of unwanted components, it is convenient to use the following definitions: Intelligible cross-talk components. — Transferred speech currents which can introduce intelligible cross-talk into certain channels at the point considered. Unintelligible cross-talk components. — Transferred speech currents which can introduce unintelligible cross-talk into certain channels at the point considered. Possible cross-talk components. — These are the transferred speech currents which, at the point considered, do not intrude into the channels of other systems but may do so elsewhere. Harmful out-of-band components. — These are the transferred currents arising from speech or pilots, which have frequencies outside the useful frequency band (relating to audio frequencies) of the carrier systems, but which may interfere with pilots. Out-of-band innocuous components. — These are the transferred currents aris­ ing from speech or pilots, which have frequencies, at all transfer points, outside the useful frequency band (relating to audio frequencies or pilot frequencies).

*) See the remark below entitled “ Definitions relating to carrier systems ”, Relative power levels at the group distribution frames and supergroup distribution frames in carrier systems of different countries

Relative power level at group Relative power level at supergroup Impedance distribution frame Basic Impedance distribution frame at group at at group Country Supergroup Receiving Sending distribution distribution Receiving Sending distribution frame frame frame N | db N | db N | db N | db

Germany (Federal Republic) — 4.2 — 3.5 B 150 ohms — 4.0 — 3.5 75 ohms balanced unbalanced Belgium, Cuba (Cuban Tele­ — 37 — 8 B 75 ohms — 35 — 30 75 ohms phone company), Denmark unbalanced unbalanced

United States of America — 42 — 5 B SUPERGROUPS ANDGROUPS (American Telephone and Telegraph Company At group or super­ — 6 — 0.2 A or B 150 ohms — 5.2 — 4.1 75 ohms group distribution balanced unbalanced frame France ■ At measuring point of supergroup dis­ tribution frame System 1 — 37 — 8 B 75 ohms — 35 — 30 75 ohms unbalanced unbalanced System 2 — 4.2 — 3.5 B 150 ohms — 4.0 — 3.5 75 ohms Italy balanced unbalanced System 3 — 5.4 — 47 — 1.1 — 10 B 150 ohms — 5.4 — 47 — 2.8 — 24 75 ohms balanced unbalanced Mexico (Telefonos de Mexico) — 5.4 — 47 — 1.1 — 10 B 150 ohms — 5.4 — 47 — 2.8 — 24 75 ohms balanced unbalanced Netherlands and United King­ — 37 — 8 B 75 ohms — 35 — 30 75 ohms dom unbalanced unbalanced Sweden ~ 5 A ~ — 47 — 1.1 — 10 B 150 ohms — 5.4 — 47 — 2.8 — 24 75 ohms balanced unbalanced Switzerland — 4.7 — 0.9 A or B 75 ohms — 4.0 — 3.0 75 ohms unbalanced unbalanced COOJ 5 4 GROUPS AND SUPERGROUPS b) Through group filters It is convenient to use the term “ desirable component ” for the audio mea­ suring frequencies of 800 c/s applied at a point of zero relative level with a power of 1 milliwatt and for the pilots with specified frequency and with the specified level at the point where they are normally injected. In the case of the transfer of a group, the ratio between the desirable compon­ ents and the different undesirable components, defined above, expressed in nepers, should be: 1° Intelligible cross-talk components . . 8 N. (70 db.) 2° Unintelligible cross-talk components. 8.0 N. (70 db.) 3° Possible cross-talk components . . . 4.0 N. (35 db.) at the out-put of the poss­ ible components. 4° Out-of-band harmful components . 4.6 N. (40 db.) 5° Out-of-band innocuous components . 2.0 N. (17 db.) Further, the possible variation of the insertion loss of the through-group filter, as a function of frequency (throughout the pass band) and as a function of tempera­ ture (between 10° and 40° C) should not exceed 0.2 N. (2 db.). c) Through-Supergroup filters This question is being studied by the C.C.I.F. d) Relative power levels at the group distribution frames and supergroup distrib­ ution frames Although the standardization of the relative power levels at the group distrib­ ution frames and supergroup distribution frames would be desirable to facilitate the establishment and maintenance of international carrier systems and the transfer of groups or supergroups from one system to another, it does not seem possible to recommend such a standardization internationally because of the diversity amongst carrier systems already in service. The following table shews, for informa­ tion, the values used in different countries. e) Adjustment of level at a point o f transfer of a basic group When a group passes-through different carrier systems, it is necessary to allow the possibility of changing levels, for example between the limits of ± 4 db (about ± 0.4 N.), at all points with translation to the basic group.

3.1.4. Group and Supergroup pilots a) Use of a group pilot Experience has shewn that in spite of the care given to the maintenance of each individual carrier system, it is not possible to guarantee a satisfactory stability for the telephone channels of a group which traverses, over a long distance, dif­ ferent carrier systems, unless a “ group pilot ” is transmitted from end to end of the “ group link ”.

*) For terminology, see the note below. GROUPS AND SUPERGROUPS 55

A a © - / sm sc S b8 SC - o o o o o o o c a c e

0 E

F ig u r e 7. — Example of the use o f a group pilot S . 24 . = 24-channel carrier telephone system on symmetrical pairs S.48 = 48-channel carrier telephone system on symmetrical pairs SC = Carrier system on co-axial pairs 1 to 12 = Numbers of the group channels.

Figure 7 above illustrates as an example, the problems which arise. A group passes successively through several different systems. At points a, b, c, d, e, f, where transfer is in the basic group band A or B it should be possible to measure the level of the pilot. Further, it would be desirable to provide automatic or manual level regulators at certain points, chosen from amongst the above points, and determined experimentally by direct agreement between Administrations. Figure 7 shews such an arrangement in which the regulator is placed at the receiving end; in figure 7 the instruments Ix and I2 for measuring the pilot, are shewn as examples. It is desirable that the amplifier A should have an automatic gain regulation, controlled by the group pilot over a range of ± db (about ± 0.4 N) with respect to the nominal value, in order to compensate only for the variations of the equi­ valent due to a particularly unfavourable combination of tolerances for each of the individual carrier systems, it being clearly understood that this amplifier A is not designed to correct automatically for faults. Where an A.G.C. amplifier is used, an alarm should be given when the amplitude of the pilot at the input of the amplifier departs from its nominal value by more than ± 4 db (about ± 0.4 N). If there is no A.G.C. amplifier it would be desirable to be able to know at any instant the levels of the pilot at the input and output of the amplifier A, for example, by means of readings on the measuring instruments Ix and I2. In this case also, an alarm should be given when the level of the pilot at the input of the amplifier departs by ± 4 db (about ± 0.4 N) from the nominal value. The level of the pilot at the output of the amplifier A should be adjusted daily to be as near as possible to the nominal value. The use of A.G.C. is much to be pre­ ferred. On long groups, it would doubtless be necessary to have several A.G.C. amplifiers, suitably spaced. Further, at the sending end 0 (figure 7) it would be desirable to have a device which gives an alarm when the amplitude of the pilot applied to the line varies by more than ± 0.05 N or ± 0.5 db. Attention of Administrations is drawn to the difficulty which could result from an appreciable reduction in the absolute power level of the pilot sent from 0; such a reduction by causing a corresponding increase 5 6 GROUPS AND SUPERGROUPS

at the A.G.C. amplifiers, is liable to cause ‘near singing’. It would be desirable to make arrangements which would allow this difficulty to be overcome.

b) Use of supergroup pilot When a supergroup passes over different carrier systems in tandem, it might be necessary to arrange a supplementary regulation of the supergroup. It is pos­ sible and desirable to use for effecting such a regulation, a “ supergroup pilot ” transmitted from end-to-end of the “ supergroup link ”, as has been recommended .above (see particularly figure 7) for a group passing over several carrier systems in tandem, but attention is drawn to the necessity for taking certain precautions so that the pilot of a “ supergroup link ” 1 does not' interfere with the pilot of another supergroup link (see paragraph d below).

c) Characteristics of the group and supergroup pilots Where it is considered to be necessary to have group and supergroup pilots, these should be permanently transmitted. Two pilots should be sent simultaneously for each group. Nevertheless, by agreement between the Administrations concerned (includ­ ing Administrations of the transit countries) only one of these pilots need be used. The frequency and level of these pilots are shewn in the following table:

Frequencies and levels of group and supergroup pilots

Group and supergroup pilots of Frequency in kc/s Absolute power level at a point of zero relative level

Basic group A ...... 35,860 — 25 db or — 2.9 N 35,920 — 20 db or — 2.3 N

Basic group B ...... 84,080 — 20 db or — 2.3 N 84,140 — 25 db or — 2.9 N

Basic S u p ergrou p ...... 411,860 — 25 db or — 2.9 N 411,920 — 20 db or — 2.3 N

It is very desirable that, before putting into service a group or supergroup connection, the Administrations concerned should agree on the frequencies of the group or supergroup pilots, within the framework of the above recommendation; these pilots facilitate the setting up of the group or supergroup connections.

*) For terminology, see the remark below. GROUPS AND SUPERGROUPS 5 7

d) Notes on the suppression o f group and supergroup pilots at certain points Note 1. — At the end of a “ supergroup link ”, in the absence of special agree­ ment between the Administrations concerned, the supergroup pilot should always be suppressed.

Note 2. — When a group is transferred from a cable section (on coaxial or symmetrical pairs) to an open-wire line, transmission over the open-wire line of the group pilot, which is an advantage as regards maintenance of the complete group, can, to a certain extent, facilitate “ tapping ” of conversations by means of radio receivers of a special type in the territory traversed by the open-wire line. However, this risk of tapping is less severe than the similar risk arising from inadequate sup­ pression of the carrier because the frequency of the group pilot is removed from the near-by carrier frequency so that the quality of the overheard conversation would be necessarily degraded.

REMARK

Definitions relating to international carrier systems

Figure 8 below represents a long group set up on several carrier systems on symmetrical or coaxial pairs in tandem. It is an example of the manner in which the expressions suggested may be applied to the constituent parts of the through group. The definitions proposed are as follows: 1. Group section. — Part of. a “ group link ” between two adjacent group distribution frames (or the equivalent).

2. Group link. — An assembly of the means of transmission using a frequency band of fixed size (48 kc/s) connecting two group distribution frames (or equivalents). It extends from the point where the group is formed to the point where it is broken down. This expression is usually applied to the combination of “ go ” and “ return ” channels.

3. Group transfer point — not used in the United Kingdom. — A “ group link ” is generally made up of several “ group sections ” connected in tandem by means of “ through group filters ” at points called “ group transfer points ”.

4. Supergroup section. — Part of a “ supergroup link ” between two adjacent super­ group distribution frames (or the equivalent).

5. Supergroup Link. — Assembly of the means of transmission using a frequency band of the specified width (240 kc/s) connecting two supergroup distribution frames (or equivalent). This extends from the point where the supergroup is assembled to the point where it is broken down. This expression is usually, applied to the combination of “ go ” and “ return ” channels. .

6.. Supergroup transfer point — not used in the United Kingdom. — A “ supergroup link ” is generally made up of several “ supergroup sections ”, connected in tandem by means of “ through supergroup filters ” at points called “ supergroup transfer points

7. Carrier line link. — In a carrier system on symmetrical cable pairs, the assembly of the means of transmission used to carry one or several groups. This can be divided into L/1OO Transfer point Transfer point Transfer point RBF RP RS RS RS RS RP RP RP RP RS RS RP RBF EMV EMP EMS EMS ETS EMS EMS EMP ETP EMG EMS ETP EMP EMS EMS EMP EMV

LO jLi -'-O— Q t — 11 (S o a o d a F QxxadaP © c x m ie r Q b a a c ia f ( w e S u e (h u e S u e

u Safe u Sn£ SUPERGROUPS ANDGROUPS

. d ecS m ^ | j Sujterysfmifi

(u J l jSufter-jSjiouft (hde -I (^toujt freeticrvi /re c tu m GUjnoujt A ectiotfl_____ L A t& jzoufi fw li

hom ier yte& fiiem ofjcum eE

EMV — Channel translating equipment (translation of the audio band- into the basic group and vice versa) EMP — Group translating equipment (translation of the basic group into the basic supergroup) EMS — Supergroup translating equipment (translation of the basic supergroup into the line frequency on coaxial pairs, and vice versa) EMG — Group modulating equipment ETS Through supergroup filter ETP — Through group filter RBF ■— Repeater distribution frame RP — Group distribution frame RS — Supergroup distribution frame (This diagram shows only one direction of transmission)

F ig u r e 8 GROUPS AND SUPERGROUPS 5 9 two or several carrier line sections, connected in tandem at intermediate points. This expression is generally applied to the combination of “ go ” and “ return ” channels. 8. Carrier line section. — Part of a “ carrier line link ” between two adjacent group distribution frames (or equivalent). 9. Coaxial line link. — Assembly of the means of transmission in a carrier system on coaxial cable pairs. This extends from the point where the supergroups are assembled, in the frequency band transmitted to line, to the point where this band of frequencies is broken down. 10. Regulated line section (symmetrical or coaxial pairs). — In a carrier system a line . section on which the line regulating pilot or pilots are transmitted from end to end without passing through an amplitude changing device peculiar to the pilot or pilots.

3.2. System having 12-carrier telephone circuits on one pair of open-wire lines

3.2.1. General characteristics *)

T he International T elephone C onsultative C ommittee,

considering,

that it is very desirable to standardize as much as possible, in the international telephone service, 12-channel carrier telephone systems on open-wire lines, using one of the basic groups already used in the carrier systems on symmetrical cable- pairs or coaxial-cable pairs (systems for which the standardization is already ad­ vanced). Carried unanimously the motion:

that the carrier systems with a large number of telephone channels on open- wire lines constructed in the future with a view to providing international telephone circuits, should satisfy the-following conditions: a) Frequency band effectively transmitted by each telephone circuit. — The audio­ frequency band effectively transmitted by each telephone circuit should be the frequency band from 300 c/s to 3 400 c/s.

b) Basic group. — The basic group should be one of the two basic groups standardized for carrier systems on unloaded symmetrical cable pairs and coaxial cable pairs, i.e.: Group A : In each direction of transmission twelve channels in the band between 12 and 60 kc/s transmitting the upper side-band for each individual channel. Group B: In each direction of transmission twelve channels in the band between 60 and 108 kc/s transmitting the lower side-band for each individua channel.

*) The C.C.I.F. has not yet defined the “ nominal maximum circuit for telephony on open wire lines ”. Until this definition is available it would seem desirable to assume that the systems specified below allow the establishment of telephone circuits which satisfy the recommendations of the C.C.I.F. over a length of about 1 000 kilometers. 6 0 CARRIER SYSTEMS — 12 CIRCUITS ON OPEN WIRES

c) Relative levels. — The relative power level at the output of the terminal equipments and the intermediate repeaters, should be, on each channel and for the frequency of this channel which corresponds to the audio frequency 800 c/s, equal to the nominal level with a tolerance of ± 1 decineper ( ± 1 db). The maxi­ mum value of the nominal level should be + 2 N (+ 17 db) at the input to the open wire line.

d) Frequencies transmitted to line. — The system should have 12 carrier tele­ phone circuits. The system should use one pair of open-wire lines. The lowest frequency transmitted to line should be high enough to allow the use of a 3-channel carrier telephone system, at the same time as the system giving 12 carrier telephone channels. Figures 9 and 10 below shew two methods of dividing the line frequency spec­ trum and the corresponding pilot frequencies available (schemes I and II). In order to ensure a degree of uniformity in the international telephone network, it is recommended that Administrations or Private Operating Companies concerned with an international carrier system, should always choose (if possible) either one or the other of these systems. The C.C.I.F. does not specially recommend either scheme I or scheme II. The Administrations or Private Operating Companies concerned in setting up a 12 channel carrier telephone system on international open-wire lines, must examine in each case, which of the two schemes is technically and economically more suitable. Further, the use on the same route of several 12-channel carrier systems (on different pairs) would involve the careful selection of the position (after modula­ tion) of the corresponding groups in the frequency spectrum. As an example, figures 11 and 12 below shew two methods (1 and 2) used respectively by the Cuban Telephone Company and by the Telephone Society of Mexico.

e) Pilot frequencies. — Each system will have an automatic gain regulator controlled by two pilots having different frequencies for each of the two directions of transmission. It is not possible to standardize throughout the international telephone service, frequencies of the pilots to be used on international open-wire carrier systems, because agreement has not been reached on the exclusive choice of a particular division of the line-frequency spectrum. It is left to Administrations or Private Operating Companies concerned in such an international connection, to take a decision on this subject. It is extremely desirable that agreement should be made between them to use the same method of division of the line-frequency spectrum, and the same pilot frequencies (i.e. either scheme I of figure 9 or scheme II of figure 10), in order to avoid modulating and demodulating equipments at the frontier repeater stations, or any other method of changing from one system to another. If agreement cannot be reached, one of two things can be done: 1° Consider the frontier repeater station where two different systems are inter­ connected, as the end of a line-regulated section i.e. stop the pilot used by the other country, which should be reintroduced into the line on the other side of the frontier. JO g 8 ? 6 5 U 3 P. 4 Voie No Frequence I----- kHz 3 0 m 50 j o 80 g o 100 110 i s o 130 1U0 ARE SSES — SYSTEMSCARRIER

^ 3 represents a telephone channel in which the audio frequencies are upright

These symbols are provisional, pending represents a telephone channel in which the audio frequencies are inverted agreement by the C.C.I.F., C.C.I.T. and the C.C.T.R.

represents a pilot frequency

T 12 F ig u r e 9. — Frequency spectrum of a twelve-channel open-wire carrier telephone system — Scheme I ICIS N PN WIRES OPEN ON CIRCUITS

/ 9 f M ? I 5 f

I 8 3 f t 5 6 f 8 9 40 II 48 JO Q8f6 5H32 4 Frequence Voie No kHz

Ho 50 60 80 go loo HO J8D 430 Jto 150 160

represents a telephone channel in which the audio frequencies are upright

These symbols are provisional, pending represents a telephone channel in which the audio frequencies are inverted agreement by the C.C.I.F., C.C.I.T. and the C.C.I.R.

represents a pilot frequency

? F ig u r e 10. — Frequency spectrum of a twelve-channel open-wire carrier telephone system — Scheme II o\ toON

Frequency band occupied by the telephone channels before group modulation

68 408 Channel No — SYSTEMSCARRIER 18 t 11 t 10 t 4 t 8 t f t 6 t 5 t U t 3 t 8 t i |

m v m 80 V___ ±_ ? j s J SB 80 96 Channel No * t t 8 t 3 t * t 5 t 6 t ? t S t 0 t /0 t // t /g Astern So j .A - | 2 ' ......

ho 8 8 - 9 5 4 3 9 12

Channel No WIRES OPEN ON CIRCUITS t^tc ts t w t a t s t / t System S O J - B - 1 2

. 36 80 93 I3f Channel No {ftgt3tfr Astern S 0 J - C 1 2 L

HO 88 98 m Channel No «t//t^totatftetfftfrt3tgty/t tsrt^ta fs t # T Siystem S O J - D -,2 ------^ ...... ‘ - . - . ■ J B A A B Frequence p -p i i l l i rj i ri i i r n i {111 111 i 111111 i I it 111 n 11111 11111111 i"i i ij ii ...... n y m 11 111 11 i 111 t r 11111 n HO 50 60 jo 80 go 100 HO 480 -430 180 kHz represents a telephone channel in which the audio frequencies are upright represents a telephone channel in which the audio frequencies are inverted

These symbols are provisional, pending

agreement by the C.C.I.F., C.C.I.T. — SYSTEMSCARRIER represents a pilot frequency and the C.C.I.R.

represents a virtual carrier frequency

B ------A A ------B carrier frequency of carrier frequency of 12 System 1st group 2nd group 1st group 2nd group WIRES OPEN ON CIRCUITS modulation modulation modulation modulation kc/s kc/s kc/s kc/s

SOJ-A-12 340 484 340 308

SOJ-B-12 340 364 340 543

SOJ-C-12 340 484 340 541

SOJ-D-12 340 364 340 306

F ig u r e 11. — Frequency spectrum of a twelve-channel open-wire carrier telephone system, using method 1

as g f A of « / j; ^ / 8 3 0 5 6 f 8 g 10 // 48 ^ IS 11 40 Q 8 f 6 5 M 3 2 i Channel No System z a h 31

n o m Channel No ZA H 3 2

System — SYSTEMSCARRIER p/ 108 158 Channel No System ZA H 5 5

^ p / iog

■ i • 11 <• i < i iii 111 111111 i i 11111 i i 11 < 11111 11 11 111 i 11 ■ 11 ^ 111 11111 m 11111 11 |l 11 111111 II l l f ri 11111 m | rr '■ i 111 i i j'lTrri i i i i | WIRES OPEN ON CIRCUITS 50 60 jo 80 go AOO 110 120 130 AM A50 160 Frequency in kc/s

—1 represents a telephone channel in which the audio frequencies are upright

These symbols are provisional, pending represents a telephone channel in which the audio frequencies are inverted agreement by the C.C.I.F., C.C.I.T. and the C.C.I.R.

T represents a pilot frequency

F ig u r e 12. — Frequency spectrum of a twelve-channel open-wire carrier telephone system, using method 2 12-CHANNEL OPEN-WIRE CARRIER SYSTEMS — TERMINAL EQUIPMENT AND REPEATERS 6 5

2° Choose pilots which, in the two systems, have exactly the same relative positions with reference to the centre of the group of telephone channels transmitted to line and the same relative levels, because it is then possible to transfer the pilots by the same process as that which transfers the groups.

3.2.2. Terminal equipments

Wherever possible the type of the equipments should be the same as those used for the systems on unloaded symmetrical cable pairs, and for systems on co­ axial pairs. In any case these equipments should satisfy the general conditions shewn in section 3.1.2 below for the following characteristics: a) Variations (as a function of frequency) o f the equivalent at the sending ter­ minal (3.1.2. d)

b) Non-linear distortion of the assembly o f terminal eqipments (3.1.2. e)

c) Cross-talk (3.1.2. f)

d) Impedance seen from the switch-board jack (3.1.2.g) Further, the equipments for open-wire lines should satisfy the following special conditions:

e) Carrier leak transmitted to line. — At a point where the relative level is zero, the absolute power level of the carrier leak transmitted to line should never be greater than the values shewn below: Measured carrier leak: for one direction of transmission and on one channel: — 3. ON (— 26 db) for the upper frequency group, and — 2.ON (— 17 db) for the lower frequency group; on the assembled channels of the system and for each direction of trans­ mission separately: — 1.7 N (— 14.5 db) Where it is desirable to guard against the risk of over-hearing conversations on the line by an ordinary radio receiver, the carrier leak must be reduced still further.

f) Stability of the carrier frequency generators. — So that the effect of the modulations or demodulations should never produce a difference greater than 2 c/s between the audio frequency applied at the input of a channel and that which is received at the corresponding end (where there is not intermediate demodulation and remodulation), the stability of the carrier generators must be such that the frequency is always exact to within 5 x 10"6 approximately. The stability of the pilot frequencies should be always exact to within 5 x l0 '6 approximately.

g) Pilot levels. — At the point of zero relative level, the nominal absolute voltage level of each pilot should be as low as possible, having regard to the type of system used. In any case, it is recommended provisionally that this absolute level should not exceed — 2.3 N (— 20 db). 66 12-channel o p e n -w i r e c a r r ie r sy s t e m s — TERMINAL e q u ip m e n t AND REPEATERS

3.2.3. Intermediate repeaters

a) Maximum gain. — Where icing of the lines is exceptional, the repeaters (in the direction in which the highest frequencies are transmitted) must have a gain of at least 5 N (43 db), for the upper frequency transmitted to line, this gain being measured between the line terminals of the repeater station equipments (which in­ cludes filters, equalizers etc.) the level regulators being in the position of maximum gain. In countries where icing of the lines is a very serious problem, it is possible to use repeaters having a maximum gain of 7.4.N (64 db), at the upper frequency transmitted to line, these repeaters also being designed to deal with the greater slope of the “ attenuation/frequency ” characteristic, under these conditions.

b) Impedance. — In practice the nominal values of the impedance of the open- wire lines varies because of the different methods of construction and range from 530 ohms to 630 ohms. The impedance of the equipment of the repeater station, seen from the terminals to which the line is connected, should be adjusted at the highest frequency trans­ mitted to line, in such a way that the modulus of the reflection coefficient at the junction between this equipment and the line, should be not greater than 0.05 in the upper part of the line-frequency spectrum, and not greater than 0.075 in the lower part.

c) Non-linear distortion. —: The non-linear distortion of a repeater should not exceed the following limits: When a power of 1 milliwatt is applied at the input to a telephone circuit, the second order harmonic distortion (ratio of the second harmonic to the fundamental) should be not less than 8.1 N (70 db); the third order harmonic (ratio of the third harmonic to the fundamental) should be not less than 9.2 N (80 db).

d) Overload level. — The “ overload level ” of a repeater should be not less than 3.8 N (33 db). This “ overload level ” is defined as the total power at the output for which an increase of a decineper or a decibel would be accompanied by an increase of 2.3 N (20 db) in the level of the third harmonic.

e) Stability. — Near singing should not occur if the line terminals are closed at each side with any impedance (from a very small value to a very high value and with any angle).

f) Minimum cross-talk ratio between repeaters in the same station. — If a distur­ bing voltage is applied to a repeater in a station (so as to include all station wiring and auxiliary apparatus), the input to another repeater in the same station being closed, with an impedance equal to the nominal impedance of the line, and the vol­ tages obtained respectively at the output of these two repeaters, compared (again 1 2 - c h a n n e l o p e n - w ir e c a r r i e r SYSTEMS — LINES 6 7

including all station wiring and auxiliary apparatus), a cross-talk ratio of not less than 8.5 N (74 db) should be obtained, the two repeaters being in their normal working condition.

3.2.4. Lines

a) Attenuation of a repeater section. — The maximum level to be transmitted on open-wire lines has been fixed at + 2 N (+17 db). The lowest level on an open line should not be allowed to fall below — 2 N (— 17 db) during normal weather conditions. These conditions are all that need to be observed if only one 12-channel carrier system is to be used on a route. Where it is desired to use several systems, there .are additional requirements to be met. The “ attenuation-frequency ” characteristic should be as near as possible to a smooth curve. For example, on a new 12-channel carrier route, deviations from a regular curve not exceeding 0.5 db, in any repeater section and throughout ’ the frequency band transmitted to line, should be obtainable.

b) Cross-talk. — Far-end cross-talk between two pairs of wires allocated to carrier systems using the same line frequency band should not be less than 7.5 N (65 db) in any repeater section (the length being about 100 kilometers), at any frequency in the frequency band effectively transmitted. Near-end cross-talk attenuation, measured at the terminal equipments or in repeater stations, should not be less than 4.8 N (42 db) at any frequency in the band of frequencies effectively transmitted to line. <

Note. — It is considered that the conditions shewn above can be met if suffic­ ient care is taken in the construction of the line. Information regarding cross-talk between circuits established on open-wire lines, as well as a detailed bibliography on this subject will be found in the article by M. Vos and C.G. Aurell “ Methods for increasing cross-talk attenation between overhead lines ” appearing in “ Ericsson Technics ” No. 6, 1936.

c) Protection against external voltages. — The French Administration uses the following methods of protection which are given for information: The line filters should be protected on the line side by fuses and lightning arrest- ors. Where the output of the audio-frequency circuit is connected directly to an open wire line, the output of the audio filter should be protected in the same way. Audio-frequency filters should be balanced and should be built to withstand a test voltage of 3 000 volts DC to frame. High-frequency filters may have a balanced first half section connected to the other filter sections, by a transformer. The first half section should be capable of withstanding a test voltage of 3 000 volts to frame. 6 8 CARRIER SYSTEMS — SYMMETRICAL CABLE PAIRS

The remainder of the filter may be unbalanced if it immediately precedes the terminal equipment. If there is a cable in between, two transformers should be used to preserve the balance and if necessary, to correct for impedance. Also for information, the Cuban Telephone Company uses these following pro­ tective methods: a) Carbon arrestors are fitted: 1) On the terminal pole (with a breakdown voltage of 750 volts); 2) Between the leading-in cable and the equipment (with a breakdown voltage of 350 volts). In 'very unfavourable conditions, these arrestors fuse and connect the line to earth. • ' [3) Thyrite arrestors are placed in the line filters to afford protection against voltages which are not high enough to operate the carbon arrestors. y) Protection by line discharge coils is also used where necessary in areas with severe lightning.

3.3. Multi-channel carrier telephone systems on unloaded symmetrical cable pairs, having 1, 2, 3, 4 or 5 groups *)

3.3.1. General a) Nominal maximum circuit on symmetrical pairs i This “ nominal maximum circuit ” is 2 500 kilometers long, and is set up on a symmetrical pair carrier system. This “ nominal maximum circuit ” for each direction of transmission has a* total of: 3 pairs of channel modulators and demodulators,, 6 pairs of group modulators and demodulators, 6 pairs of supergroup modulators and demodulators**. Figure 13 opposite is an outline diagram of the “ nominal maximum circuit on symmetrical pairs ”. It will be seen- that there is a total of 30 modulations and demodulations for each direction of transmission, supposing that each modulation or demodulation is effected by a single stage**. The distance between two points o f successive modulation is assumed to be always between about 300 and 500 kilo­ meters.

*) The systems referred to provide groups of 12 long-distance telephone circuits using sym­ metrical pairs in two different cables for each direction of transmission. Systems (12 + 12) are not included but are dealt with in section 3.5.2 below. **) Where systems have 1, 2, 3 or 4 groups, it is possible to have a smaller number of modula­ tions and demodulations, but this does not detract from the usefulness of the idea of “ nominal maximum circuit on symmetrical pairs.” BSOO Km £ WOO mites) ’I

f lo o d — M ------flo o fl— flo ~ > o fl------0

Basic Line Audio Group frequency frequency frequency

Channel translating equipment (translation of the audio-frequency band into the basic group and vice versa)

_ n _ Group translating equipment (translation of the basic group into the basic supergroup ^ and vice versa)

Supergroup translating equipment (translation of the basic supergroup into the line frequency and vice versa)

F ig u r e 13. — Diagram o f the principle of the nominal maximum circuit on symmetrical cable pairs 7 0 CARRIER SYSTEMS — SYMMETRICAL CABLE PAIRS

b) Total noise For the establishment of the circuits on symmetrical cable pairs, it is necessary to divide the total allowable noise between the three causes of this noise. — background noise — intermodulation noise — cross-talk noise. This is being considered as a new question.

c) Matching of the repeater impedance to that o f the line It is desirable to limit the reflection coefficient at the ends of the repeater section so that the effect of the reflected near-end cross-talk does not contribute excessively to the total far-end cross-talk. For example, in a cable with a near-end cross-talk ratio of 6.5 N and which has a far-end cross-talk ratio (direct far-end crosstalk) equal to or greater than 8 N (the cable being between impedances equal to its characteristic impedances), the contribution of the reflected near-end cross-talk would be insignificant compared with the effect of the far-end cross-talk at the maximum frequency transmitted, if the reflection coefficients between repeaters and line have the following values: (a) the input (or output) impedance of the repeater (in its normal operating condition and including line transformers and equalizers) measured between the line terminals at a frequency f and (b) the nominal value of the impedance at the frequency / of the cable pair connected to the input (or output) of the repeater, should not exceed the value given by the formula:

0.15 w i* a maximum value of 0.25

where f max. = maximum frequency to be transmitted. The following equivalent formula may also be used: 1 f msL% a — + 1.9 — — logg N with a minimum value of 1.4 N 2 j or

a = + 16.5— 10 log,/-— db with a minimum value of i2 db

where a is the minimum admissible value for the return loss between the cable and the repeater, the value taken for the impedance of the cable and of the repeater being the same as in the definition of reflection coefficient. An Administration or Private Operating Company ordering repeaters should specify the nominal value of the line impedance (modulus and argument) for the various frequencies in the band of frequencies effectively transmitted by the line. d) Line regulating pilots (see figure 14) For the regulation of the line in symmetrical cable pairs, one or. other of the two following methods may be used: CARRIER SYSTEMS — SYMMETRICAL CABLE PAIRS 71

Variant A Variant B 60 16 56

A 1-group M 18 '0 systems 18 60 kc/s kc/s

I*0 118 16 10k- k

\T 2-group 18 ( systems 18 60 108 ® 108 kc/s kc/s

(10 160 16 10k- 158 * A A A

3-group M x K . 18 60 106 156 systems 18 60 108 156 kc/s kc/s

60 8 08 16 118 200

4-group 1 - n K n 18 6 0 108 156 8 o k systems 18 60 108 156 80k kc/s kc/s

&1 856 16 118 m

5-group 18 60 108 156 m 858 systems 18 60 108 156 80k 858 kc/s kc/s Absolute power level (at a point of zero relative level) — 1.73 N (— 15 db) — 2.0 N (— 17 db)

represents a group of 12 telephone channels with virtual I carrier frequencies spaced at 4 kc/s and in which the audio 1— frequencies are upright in the different telephone channels. These symbols are provisional; fv. represents a group of 12 telephone channels with virtual graphical symbols have to be 118 carrier frequencies spaced at 4 kc/s and in which the audio standardized by agreement be­ frequencies are inverted in the different telephone channels tween the C.C.I.F., the C.C.I.T. and the C.C.I.R. r represents a pilot

Note. — The group shewn dotted i ** ^ can be inverted without changing the frequencies v .--" recommended for pilots.

*) Recommended frequency; there are systems using 253 kc/s.

FIGURE 14. — Line-regulating pilots in carrier systems on symmetrical pairs 7 2 CARRIER SYSTEMS — SYMMETRICAL CABLE PAIRS

A. 1° A pilot at 60 kc/s, with an absolute power level o f— 1.73 N (— 15 db) (referred to a point of zero relative level deduced from the level diagram of the tele­ phone circuits), this frequency being in the gap between the basic groups A and B and it being understood that this pilot would be used for the regulation of the line on all regulating line sections, whatever their length, and also for synchronization or control of frequencies. 2° Where necessary, and especially for long line-regulated sections, an addi­ tional line regulating pilot 4 kc/s above the maximum frequency transmitted to line and with an absolute power level of — 1.43 N (— 15 db) (referred to a point of zero relative level deduced from the level diagram of the telephone circuits). Note. — There are in existence, systems with 5 groups in which this pilot is only 1 kc/s above the maximum frequency transmitted. The point 2° does not apply to systems with a single group.

B. Two pilots situated in the basic group (B) at 64 kc/s and at 104 kc/s trans­ mitted with an absolute power level of — 2 N or — 17 db at a point of zero relative level. On the high-frequency line, it is possible to have two pilots per 48 kc/s of trans­ mitted band and from amongst these pilots 16 kc/s and the maximum transmitted frequency less 4 kc/s, are selected. For systems with two or more groups, a third line-pilot is used and located between top and bottom pilots. In 2-group systems, the frequency 64 kc/s is used, and in 5-group systems, the frequency 112 kc/s. Use of one or other of these methods at the discretion of the Administrations concerned should not give rise to difficulties, because these pilots must be suppres­ sed effectively at the end of a line-regulated section. e) Relative levels Nominal values. — The relative power level, measured at the end of the repeater section crossing the frontier, at the input to the repeater, for the various measuring frequencies selected, should always be greater than — 6.5 N or — 56.5 db, when a power of 1 milliwatt is applied (in the terminal equipment) at the origin of each audio telephone circuit. (Any equalizers are considered as repeaters). The nominal value of the relative power level, measured at the input of the repeater, under the same conditions, is shewn in paragraphs 3.3.2 a), 3.3.3 a) and 3.3.4 a) below, for the various systems. Verification measurements. — The recommendation appearing in paragraph 3.1.2 d) above should, in most cases, do away with the need for level measurements at frontier repeater stations. • If such measurements are found necessary, they will be made at frequencies which have been chosen for lining-up and for “ reference ” (see Maintenance In­ structions in the 6th part of the present work, section 2). These are well-defined frequencies used for drawing the attenuation frequency ” characteristic of the line. For information, frequencies used are spaced at: SYMMETRICAL CABLE PAIRS — 1, 2 , OR 3 GROUPS 7 3

4 kc/s between 12 and 60 kc/s, 8 kc/s between 60 and 108 kc/s, 12 kc/s between 108 and 252 kc/s. The conditions under which the measurements at the frequencies of line-pilots will be made, should be agreed by the Administrations concerned. Level measurements at the frequencies chosen will be made, particularly at frontier stations, at the output of each line repeater. The value measured at each of the frequencies should normally be + 0.5 N (4.5 db) for systems with 1.2 or 3 groups and + 0.2 N (1.75 db) for systems with 4 or 5 groups (except where special cables are concerned, such as submarine cables, or where a special method of equali­ zation). No value of the relative power level thus measured should depart from the nominal value given above by more than — 0.2 N (+ ' 2 db).

3.3.2. Recommendations for systems having 1, 2 or 3 groups a) Relative level at the output of the repeaters The nominal value of the relative power level, measured at the output of the repeater for the various measuring frequences chosen (see paragraph 3.3.1 e. above), is + 0.5 N (+ 4.5 db). b) Frequencies transmitted to line The line-frequency spectrum should conform to the following rules: 1. Systems having one group on symmetrical-cable pairs. The frequency band used in each direction of transmission extends from 12 kc/s to 60 kc/s. In this band, twelve contiguous channels are arranged. The upper sideband, corresponding to the following virtual carrier frequencies, is transmitted to line: 12, 16, 20 v ...... 56 kc/s (A virtual carrier frequency is the frequency which would be transmitted to line if zero frequency is applied at the audio-frequency input of the telephone channel concerned.) ' 2. Systems having two groups on symmetrical cable pairs. The frequency band used in each direction of transmission extends from 12 kc/s to 108 kc/s. In this band, two groups of twelve contiguous channels are arran­ ged, that is to say: Group A. — Twelve channels between 12 and 60 kc/s transmitting the upper side band for each channel. Group B. — Twelve channels between 60 and 108 kc/s transmitting the lower side band for each channel. Group A is identical with the group used in the 12-channel carrier telephone systems in cable. IS 60 108 156 (a) Systems having l,-2 or 3 groups

Group

Channel No. 60 108 156 Scheme No. 1 (recommended)

Super­ /* group A______

Group

Channel No.

Scheme No. 1 bis (which may be used by agreement between Administrations) (b) Systems having 4 groups

Super­ group

Group

Channel No.

Scheme No. 2 (recommended)

/*' Super­ group

Group

Channel No.

Scheme No. 2 bis (which may be used by agreement between Administrations) (c) Systems having 5 groups represents a group of 12 telephone channels having virtual carrier frequencies placed at 4 kc/s and in which the audio These symbols are frequencies are upright on the different telephone channels. provisional m til agreed by the C.C.I.F., represents a group of 12 telephone channels having virtual C.C.I.T. and C.C.I.R. carrier frequencies placed at 4 kc/s and in which the audio frequencies are inverted on the different telephone channels

F ig u r e 15. — Line-frequency spectrum for international carrier systems on symmetrical cable pairs SYMMETRICAL CABLE PAIRS — 4 OR 5 GROUPS 7 5

3. Systems having 3 groups on symmetrical cable pairs. The C.C.I.F. recommends that the line-frequency spectrum (when connecting up existing cables or laying new cables) should be in accordance with figure 15 a.

3.3.3. Recommendations for systems having 4 groups a) Relative level at the output of the repeaters The nominal value of the relative power level, measured at the output of the repeater for the various measured frequencies chosen, (see paragraph 3.3.1 above) is + 0.2N(+ 1.75 db). b) Frequencies transmitted to line The C.C.I.F. recommends that the line-frequency spectrum (when connecting up cables or laying new cables) should be in accordance with figure 15 b opposite. Remark. — By agreement between the Administrations concerned, it is possible to omit one group of supergroup 1 shewn in scheme No. 2, figure 15 c for systems with 5 groups; if this is done the scheme No. 1 bis of figure 15 b is obtained.

3.3.4. Recommendations for systems having 5 groups a) Relative level at the output of the repeaters The nominal value of the relative power level, measured at the output of the repeater for various frequencies, is + 0.2 N (+ 1.75 db). b) Frequencies transmitted to line The C.C.I.F. recommends the use normally of scheme No. 2 of figure 15 c opposite. Note 1. — Where there is direct interconnection between a system with 5 groups on symmetrical pairs and systems with a smaller number of groups, by agreement between Administrations and Private Operating Companies concerned, the system with 5 groups, shewn in scheme No. 2 bis of figure 15 c may be used. Note 2. — When it is desired to interconnect, in the band of frequencies of the basic supergroup (312 — 552 kc/s), a carrier system on coaxial pairs, either with a system with 5 groups on symmetrical pairs and using scheme No. 2 bis, or to a system with 4 groups using scheme No. 1 of figure 15 b, by agreement between the Administrations or Private Operating Companies concerned, a similar super­ group may be transmitted on the coaxial system using the arrangement of groups within supergroup shewn in figure 16 below. The remark below shews a simple method for assembling basic group B into a supergroup conforming to one or other of the schemes of figures 17 (paragraph 3.4.1 below) and 16, and vice versa. 7 6 SYMMETRICAL CABLE PAIRS — 4 OR 5 GROUPS

Super­ group

Group

12 18 12 Channel IS__ __ 32__ 12__ No. S6k 156

(a) Arrangement of groups and channels (supergroup 3 has “been shewn as an example)

Super­ 3 ' group 5 0 3 2 1 I 2 3 0 S ABCDE 5 0 3 2 1 etc Group KKNNK /1A W 1 ^TNJXKrv NNKKN ___ 60 300 318 662 66k Sok 812 1052 kc/s Super­ group 8' No. Group 6 0 3 2 1 6 D C B A 6 0 3 2 1 5 0 3 2 I etc KNJNJNTx yVIXIXK INNKKTv KTxfxKfy 60 300 312 " 562 66k 80k 812 1052 kc/s

(b) Example of possible positions, in the band of frequencies transmitted on the coaxial pair of the supergroup corresponding to scheme No. 2 bis Figure 16. — Arrangement of groups in a supergroup, which may be used on carrier systems on coaxial pairs interconnected with systems on symmetrical pairs SYMMETRICAL CABLE PAIRS — 4 OR 5 GROUPS 7 7

REMARK

Method proposed by the Belgian Telephone Administration for the transfer from a coaxial cable to a symmetrical-pair cable system

In the systems providing 1, 2, 3 and 4 groups on symmetrical pairs, additional groups beyond the first are not usually installed until all the cable pairs (in practice 14 or even 24) are taken up. Usually the cable pairs are equipped with 12 channels per pair in each .direction of transmission, thus providing 12 x 24 = 288 circuits in a cable with 24 pairs; if the number of circuits required exceeds this number, the cable is equipped with 24 channels per pair, which gives 288 additional circuits, then with 36, 48 and 60 channels per pair. In short, the highest frequency transmitted is kept low, so as to simplify tfie equipment and avoid the construction of intermediate repeater stations for as long as possible, having regard to the requirements for circuits. As a result there is an arrangement of frequencies within the groups, which is in accord­ ance with the recommendations of the C.C.I.F. for systems with 1, 2, 3 and 4 groups, and now allowed (by agreement between the Administrations concerned) for systems with 5 groups. This arrangement is such that the frequency spectrum of the first group is inverted as compared with the frequencies in the other groups, and the systems have developed in such a way that it is virtually impossible to change this arrangement. On the other hand with coaxial systems where there is only one pair, the initial installa­ tion is for at least 60 circuits or a basic group, and as the system is built up an orderly arrangement of frequencies of different groups within the supergroups, is obtained. To interconnect These two apparently incompatible arrangements, the following is necessary:

(1) If only single groups are transferred from one system to another, there is no difficulty because it is necessary to use, in both directions, a basic group, e.g. the basic group B (60-108 kc/s). From this basic group the stages of modulation can be chosen so as to provide the correct arrangement of frequencies transmitted to line on both the coaxial and the symmetrical pair systems.

(2) Where it is desired to transmit, on the symmetrical pair, a band from 12 to 204 or 12 to 252 kc/s, in which the band 12-60 kc/s is inverted with respect to the others, there is nothing to prevent the translation of this band into the band 312-552 kc/s by a modulation of the whole. But, in the supergroup thus obtained, the part of the band between 504 and 552 kc/s is inverted with respect to the others. When demodulating this supergroup it is necessary to use a frequency of 444 kc/s, instead of 612 kc/s to obtain the groups with correct orientation.

For the other direction of transmission, i.e. for the modulation process, where the complete supergroup (5 groups) or incomplete supergroup (4 groups) are intended then to be transmitted over a symmetrical pair system, it is necessary to use the same special modulating frequency for one of the five groups so as to obtain a supergroup of 48 to 60 circuits which will suit existing symmetrical pair equipments. The only complication which results from so doing, is the need to provide on all super­ group racks, a sixth frequency (444 kc/s) and in certain cases, the replacement or preferably the modification of the 504-522 kc/s band filter, so as effectively to suppress any side bands above and below these frequencies. This changed filter seems to be only necessary for the modulation process.

3.3.5. Intermediate terminal repeaters

a) Maximum gain The complete equipment of an intermediate repeater station should have a maximum gain of 7 N (61 db) measured at the highest frequency transmitted. 7 8 SYMMETRICAL CABLE PAIRS — REPEATERS

The above value is a nominal value and a factory tolerance of + 0.1 N (+ 1 db) throughout the band of frequencies effectively transmitted, is allowed.

b) Non-linear distortion (see remark below) The non-linear distortion of the repeaters should not exceed the value corres­ ponding to the limits below. The second-order harmonic distortion attenuation should be at least equal, to 8.9 N (77 db) for the repeaters of systems with 1, 2, or 3 groups and 9.2 N (80 db) for the repeaters of systems with 4 or 5 groups. The third-order harmonic distortion should be at least equal to 9.7 N (84 db) for the repeaters of systems with 1, 2, or 3 groups and to 10.4 N (90 db) for the repea­ ters of systems with 4 or 5 groups. These values are measured by transmitting a power of 1 milliwatt into any telephone channel, the repeaters being fitted with valves with average electrical characteristics. c) Overload level (see remark below) The absolute level of the “ overload level ” of a repeater should be not less than 3.2 N (28 db). This “ overload level ” is defined as the total power at the output for which an increase of one decineper or one decibel is accompagnied by an increase of 2.3 N (20 db) in the absolute power, level of the third harmonic. d) Minimum value o f the cross-talk ratio between repeaters in the same repeater station The cross-talk ratio between two repeaters in the same station should be not less than 8.5 N (74 db). This value is applied to the whole of the equipment of the repeater station from the input transformer to the output transformer. e) Background noise For systems having riot more than 3 groups, the background noise of an inter­ mediate repeater should be not greater than 0.2 N (2 db), above the terminal agitation noise, where the repeater has a level “ gain-frequency ” characteristic. The C.C.I.F. is studying limits for systems with more than 3 groups.

REMARK

Methods used for measuring the non-linear distortion of the negative feed-back amplifiers of systems having at least 12 carrier telephone channels

As regards acceptance tests in the factory, or tests of amplifier prototypes, one effect of non-linear distortion is the appearance of harmonics which did not exist in the wave applied at the input of the amplifier. A general method, similar to that employed in the Bell System of the United States of America, for characterizing this aspect of non-linear distortion, consists in expressing the distortion by shewing separately the amplitudes of SYMMETRICAL CABLE PAIRS — LINES 7 9 the harmonics or other products of modulation which are generated when one or two sinusoidal frequencies are transmitted through the amplifier. Often, only a single frequency is used to load the amplifier; in this case, the distortion is expressed by the amplitudes of the harmonics, the second and third harmonics being the most important. Two sinusoidal frequencies of equal power are sometimes used to load the amplifier, for example when the harmonics fall outside the frequency band transmitted or when the non-linear distortion varies as a function of frequency. When the amplifier is loaded with two sinusoidal fre­ quencies, the non-linear distortion is expressed by the amplitudes of the modulation products of frequency f x ± f 2 and 2 /j ± f 2 where f and / 2 are the frequencies of the applied signals. To give a complete description of the non-linear distortion, a curve representing the amplitude of a modulation product, as a function of the power of the sinusoidal measuring frequency is used. It is usual to express the value of a modulation product by its logarithmic ratio (decibels or nepers) with respect to the value of the power of one of the sinusoidal frequencies used to load the amplifier. The fixing of the admissible limits for the values measured by this method on various types of negative feed-back amplifiers, for systems with at least 12 carrier telephone circuits, is being studied by the C.C.I.F. These limits should be in co-relation with the admissible limits for cross-talk and for the circuit noises fixed by the C.C.I.F. for the whole of an international telephone circuit. This co-relation is moreover, not easy to establish and may necessitate in addition to the objective measure­ ments, appreciation tests made under long-distance telephone service conditions. In order to portray the second effect of non-linear distortion, it is necessary to draw a curve representing the variation of the gain of the amplifier as a function of the output power. One of the effects of the non-linear distortion of an amplifier may be characterized by its “ overload level ”, defined as the total power at the output for which an increase of one decineper or one decibel would correspond to an increase of 2.3 N (20 db) of the absolute power level of the third harmonic.

3.3.6. Lines

A. Cable specification

Essential clauses o f a model specification for the supply o f star quad cable designed to provide 12, 24, 36, 48 or 60 carrier telephone channels on each quad pair. a) Types o f cable The new cables which will be laid in the European international telephone network, will have unloaded symmetrical pairs, designed to be used for 12, 24, 36, 48 or 60 carrier telephone channels on each pair. These pairs should be laid up in star quads and all the unloaded pairs of the same cable should be of one of the three types the characteristics of which are shewn in the table below:

Type I Type II Type III

Diameter of the conductors (in m m )...... 0.9 1.2 1.3 0.0355 0.0475 0.051

Effective capacity (in microfarads) per kilometer . . . 0.033 0.0265 0.028 per m i l e ...... 0.053 0.0425 0.045

Characteristic impedance (in ohms) measured at 60 kc/s 153 178 170 120 kc/s 148 174 165 240 kc/s 172 163 8 0 SYMMETRICAL CABLE PAIRS — LINES

It is essential that a repeater section crossing a frontier should be of a uniform type throughout its length. When a frontier section is between a large and a small country, the Administration of the larger country should do everything possible to use whichever of the three types has been adopted by the smaller country, so as not to cause the Administrations of small countries to use sections of international cable of a different type from that of their national cables. b) Regularity of factory lengths The regularity may be characterized by one or other of the equivalent methods below, the choice of which is left to the Administrations concerned. b 1) Effective capacity. — The “ effective capacity ” is measured between the two conductors of the pair, all other cable conductors being connected together and to the sheath.

Ratios of the effective capacity Type I Cable. — The average of the effective capacities of all the pairs in any factory length should not differ from the nominal value by more than 5 %. In any factory length, the ratio between any individual value of effective capa­ city and the average value obtained for this factory length, should not exceed 7.5%; the arithmetic mean of the absolute values of these ratios should not exceed 2.5%. Types II and III Cables. — The average effective capacity of any length should not differ by more than ± 3 % from the nominal value. In any length, the difference between the effective capacity of a pair and the average capacity for the cable length should not exceed ± 5 %. b 2) Impedance (type II and III cables). — The real part of the characteristic impedance of any circuit, measured with a frequency of 120 kc/s should not depart by more than ± 5 % from the mean value of all the pairs of the first manufacturing batch of each type. This mean value should not depart by more than ± 5 % from the nominal value at 120 kc/s. The impedance will be measured on the factory lengths using a bridge, the circuits being terminated by an impedance equal to that which is measured by the bridge. c) Crosstalk The quality of the cable from the point of view of crosstalk, may be charact­ erized by one or other of the two equivalent methods below, the choice of which is left to the Administrations concerned. c 1) Direct measurements of crosstalk. — For a factory length of 230 meters, the crosstalk between any two side circuits should satisfy the following conditions: — far-end crosstalk ratio should be greater than 7.8 N (68 db), — near-end crosstalk attenuation should be greater than 6.4 N (56 db). SYMMETRICAL CABLE PAIRS — LINES 81

For cables to be used with 5 groups, these values should be obtained up to 240 kc/s; and for cables,with two groups, up to 120 kc/s. During these measurements, the circuits will be terminated by the real part of the nominal impedance for the frequency considered. For factory lengths greater than 230 meters (250 yds) the above limits will be reduced by log ^ N (20 log10 ^ db), L being the length in meters. Lengths shorter than 230 meters should satisfy the same conditions as lengths of 230 meters. c 2) Capacity unbalance and mutual inductances. — All the capacity unbalance tests should be made with an alternating current of 800 c/s. The mutual impedance tests should be made with an alternating current of 5000 c/s. All the tests should be made at ambient temperature, without applying corrections; but in case of dispute, the results obtained at 10° C. will be considered as final. All the conductors, other than those under test, should be connected to the cable sheath. For a factory length of 230 meters (250 yds), the capacity unbalance should not exceed the values given in table I below and the mutual inductances should not exceed the values given in table II. These tables shew different values for type I cables on the one hand, and for types II and III on the other hand. For lengths greater than 230 meters it is necessary to apply the following rules: The average values from pair to pair given in the preceding table, should be multiplied by the square root of the ratio between the length in question and 230 meters. All the maximum values, as well as the average values between a pair and earth, should be multiplied by the ratio between the length in question and 230 meters. Lengths shorter than 230 meters should satisfy the same conditions as lengths of 230 meters.

Table I

Capacity Unbalance

Mean of all readings, Maximum individual reading (ignoring signs)

Types II Types II Type I and III Type I and III

p,p.F (J.P.F HpiF UlxF Capacity unbalance: between pairs of the same quad. . . . 33 17 125 60 between pairs of adjacent quads in the same la y e r ...... 10 5 60 25 mean value not speci­ between pairs in non-adjacent quads in fied because all possible the same l a y e r ...... combinations are not measured 20 10 between pairs in quads in adjacent 10 5 60 25 la y e r s ...... between any pair and earth ...... 100 100 400 400

Rem ark. — The limits shown for the mean values do not apply to cables which have 4 or less quads. 8 2 SYMMETRICAL CABLE PAIRS — LINES

T able II Mutual inductances

Mean of all readings, Maximum individual reading (ignoring signs)

Type I Types II Types II and III Type I and III

nano H nano H nano H nano H Mutual inductances: between pairs of the same quad. . . . 150 125 600 500 between pairs of adjacent quads in the same la y e r ...... 100 40 400 150 between pairs in non-adjacent quads . 50 20 350 150 between pairs in quads in adjacent layers ...... 100 40 600 250

Remark. — The limits shown for the mean values do not apply to cables which have 4 or less quads. d) Insulation resistance In a length of cable, the insulation resistance measured between a conductor and all the other conductors connected together, and to the earthed sheath, should not be less than ten thousand (10000) megohms per kilometer (approximately 6200 megohms per mile), the potential difference used being at least 100 volts and not greater than 600 volts. The reading is to be made after electrification for one minute, the temperature being at least equal to 15° C. (approximately 60° F.). e) Dielectric strength. — When specially requested, cables will have a construc­ tion such that the insulation of any cable length should be capable of withstanding, without breakdown, a potential difference, specified in each particular case, but not exceeding 2000 volts r.m.s., applied for at least 2 seconds between all the con­ ductors connected together and the earthed sheath. The test is to be made with a 50 c/s alternating current. The value of the test voltage should not exceed by more than 10 per cent the maximum value of a sinusoidal voltage having the same r.m.s. value.

B. Specification of a repeater section

Essential clauses of a model specification for the installation of a repeater section of a cable with unloaded symmetrical pairs suitable for carrier systems with up to 60 telephone channels on each pair. a) Maximum attenuation of a repeater section The attenuation of the cable in a repeater section should not normally exceed 6.5 N (56.5 db) for the highest frequency. For 20% of the sections, a maximum value of 7.0 N (61 db) may be allowed. b) Crosstalk Far-end crosstalk between circuits in the same direction, measured on the repeater sections of a carrier system on unloaded symmetrical pairs, terminated SYMMETRICAL CABLE PAIRS — LINES 83 at their two ends by impedances equal to their characteristic impedance, should not be less than the values shewn below (which allow for any crosstalk balancing networks). 8.0 N (69.5 db) for repeater sections of 12-channel systems. 7.5 N (65 db) for repeater sections of 24, 36, 48 or 60-channel systems. c) Uniformity of impedance

The impedance in any circuit on a repeater section forming part of a carrier system on unloaded symmetrical pairs, should not differ from the nominal value by more than the values shewn below: ± 5 % (value measured at 60 kc/s) for a repeater section forming part of a 12 channel system; ± 8 % (value measured at 108 kc/s) for a repeater section forming part of a 24 channel system; ± 8 % (value measured at 120 kc/s) for a repeater section forming part of a 36 or 48 channel system: ± 8 % (value measured at 240 kc/s) for a repeater section forming part of a 60 channel system. d) Balancing of repeater sections

As regards methods of balancing repeater sections of unloaded symmetrical pair carrier cable systems, various methods have been applied with success in dif­ ferent countries, both to deloaded old-type audio cables and to more modern symme­ trical pair carrier cables. The C.C.I.F. does not specially recommend any one of these methods, it being understood that for a repeater section crossing a frontier, the Administrations or Private Operating Companies concerned should agree in each case upon the choice of the balancing method to be used. For information, the following annexes appearing in the Book of Annexes to Vol. Ill of the Green Book give the methods of balancing which have been success­ fully used in Great Britain (Annexe 5) in the Netherlands, (Annexe 10), in France (Annexe 7) and in Mexico (Annexe 8) on new unloaded symmetrical pair carrier cables. Note. — The deloading (and balancing of far-end crosstalk) is one of the me­ thods of obtaining carrier telephone circuits. Where old cables are concerned, it may happen that with the best balancing possible, a large percentage of the cable pairs may be unsuitable for carrier working, and that on each of the suitable pairs, only a limited number of carrier channels may be worked; it is possible that in countries with a high telephone density, for routes between towns where a high level of traffic is expected in the near future, it. is sometimes cheaper to lay a new cable (having a wide frequency band) for the long distance connections and to cut up an old cable for extension circuits. 8 4 SYMMETRICAL CABLE PAIRS — LINES

When it is a question of an old cable connecting two cities a long distance apart, and if no immediate station is to be served, it is possible that deloading is the cheapest solution. As each case needs special consideration, the C.C.I.F. has made no general recommendation on this subject. e) Crosstalk balancing networks The C.C.I.F. does not judge it opportune to recommend the use of a particular type of crosstalk balancing network. Some Administrations moreover, are of the opinion that it is sufficient during construction of the cable to balance out cross­ talk very carefully by means of crosses at joints, and that if this is done there is no need for crosstalk balancing networks. Three types of balancing networks have been used successfully: 1. networks having only capacitors; 2. networks having capacitors and a resistor; 3. networks having a capacitor and a variable mutual inductor. As regards the two first types, which have been used mainly in Europe, the C.C.I.F. point out that it is possible (by means of suitable crosses effected between conductors at the joints) to reduce the number of resistances, and thereby achieve a slight saving, although it is necessary (for each network) to leave room for a resis­ tor in the far-end crosstalk balancing frame. For information, annexes 9 and 10 of the Book of Annexes to Vol. Ill of the Green Book reproduce the essential clauses of specifications which are used in Great Britain and Mexico for the supply of far-end crosstalk balancing networks. f) Location of the far-end crosstalk balancing frames In spite of the theoretical advantages of intermediate points in the repeater section for the location of far-end crosstalk balancing frames, Administrations and Private Telephone Companies with experience, are unanimously of the opinion that it is sufficient to place them at one end of the repeater section, which avoids a cabinet (or small building) which would otherwise be necessary at an intermediate point. If pairs have to be taken out at an intermediate point of a repeater section, it may be better to locate the balancing network at the beginning, rather than at the end of this section. On the other hand, for submarine cables, it may be necessary to have two crosstalk balancing frames, one at each end of the submarine section, because it is not practicable to make the crosses between wires in different layers, and because the different circuits do not have the same phase propagation time in the submarine cable. CARRIER CURRENTS — COAXIAL PAIRS 8 5

3.4. Multi-channel carrier telephone systems on coaxial pairs

3.4.1. General characteristics a) Frequency spectrum The frequency spectrum between 60 kc/s and about 4 Mc/s should be in accor­ dance with the diagram opposite (figure 17). It is very desirable to be able to set up large groups of long distance international circuits, on carrier systems on coaxial pairs, with a minimum of intermediate demo­ dulations and remodulations, by avoiding intermingling these groups with those used for setting up shorter circuits. Therefore, the C.C.I.F. recommends preferably the use of supergroups 4 to 12 (inclusive) to set up these large groups of long-distance international circuits. It is convenient to use supergroups 1 to 3 for short circuits. Administrations or Private Operating Companies needing a greater number of shorter circuits, should agree to omit one of the supergroups higher than 12, in order to facilitate the deri­ vation of the others. Note. — Supergroups 1 to 3 and 13 to 16 have the same quality as the other supergroups and may well be used for long-distance circuits. Their use is recommend­ ed for these short circuits because they can be extracted from the line (or reintroduced) by simple filters (assuming in the present state of the art, the sacrifice of a supergroup when the higher supergroups are used), without demodulation or remodulation of the supergroups which are not derived. b) Stability of the virtual carrier frequencies It being understood that any international telephone channel should be suitable for voice-frequency telegraphy, the stability of the virtual carrier frequencies should be such that, between an audio frequency applied at the origin of a circuit and a corresponding audio frequency at the other end, there should be a maximum differ­ ence of 2 c/s irrespective of the make-up of this circuit, that is to say whether there are, or are not, any intermediate modulation and demodulation processes. It is considered necessary to specify for the master oscillators of carrier systems, a stability of at least 10'7. It being understood that the coaxial pairs recommended by the C.C.I.F. should be able to transmit effectively a band of frequencies up to about 4 Mc/s and that for these voice-frequency telegraph transmissions on a telephone channel, the differ­ ence between the audio frequencies should not exceed 2 c/s, it is necessary that the master oscillators referred to above, should have a stability of at least 2.5 x 10'7 when only two of them are involved in the circuit. 8 6 CARRIER CURRENTS — COAXIAL PAIRS

Basic Group A Basic Group B

Telephone Channel No. i e 3 U 5 6 ? 8 0 » II 18 IS II 40 0 8 „ 1 .5 . 5 „ U .3 e . I

Frequency kc/s

Numbering of the channels in groups

(basic supergroup)

Supergroup No. Group No. K* K* K3 K . 3 . 5 KS K* K3 KS K1 * - N W \ N AAAAA L kL N L Frequency kc/s SO log ISS Sou 852 300 318 350 la g USC 5oU SSS S6U SIS S60 J o g f i g 80U

Numbering of groups in supergroups

Supergroup No. 40 11 IP 13 /// 15 16 uogp - 4 556 80S 4058 JSOU 1558 IgOO w<8 esg6 aswiT ' a$8 3o«0am 38*8 3536 3^Wl * V t ? * * ♦ t + | * * * * 8^8,8^ 30363oWf J28fr]3B0g 3538 3SUO 3ft> 3p8 MSS kc/s

Basic supergroup transmitted directly to line

represents a supergroup of 60 telephone channels, the virtual carrier frequencies of which are spaced at 4 kc/s and in which the audio frequencies are upright in the different telephone channels.

represents a supergroup of 60 telephone channels, the virtual carrier frequencies of which are spaced at 4 kc/s and in which 1 the audio frequencies are These symbols are provisional; inverted in the different telephone channels • until agreed by the C.C.I.F., C.C.I.T. and C.C.I.R.

represents a measuring frequency

F ig u r e 17. — Line frequency spectrum for international coaxial cables used for carrier telephony CARRIER CURRENTS — COAXIAL PAIRS 8 7 c) Pilots In systems with a large number of carrier telephone channels on coaxial pairs, regulating pilots are required to maintain the equivalents at the specified values and to ensure an attenuation equalisation which is always satisfactory: also synchro­ nising pilots are needed either to synchronise the master oscillators at the two ends or to enable their frequencies (and if necessary their phase) to be compared from time to time. Finally, it is desirable to allow for switching pilots which can be used in particular to change over from a working to a reserve repeater or section of line. It is possible that the same pilot may be used to achieve more than one of these three functions. 1° Line regulating pilots. — In order to reduce variations of level and of equi­ valent on long routes, it is very desirable that the line regulating sections, over which the control of the levels and attenuation equalisation is exercised by pilots transmitted from end to end, should be as long as possible. In practice, a large number of regulating sections will be terminated at inter­ national exchanges. Administrations or Private Operating Companies concerned will agree the limits of all regulating sections for each particular case. There should be two line regulating pilots; these pilots could, for example, be used to displace bodily the “ gain-frequency ” characteristic of the repeater, in order to compensate for variations of attenuation in the preceding cable section, and if necessary, to readjust the attenuation equalisation. There are also other methods of regulating, using two pilots. The frequency spectrum recommended for international coaxial cables (see section 3.4.4 below), allows the use of 16 carrier telephone supergroups (Figure 17). These 16 supergroups occupy the band of frequencies from 60 kc/s to 4028 kc/s. To achieve the required regulation, it is necessary to have one pilot in the lower part and one in the upper part of this frequency band. The International Telephone Consultative Committee recommends the use of the following frequencies:— a) 60 kc/s or 308 kc/s for the lower line regulating pilot; P) 4028 + 64 = 4092 kc/s for the upper line regulating pilot. By agreement between the Administrations or Private Operating Companies concerned, it is possible to use instead, for the upper regulating pilot, the frequency 2540 + 64 = 2604 kc/s, i.e. a frequency 64 kc/s above the maximum frequency of the tenth super group. The development of the international coaxial cable network could involve the need for intermediate line regulating pilots: but it would be premature to fix the frequencies to be used for such pilots. 88 CARRIER CURRENTS — COAXIAL PAIRS

In any case they should be selected to avoid interference with television trans­ missions using the coaxial pairs on which these pilots would be transmitted. The frequencies used for the complementary line regulating pilots should be selected from the following list:— 60, 308, 556, 808, 1056, 1304, 1552, 1800, 2048 and 2296 kc/s for carrier systems on coaxial pairs with 10 supergroups: 60, 308, 556, 808, 1056, 1304, 1552, 1800, 2048, 2296, 2544, 2792, 3040, 3288, 3536 and 3784 kc/s for systems with 16 supergroups. The absolute power level of these pilots (referred to a point of zero relative level, deduced from the level diagram of the telephone circuits set up on the carrier system considered) should be adjusted at the output of the send repeater to have a nominal value and tolerances as follows:— For the lower line regulating pilot, the nominal value is — 10 db (with a tolerance of dt 0.5 db for the initial line-up) or — 1.2 N (with a tolerance of ± 0.05 N for the initial line-up). In addition, these levels, after the initial line-up, should not vary with time by more than ± 0.03 N (or ± 0.3 db). The same nominal value and the same tolerances are applicable provisionally to the upper line regulating pilot. Note. — The C.C.I.F. is studying whether it is possible to have a higher nominal level for the upper line regulating pilot, in order to permit the use of simple regulators in each repeater station.

2° Synchronising pilot: frequency control pilot. — The European coaxial cable network not yet having been completed, it is not possible to have a fundamental frequency standard for all Europe, from which would be derived synchronising pilots for all carrier systems: moreover at the present time, this does not seem necessary. During the next few years only “ partial ” coaxial cable networks will be established and it is necessary that within the same partial network, “ master oscillators ” in stations where frequencies are generated, should be “ co-ordinated ”. This “ co-ordination ” could be a dependence of one oscillator on another by the following methods: 1) synchronisation, i.e. locking of frequencies and phase; 2) isochronisation, locking of frequencies only; 3) differential control to correct, from time to time, differences between the frequencies. Also it is possible to install devices to give an alarm, if the difference between the frequency of the frequency control pilot, and the frequency of a local oscillator exceeds a certain fixed value. It is not necessary to recommend at the present time any particular method of comparing or controlling the master oscillators at different stations, and it is sufficient to have “ periodic frequency comparison ” of the master oscillators, this comparison being followed if necessary by automatic or manual regulation, it being understood CARRIER CURRENTS — COAXIAL PAIRS 8 9 that in each partial network, the master oscillators will be compared periodically with a national frequency standard (see Note 1 below) and that the various national frequency standards are regularly compared one with another, internationally. It is possible that these periodic comparisons will be sufficient, if experience shews they are not, the C.C.I.F. envisages the use of a synchronising pilot, trans­ mitted over whole of the European network. The periodic comparison of the frequencies generated by the master oscillators is made by means of a pilot transmitted to line for this purpose. It is necessary to compare phases. To make this periodic frequency check, a frequency of either 60 kc/s or 308 kc/s may be used for the frequency control pilot. If a country has a national frequency standard which may be distributed (by radio or line) throughout the country, and if the master oscillators of the carrier systems have an adequate frequency stability (see Note below), it is sufficient to compare these oscillators from time to time with the national frequency standard. But the case may arise, either of a country which has a national frequency standard with no facilities for distributing it throughout the country, and particularly in an area in which a coaxial carrier system is to be set up, or in a country which has no national frequency standard. In this case, the neighbouring country (having a national frequency standard and facilities for its distribution) could send a synchro­ nising pilot to the first country from the nearest master oscillator to the common frontier. This would permit interconnection between the networks of the adjoining countries considered. The absolute power level of a frequency control pilot (referred to the point of zero relative level deduced from the level diagram of the telephone circuit set up on the carrier system considered) should be adjusted at the output of the send repeater, to a nominal value of — 1.2 N (— 10 db). Note 1. — A national frequency standard consists of oscillators and means for measuring their frequency with respect to time given by astronomers, which is the fundamental base. The frequency standard determined by this equipment is transmitted by a “ distribution oscillator ” to the oscillator which is to be compared with the national frequency standard. Except for small fluctuations (generally less than 10"8 in relative value), the frequency of this “ distribution oscillator ” varies slowly as a function of time: the value of this slow variation is known and corrected. If this correction is made systematically and continuously, or if the slow variation is always sufficiently small, (between 10‘7 and 10'8), no special precautions need be taken. If this slow variation of the standard frequency distributed exceeds the limit quoted above, and is only corrected periodically, it is necessary to inform the maintenance service (of the telephone Administration owning the oscillator to be compared with the national frequency standard), of the times when this correction will be made, in order that

*)The frequency 1 800 kc/s is provisionally reserved to allow, if necessary, international fre­ quency comparisons. If the Administrations concerned so desire, this frequency 1 800 kc/s may be used for the frequency control pilot. 9 0 CARRIER CURRENTS — COAXIAL PAIRS

the comparison of the frequency of this oscillator with that of the national frequency standard, may be made at a time when this variation has just been corrected. Note 2. — The annexes Nos 11 to 15 of the Book of Annexes to Vol. Ill of the Green Book describe, for information, the methods followed in different countries for the “ coordination ” of the master oscillators between themselves and with the national frequency standards.

3° Automatic switching pilots. — The provision of a reserve section of line, to be used when the corresponding section of the line normally used develops a fault, is an insurance against the risks of interruption of the telephone service, and it is necessary to consider the cost of this insurance. Some comments on this question are given below: It is necessary to note that the systematic provision of a reserve section of line is not the only means of providing against an interruption of service when there is a major or minor fault, and that it does not give complete security. On the one hand effective security measures can be taken to design repeaters to avoid interruption of services due to a repeater fault. On the other hand when a working coaxial pair would be affected by a cable fault, it is unlikely that a reserve pair in the same cable would be unaffected and remain so for a period long enough for it to be used to replace the working pair, thereby rendering the coaxial reserve pairs less effective. An alternative is to consider the installation of a complete set of supergroup translating equipment (translating basic supergroups into the coaxial line frequency band and conversely), on all coaxial systems, so that all the circuits can be switched on to reserve supergroups set up, either on a second carrier system using a coaxial pair in the same cable (e.g. a system used mainly to provide national circuits) or preferably on a coaxial pair in a cable on another route. It would perhaps be possible, by this method, to obtain reserve circuits more economically than by establishing additional sections of coaxial line, which would normally be unused. With manual or semi-automatic working it is not necessary to arrange for a switching pilot to “ engage ” circuits at the out-going exchange when there is a fault on a coaxial system.

4° Multi-purpose pilots. — Administrations or Private Operating Companies concerned with an international carrier system on coaxial pairs, may agree to use (if they consider it desirable) one of the lower line regulating pilots (either 60 or 308 kc/s), for the level control as well as for frequency control. In any case, it is desirable that one of the two following solutions should always be used to allow the simultaneous use of the line regulating pilots for frequency control:— — either there is, in each line regulating section, a master oscillator which is regularly compared, directly or indirectly with a national frequency standard; CARRIER CURRENTS — COAXIAL PAIRS 91

— or, there is no master oscillator in a line regulating section, the lower pilot of the line regulating pilot coming from the adjacent section is reintroduced, with a stabilised level beyond the junction between the two line regulating sections considered. Generally speaking, it is possible for one pilot to have two or more functions if the Administrations or Private Operating Companies concerned so decide. For information the annexes 16 to 18 of the Book of Annexes to Vol. Ill of the Green Book shew the multi-purpose pilots now in use in the carrier systems on coaxial pairs of various countries.

5° Additional measuring frequencies. — For periodic maintenance measure­ ments of a carrier system on coaxial pairs, it is necessary, in addition to the pilots mentioned above, to have pilots at a specified level, called “ additional measuring pilots ”. The frequencies which may be used for these are the following: 60, 308, 556, 808, 1056, 1304, 1552, 1800, 2048 and 2296 kc/s for the carrier systems on coaxial pairs having 10 supergroups; 60, 308, 556, 808, 1056, 1304, 1552, 1800, 2048, 2296, 2544, 2792, 3040, 3288, 3536 and 3784 kc/s for systems having 16 supergroups. The absolute power level of these additional measuring pilots (referred to a point of zero relative level deduced from the level diagram of the telephone circuits set up on the carrier system considered) should be regulated, at the output of the send repeater, to the following nominal value with the following tolerances:— The nominal value is fixed at — 10 db (with a tolerance of ± 0.5 db for the initial line-up) or — 1.2 N (with a tolerance of ±).05 N for the initial line-up). In addition, these levels, after the initial line-up should not vary by more than ± 0.03 N (±0.3 db) as a function of time.

6° Precautions to be taken with pilots at a supergroup transfer point. — At a point B where one or several supergroups on a line section AB are transferred into another line section BC (see figure 18), special precautions should be taken with respect to the pilots defined above, so that these pilots are transmitted to certain line sections where it is desired to route them, and that, on the contrary, they do not interfere on other sections, with the pilots of the same type transmitted on these sections.

C

F ig u r e 18 to

2500 km (Woo wiUeo) ARE CRET — OXA PAIRS COAXIAL — CURRENTS CARRIER -DflqooD

QdCUAxLe ' ( j ^ a u d e (J c k z w d e jdeo jze*jueuced /Leo jt&jueuceo zdeO jt&fueuced deo ftequmceo 1kaucnmc>eo o u r fto c c d e to /shi (S/zoufte pMmcu/ie /la ji&ufie />ecDudmfi£ M i fu m e c o a a c io le yde baoe &te baoe

Basic Basic Line Audio group supergroup frequency frequency frequency frequency

—C— Channel translating equipment (translation of the audio frequency band into the basic group and vice versa) —0 — Group translating equipment (translation of a basic group into a basic supergroup and vice versa)

—f~|— Supergroup translating equipment (translation of a basic supergroup into the line frequency band and vice versa) Li

Figure 19. — Diagram of the nominal maximum circuit on coaxial pairs CARRIER CURRENTS — COAXIAL PAIRS 9 3

Line regulating pilots. — If it is not desired to connect the line regulation of section AB to that of the other sections, the point B, is by definition, the end of a line regulating section AB and, the line regulating pilots of this section AB should be stopped in such a way that, on the sections BC and BD, they are 4.6 N (40 db) below the pilots used on these sections. If, on the contrary, the point B is not the end of the line regulated section AB (for example the line regulated section extends from A to D) and one (or two) line regulating pilots are in the band of frequencies of the channels transferred towards BC, or at the edge of this band, the line regulating pilots of the section AB should be stopped, as above, so as not to be transmitted on the section BC. It may be neces­ sary to take special precautions at point B so that one of these pilots (or both) should be transferred beyond B on to section BD.

Additional measuring frequencies. — At a station where supergroups are derived, and which is within a line regulating section, the additional measuring frequencies within the derived frequency band are derived with the supergroups. Administrations concerned should, in each particular case, fix the points where the additional measuring frequencies transmitted on a part of the system should be blocked, so as not to interfere with the operation of the other parts. It may not be possible to use additional measuring frequencies oh the edges of a derived frequency band because the amplitudes of these frequencies are affected by the transfer filters. It could therefore be desirable in certain cases to fix the “ measuring sections ” over which these additional measuring frequencies would be used. .The fixing of such measuring sections is left to the discretion of the Administrations concerned.

Other pilots. — No recommandation on this subject has been issued: the question is being studied.

Note. — Similar arrangements must be made for the interconnection of symme­ trical pairs and coaxial systems.

d) Nominal maximum circuit on coaxial pairs

This “ nominal maximum circuit” is 2500 kilometers (1550 miles) long and set up on a carrier system on coaxial pairs. This “ nominal maximum circuit ” has, for each direction of transmission, a total of:— 3 pairs of channel modulators, each pair including translation from the audio frequency band to the basic group and vice versa: 6 pairs of group modulators, each pair including translation from the basic group to the basic supergroup and vice versa: 9 pairs of supergroup modulators, each pair including translation from the basic supergroup to the frequency band transmitted on the coaxial pair and vice versa. 9 4 CARRIER CURRENTS — COAXIAL PAIRS

Figure 19 opposite shews the outline of the “ nominal maximum circuit on coaxial pairs It will be seen that there is a total of 36 different modulations for each direction of transmission, assuming that each modulation is made in a single stage. The distance between two successive modulation points has an average value of 280 kilometers: it is assumed that this distance does not exceed 500 kilometers. e) Noise

For setting up new international coaxial systems the following basis should be used provisionally:— The psophometric power of the “ nominal maximum circuit on coaxial pairs ” should not exceed 10,000 picowatts at the point of zero relative level, it being under­ stood that this limit should not be exceeded for more than 1 % of the time, the mea­ surements being made during a period (generally the' busy hour) when the noise level is expected to be at its highest.

Division of the allow'able noise between the modulating equipments and the high- frequency line. — It is agreed provisionally as a guide in establishing new coaxial systems, that the psophometric power corresponding to the noise produced by all the modulating equipments mentioned in the definition “ nominal maximum circuit on coaxial pairs ” (if all these equipments are of the type used in the country where the project is being considered) should not exceed one quarter of the limit fixed above, i.e. 2500 picowatts at the point of zero relative level. This lea­ ves a psophometric power (at the point of zero relative level) of a least 7500 pW for the “ high frequency line ” i.e. for the 2 500 kilometers of coaxial pair and all the associated intermediate repeaters.

Noise per unit length of the high frequency line. — As a guide for the construction of international carrier systems on coaxial pairs, it is provisionally recommended that the psophometric power produced on the section of line within each country, should not exceed a total value based on an average value of 3 picowatts per kilometer (referred to a point of zero relative level), it being understood that in the case of a section crossing a frontier the Administrations or Private Operating Companies concerned come to an agreement on this subject.

Note. — The contribution of the high-frequency line to the “ psophometric power ” (at a point of zero relative level) produced on the whole of the circuit, is not strictly proportional to the length of the line, because certain intermodulation products (odd harmonics) tend to add on a voltage, instead of on a power basis: but this assumes that the phase distortion is relatively small, and this process is in fact interrupted by the modulating operation. As it has been stated in the definition of the “ nominal maximum circuit on coaxial pairs ”, that the maximum length of line between two successive modulating points is 500 kilometers (310 mis) it can be assumed that the psophometric power at a point of zero relative level is approximately propor­ tional to the length of the line, so that the limit shewn above, of 7 500 picowatts for a line of 2 500 kilometers (1 550 mis), corresponds for example, to a limit of 1 500 picowatts for a line of 500 kilometers (310 mis). CARRIER CURRENTS — TERMINAL EQUIPMENTS AND REPEATERS 9 5

3.4.2. Terminal equipments

Stability of virtual carrier frequencies

All stations with coaxial terminal equipments should have master oscillators with a frequency stable to about 10'7.

3.4.3. Repeaters

Return current coefficients between the impedance of a coaxial pair and the input and output impedances the repeaters used on this coaxial pair ZL is the characteristic impedance of the line (for any frequency / effectively transmitted), this impedance being the ordinate for the frequency / of a smooth curve, agreed by the Administrations or Private Operating Companies concerned as being representative of the average “ impedance/frequency ” characteristic of the type of coaxial pair concerned:— ZR is the worst value of the input impedance (for the frequency /) of the equip­ ment of a repeater station, as seen from the line (see figure 20): ZE is the worst value of the output impedance (for the frequency /) of the equipment of a repeater station, as seen from the line: A = al the total image attenuation (at the frequency / ) of the line between two adjacent repeater stations, a being the average attenuation per unit length and I the average length between two adjacent repeater stations.

Figure 20. — Repeater section of coaxial pair

Then N (in decibels or nepers) is defined by the formulae:

ZE + ZL z L + ZR N — 2A -j- 20 logio + 20 logio (decibels) ZE -ZL ZL :— ZR or

Ze + ZL ZL + z R N — 2A -f- loge -j- In (nepers) Ze -ZL ZL -ZR 9 6 COAXIAL PAIRS — LINES

For a carrier telephone system on coaxial pairs, N should be at least equal to 4.6 N (40 db), for all frequencies below 300 kc/s because it is difficult at these frequen­ cies (where the cable has a relatively small attenuation) to obtain higher values of N with practical values for the reflection coefficient at the input of the repeater, if it is assumed that the output impedance of the repeater is completely unbalanced with respect to the line impedance. With this value of N, it can only be hoped that it will not produce rolls in the “ attenuation/frequency ” characteristic, with an amplitude greater than 0.1 N (1 db), (considered to be a reasonable limit) to the end of a line section of about 700 kilometers (435 miles). This appears sufficient as it is very improbable that these very long-distance interna­ tional circuits will be established, throughout their length, on channels of super­ group no. 1 (60 to 300 kc/s) because it is very easy to derive this supergroup at inter­ mediate points, which makes it particularly suitable for relatively short circuits. At frequencies higher than 300 kc/s, to obtain at the end of a telephone circuit of 2500 km (1550 mis) (based on the “ nominal maximum circuit on coaxial pairs ”), when this telephone circuit is carried by supergroups transmitted to line with such frequencies, rolls of the attenuation “ equivalent/frequency ” characteristic having an amplitude not greater than 0.1 N (1 db), it is desirable to adopt for N the value of 5.2 N (45 db). Note 1. — Even if a telephone circuit of 2500 km (1550 miles) based on the “ nominal maximum circuit on coaxial pairs ” is carried by supergroup N° 1 in some of the 9 sections of the line between the successive modulation points (see figure 19 above) it is likely that amplitude of the rolls of the “ attenuation/frequency ” characteristic will not exceed 0.1 N (1 db) because in repeater sections of other line sections where this “ nominal maximum circuit ” is carried on channels of other supergroups, there will probably be values of N which are much higher than the limit of 5.2 N (45 db). Note 2. — The C.C.I.F. has only defined the admissible limits for the quantity N (involving three terms) (see the formulae above). It is recommended that the Admi­ nistrations or Private Operating Companies concerned with a section of coaxial pair crossing a frontier, should agree to fix in this particular case the admissible values for each of these three terms, while still respecting the above conditions, i.e. to use a very good match at the ends of the repeater section or a deliberate unbulance.

3.4.4. Lines

A. Cable specification

Essential clauses of a model specification for the supply of cables including coaxial pairs designed to provide a large number of carrier telephone channels

a) Type o f coaxial pair. — It is very desirable to have throughout the European network the same type of coaxial pair and having the following characteristics:— COAXIAL PAIRS — LINES 9 7

The centre conductor should be a solid copper wire of 0.104 ins (2.6 mm) diameter. The outer conductor should be a soft copper tape, in the form of a cylinder around the insulation, the axis of this cylinder being the axis of the centre conductor: the thickness of the copper tape used for the outer conductor, should be 0.010 in (0.25 mm): the interior diameter of the outer conductor should be 0.375 ins (9.5 mm). It is desirable for cross-talk reasons, to surround each outer copper conductor with two open helical soft steel tapes.

b) Impedance. — If polythene or ebonite discs are used for the insulation, the ordinate of the average impedance/frequency curve at a frequency of 2.5 Mc/s should be between 73.5 and 76.5 ohms. The regularity of the impedance of the coaxial pair has only been specified for a repeater section in the following form:— At each end of the repeater section, the impedance throughout the band of frequencies to be transmitted is measured, the other end being terminated with an impedance such that it does not produce an appreciable reflexion at this remote end. A curve is drawn with frequency as abscissa and the resistive component of the impedance measured at the appropriate frequency as ordinate. This curve will have rolls and a smooth curve is drawn through it. For all types of coaxial pairs used for telephony, the difference between the measured curve and the smooth curve, should not exceed 3 % If a coaxial pair is to be used, not only for telephony but also for television, it may be desirable to specify more stringent limits for impedance regularity: this question is being studied by the C.C.I.F.

c) Dielectric strength. — The insulating material should withstand for two minutes a voltage of 2000 volts r.m.s. 50 c/s applied between the centre conductor and the outer conductor connected to the sheath. This dielectric strength test should be made on each factory length and on the repeater section.

d) Insulation resistance. — The insulation resistance between the centre and outer conductors of the coaxial pair, measured with not less than 500 volts, should not be less than 5000 megohms per kilometer after electrification for one minute, at a temperature not lower than 10° C (50° F) the readings of the galvanometer during this test showing perfectly steady electrification. The measurement of the insulation resistance should be made after the breakdown test. This measurement should be made on each factory length and on the repeater section.

e) Attenuation. '■— When polythene or ebonite discs are used, the attenuation of the coaxial pair, measured at 15° C (59° F) and at a frequency of 2500 kc/s, should not exceed 0.47 N (4.1 db) per kilometer. The measured value of the attenuation should be corrected for the average cable temperature, using an attenuation/temperature coefficient of 0.0021 per degree C at any frequency between 300 kc/s and 2500 kc/s 98 SYSTEMS ON COAXIAL PAIRS

B. Specification of a repeater section Essential clauses of a model specification for the supply of coaxial cables designed to provide a large number of carrier telephone channels a) Regularity of impedance. — At each end of the repeater section, the impedance throughout the band of frequencies to be transmitted is measured, the other end being terminated by an impedance such that it does not produce an appreciable reflection at this remote end. A curve is drawn with frequency as abscissa and the resistive component of the impedance, measured at the appropriate frequency, as ordinate. A curve is obtained having rolls, and a smooth curve is traced through it. For all types of coaxial pairs used for telephony the deviation of the measured impedance with respect to the value deduced from the smooth curve, should not exceed =t 3 %. Note. — The C.C.I.F. considers that it is not necessary to standardise interna­ tional pulse tests, for coaxial pairs to be used only for international telephone circuits. Administrations or Private Operating Companies concerned within the International line should decide whether these tests should be made and what the limits should be.

b) Crosstalk. — The far-end crosstalk between two coaxial pairs of a cable should be at least 9.8 N (85 db) at any frequency in the band of frequencies effectively transmitted.

Note. — It is not considered necessary to specify a limit for the near-end crosstalk ratio, because recent tests have shewn that the near-end crosstalk ratio, under service conditions, is greater than the far-end crosstalk ratio. In a repeater section (with a length of about 9 kilometers), a near-end crosstalk ratio E, corresponding to the empirical formulae below will be obtained:—

E = 80 + 50.5 \ / f decibels / in Mc/s or E = 9.2 + 5.8 I/T nepers / in Mc/s up to maximum of 135 decibels or 15.5 nepers.

3.4.5. Design, supervisory arrangements and power feeding of a carrier system on coaxial pairs a) Definitions o f the constituent parts of a carrier system on coaxial pairs ' * ' • Repeater stations of a carrier telephone system on coaxial pairs can be classified as follows:—

1. Stations are classified as attended or unattended, according to whether they are normally staffed or not.

2. Power feeding stations have a local electricity supply and feed power over the cable conductors to other stations. Power fed stations receive power from other SYSTEMS ON COAXIAL PAIRS — FEEDING 9 9 stations over the cable conductors. There are other stations having a local power supply which do not feed power to other stations (autoalimentees). 3. As regards regulation of levels and equalization, a station having a local regulator (automatic or manual) is called a regulated station (station regulee), whilst a station which has a regulator remotely controlled from another station is called remotely regulated (teleregulee) and a station having no regulation is called non­ regulated (non regulee). Amongst regulated stations, there is one which controls the regulation of one or several remotely controlled stations; this is called a regulating station. It is possible to classify coaxial line sections as follows: A repeater section (section elementaire d’amplification) is a section of line between any two adjacent stations. A “ section principale d’amplification ” is a section of line (with intermediate amplifiers) between any two adjacent attended stations; A line regulated section (section de regulation de ligne) is defined under 10 in the note at the end of section 3.1 above. b) Supervisory arrangements Alarms. — Experience gained by Administrations which have had in use carrier systems on coaxial pairs, shews that it is necessary to transmit to all interested stations the following information:— 1° Failure of an amplifier or amplifier valve and effective substitution of a reserve amplifier or valve; 2° Failure of the normal source of power and effective substitution by a reserve source; 3° Indication of the repeater section in which a fault has occurred on the conductors of the coaxial pair. To transmit this information interstice pairs in the coaxial cable should be used. It is recommended this information should be transmitted automatically (by interstice pairs in the cable) to the attended station responsible for clearing the type of fault in question. Speakers. — Provision of two speaker circuits is recommended. The first circuit, called omnibus speaker should serve as an omnibus line between all attended stations. The second line, called through speaker is used to inform the control station (or sub-control station) concerned when one of the three faults have been automatically signalled to an attended station. c) Power feeding The advantages of alternating current for power feeding over a coaxial cable system are the following:— 1. There is in all countries a widespread alternating current power distribution network which can be used to supply the coaxial cable network. 2. Power may be transmitted at the most suitable voltage, which can be stepped up if necessary in an intermediate repeater station before transmission to the next station. 1 0 0 SYSTEMS ON COAXIAL PAIRS — FEEDING

3. Apart from the need for having indirectly heated valves, there are no other special requirements for the valves used in repeater stations.

4. In repeater stations any suitable anode voltage can be produced.

«• 5. There is no risk of electrolytic corrosion, even if earth return is used.

The disadvantages of alternating current are the following:

1. The transmission efficiency is less than with direct current.

2. The filtering of the power supply to each repeater station is more difficult than with direct current.

3. For a given maximum voltage, the effective voltage is lower.

4. Another point to consider when using alternating current is the noise produced on any voice-frequency circuits in the same cable.

After having weighed these advantages and disadvantages, the C.C.I.F. recom­ mends the use of alternating current at the normal supply frequency (in Europe 50 c/s). Various methods of power feeding over the cable which have been used are:—

1. In the United States of America, Great Britain and Italy, the power feeding circuit consists of the two centre conductors of two coaxial pairs; this provides a circuit balanced with respect to earth. In Great Britain and in Italy, this circuit is fed with constant voltage. In the United States of America, the current, (at 60 c/s) is monitored and is not allowed to fall below 80 % of its nominal value; also the differ­ ence between the currents in the two conductors is monitored and is not allowed to reach a value of 25 %.

2. In France, the power feeding circuit consists of the centre and outer conduc­ tors of the same coaxial pair, each outer being insulated from the other and from earth. There are thus as many power feeding circuits as there are coaxial pairs in the cable. This property is used by the French Administration in its equipments to augment reliability in the following manner: each line amplifier has two “ amplification paths ” with common negative feed-back. Each power feeding circuit on a coaxial pairs provides power for the operation of the valves in one “ amplification path ” of the corresponding line amplifier and one “ amplification path ” of the line amplifier of the coaxial pair used for the other direction of transmission. In this manner, power failure or a break in the power feeding circuit does not noticeably interfere with transmission, each amplifier continuing to operate with a single path. Constant current power feeding is used. The C.C.I.F. considers that it is unnecessary to recommend one or other of these methods exclusively, as the “ power feeding sections ” will always be relatively short, INSULATION OF COAXIAL PAIRS 101 and as a result their construction and maintenance is primarily a national question. If special agreement is not reached between the Administrations or Private Operating Companies concerned with a power feeding section crossing a frontier, it is recom­ mended that each Administration should power feed the repeater stations in its own territory. Stand-by power supply. — In countries where the mains power supply is unreliable and where it is the normal source of supply for the coaxial system, it is recommended that in each power feeding station there should be equipment to transfer from the normal source of supply to a stand-by source or vice versa in such a manner that there is not a break in transmission exceeding 150 ms approximately on voice-frequen­ cy telegraph circuits or on telephone circuits with automatic signalling carried by the system. d) Spacing of power feeding stations and attended stations A power feeding station is not necessarily an attended station if there is an adequate alarm and remote control system. In this case it is probable that the location of the attended station will be determined by local telephone requirements. e) Earthing of coaxial cables The practices followed vary according to the country and the systems used for power feeding. The British Administration always earths the outer conductor of each coaxial pair at each point where the pair is terminated. The French Administration on the other hand, which uses a power feeding circuit consisting of centre conductor and outer conductor of the same pair, keeps the tubes insulated one from another and from earth, for the reasons given in the Annex below.

ANNEX

Method used by the French Administration from insulating coaxial tubes from earth

The French Administration insulates the coaxial outer conductors from each other and from earth for the following reasons: 1. The power feeding circuit formed by the centre conductor and the outer conductor of the same coaxial pair, is completely independent of the adjacent circuit formed by another pair of the same cable; there are thus as many independent, power feeding circuits as there are tubes in the cable. The French Administration takes advantage of this to reduce the liability of failure, as shown on page 93 above. 2. Difficulties which can result from large differences of potential between “ earths ” at adjacent repeater stations, are avoided. In certain cases (in particular parallelism with direct current traction systems) these potential differences cause currents which flow in parallel along the outer conductor and 1 0 2 SYSTEMS ON COAXIAL PAIRS — INTERCONNEXION the centre conductor of the coaxial pair and through the power feeding transformers, and which reach such values that the operating conditions of these transformers are seriously affected. 3. The insulation of the outer conductors with respect to earth may be permanently “ watched ”. The protection of staff is ensured as follows: (a) Whenever the outer cover of a coaxial pair, is earthed (at a station or within the cable), the power supply on this pair is cut off. The following instructions are given to jointers and others working on the cable: (1) remove the U-links at the cable terminals, (2) earth the outer conductor of the cable pair before doing anything else. (b) The outer conductors of the coaxial pair in the cable are insulated by layers of paper, the specification requirements being: “ Dielectric strength. — The factory lengths should be capable of supporting, without rupture, for one minute, an alternating current voltage of 50 c/s with an r.m.s. value as follows: * (1) 2,000 volts between the centre conductor and the outer conductor of each coaxial pair; (2) 2,000 volts between (a) conductors of the quads and pairs surrounding the coaxial pairs and (b) the sheath and the outer conductors of the coaxial pairs, connected together; ' (3) 1,000 volts between the outer conductors of the coaxial pairs.” No special arrangements therefore need to made to earth all outer conductors simul­ taneously.

3.4.6. Interconnection of carrier systems on coaxial pairs using different techniques

T he International T elephone C onsultative C ommittee, Considering, that there are in use several multi-channel carrier telephone systems satisfying the general conditions recommended by the C.C.I.F. in sections 3.4.1 to 3.4.5 above, that until there is a more complete standardisation of these systems, special arrangements have to be made each time it is desired to establish an international connection on two different systems, carried unanimously the motion: that whenever it is desired to have a direct connection at high frequencies across a frontier between carrier systems on coaxial pairs using different techniques, the following conditions should be met: a) Pilots

Each line regulating pilot should be transmitted on the two systems to be inter­ connected, at the same absolute power level (measured at a point of zero relative level). If the two systems do not use the same frequencies for the pilots, each of the stations situated at the ends of the line regulated section crossing the frontier, should be equipped to send all the pilots needed by both systems. SYSTEMS ON COAXIAL PAIRS — INTERCONNEXION 103 b) Transmission conditions The following is suggested as a starting point for discussion between the Admi­ nistrations or Private Operating Companies concerned: Each of the Administrations should accept on the “ receiving ” side, the transmission conditions (levels, matching of the impedances of the repeater to those of the line) which are normally used on the incoming system. Further, the recommendation of the C.C.I.F., in subsection 3.4.3 above, for the matching of the impedances of the repeater to that of the line in all repeater sections, should also be applied in this particular case of a section crossing a frontier. It s however understood that the Administrations concerned could always agree to adopt other arrangements if preferred in certain particular cases. The Administrations concerned should agree on a graph, giving, for the high frequency line, the nominal value of the level at the output of the frontier repeater for the various frequencies, it being understood the value of the relative power level, should be within ±0.2N (±2 db) of the values fixed by this graph, which graph depends on the system considered.

Remark. — The question of tolerances, with respect to the nominal level, at the frequencies of the line regulating pilots can only be studied in each country by taking into consideration the regulating systems used.

c) Power feeding If a special agreement is not reached by the Administrations or Private Operating Companies concerned in a power feeding section crossing a frontier, it is recommended that each Administration power feeds only the repeater stations in its own country.

d) Supervision and alarms In each particular case, these points should be agreed by Administrations concerned.

e) Characteristics The C.C.I.F. has standardised the dimensions of the coaxial pair to be used in the future international European telephone network (see section 3.4.4 A above). Nevertheless, this standardisation allows certain variations so that the coaxial pairs manufactured by different contractors in different countries may not have exactly the same characteristics. To ensure uniformity throughout the frontier repeater section, it is strongly recommended that, by agreement between the two Administrations or Private Operating Companies concerned, the manufacture of the whole section should be entrusted to the same firm. If the same contractor does not supply the whole section, the two Administrations or Private Operating Companies concerned must very carefully coordinate their detailed specifications and their methods of laying and jointing, to ensure that the conditions recommended by the C.C.I.F. for the complete repeater section, are met. As regards matching of the impedance of this repeater section, to the impe­ dances of the two adjacent receive amplifiers, in the general case of a coaxial cable 10 4 SYSTEMS (1 + 3 ) ON OPEN WIRE LINES

section between two adjacent repeaters, the C.C.I.F. has only fixed allowable limits (for telephony and for television), for the sum N of the three terms defined in the paragraphs entitled “ Reflection coefficients between the impedance of a coaxial pair and the input and output impedances of the repeaters used on this coaxial pair ”, which appear in sub-section 3.4.3 above and in the 4th part below. It is recommended that the Administrations or Private Operating Companies concerned with a frontier repeater section, agree to fix the allowable values for each of these three terms, while still meeting the above condition, i.e. to agree on having as good a balance as is possible or a systematic unbalance at the ends of the repeater section. It is also very desirable that throughout the length of a carrier system on coaxial pairs, the Administrations or Private Operating Companies concerned agree always to use the same methods, principally as far as matching of impedances is concerned, so as to facilitate maintenance of the system.

3.5. Other modem carrier systems

Even though the systems described in the present section are modern, particularly because the audio frequency band effectively transmitted for each telephone channel is from 300 to 3 400 c/s, the general recommendations of section 3.1 above cannot be applied to them entirely, having regard to certain peculiarities of their make-up. For this reason the following special arrangements have been made.

3.5.1. Systems providing three carrier telephone circuits on a pair of open wire lines *)

The particular system described below may be used in areas with low telephone traffic. This two wire system provides three good quality telephone circuits in the frequency band above the existing circuit. (This system should be placed below the frequency band shewn in diagram I, section 3.2.1 above, for a system providing 12 telephone circuits). The most important consideration is to recommend a line frequency spectrum such that at the junction point between two' systems, it is not necessary to use modulators, as this junction point may be in a completely uninhabited region. Further, it is necessary that each of the three carrier telephone circuits, which the system provides may be used for 18 to 24 channel voice frequency telegraph systems; also it should be possible to obtain with this system, either a

*) The C.C.I.F. is studying the revision of the existing specifications for systems providing a small number of carrier telephone channels on open wire lines ( Yellow Book, Vol. IIIA/j, pages 67 to 74) to allow the effective transmission of frequencies from 300 to 3 400 c/s. With the exception of the model questionnaire which appears at the end of the present section, this text of the Yellow Book has therefore not been reproduced. SYSTEMS (1 + 3 ) ON OPEN WIRE LINES 1 0 5 carrier telephone circuit and a both way programme circuit (old type), or a programme circuit (new type) in addition to the audio frequency telephone circuit. The specification below has been designed for the above particular case. a) Frequency band transmitted The carrier frequency spacing should be 4 kc/s as in all other recommendations of the C.C.I.F. for modern carrier systems. It is recommended that the lower band transmitted to line, for one direction of transmission, should be between 4 and 16 kc/s and that the upper band, used for the other direction of transmission should be, either between 18 and 30 kc/s, or between 19 and 31 kc/s, so as to allow the use of staggering of the carrier frequencies, if it is later decided to use a second similar system on the same pole line. b) Relative power level The relative power level at the output of the terminal equipments and interme­ diate repeaters, on each channel and for the frequency of this channel which corres­ ponds to the audio frequency 800 c/s, is at the most ± 2 N (± 17 db). c) Pilots The frequency of the pilots should be 16, 110 kc/s for the lower band transmitted to line and 31, 110 kc/s for the upper band. This recommendation is applicable for the four frequency spectra shewn below. The absolute power level of the line pilots should be — 1.73 N (— 15 db) at a point of zero relative level. d) Variations (as a function of frequency) o f the equivalent at the output o f the sending terminal. See 3.1.2 d) above. e) Non-linear distortion of terminal equipments See 3.1.2 e) above. f) Crosstalk in terminal equipments See 3 .1 .2f) above. g) Impedance (as seen from the switchboard) See 3.1.2 g) above. h) Stability of the carrier generator frequency So that the effect of the modulations and demodulations never gives rise to a difference greater than 2 c/s between the audio frequency input channel and the audio frequency output at the remote end (where there is no intermediate demodulation and modulation), the stability of the carrier frequency generators should be not less than 2.5 x 10'5 approximately. i) Carrier leak At a point of zero relative level, the absolute power level of the carrier leak should not be greater than: 10 6 SYSTEMS (1 + 3 ) ON OPEN WIRE LINES

2.0 N (— 17 db) for a channel and for each direction of transmission, — 1.7 N (— 14 db) for all channels of the system taken together and for each direction of transmission. Note. — It remains to be examined how it would be possible to use systems corresponding to the particular specification above on main routes. In this connec­ tion the C.C.I.F. has studied the frequency spectra which could be used for the 1st, 2nd, 3rd and 4th systems set up on the same route, as well as the possibility of using staggered frequencies and inversion of the side bands. , The C.C.I.F. recommends the four frequency spectra in figure 21 below: no order of preference has been fixed between them, and Administrations concerned will select, in each particular case, the most appropriate scheme or schemes. Remark. — Also, by agreement between Administrations or Private Operating Companies concerned, the lower frequency band transmitted to line in schemes Nos. 2 and 4, may be inverted.

Figure 21. — Frequency spectrum for four systems, each providing three carrier telephone circuits, which can be established on the same pole route SYSTEMS ( 1 2 + 12 ) ON CABLES 1 0 7

Establishment o f a model questionnaire concerning preliminary information which should be obtained relating to existing open wire lines by Administrations or Private Telephone Companies wishing to establish multi-channel carrier .telephone systems

T he International T elephone C onsultative C ommittee

carried unanimously the motion that the following questionnaire should be used:

1. Which communication channels should be set up on carrier systems?

2. Which lines are available for carrier working? a) length of these lines, b) gauge, nature of wire,.distance between wires, c) existing cable sections, (location, type and length of these cables), d) existing transpositions, e) amongst available lines, are there two or more identical circuits which could be interchangeable, and from amongst which reserve circuits could be nominated?

3. What suitably located buildings are available for installation of the repeaters ? Where are the audio frequency repeaters located on the lines to be equipped for carrier working?

4. What are locations of radio transmitting stations liable to interfere with the carrier channels? What power and frequency is used by the various trans­ mitters?

5. Are the new carrier circuits to be connected to other lines permanently or temporarily?

6. Certain lines and offices having been selected as a result of the answers to the above questions, Administrations or Private Operating Companies should obtain the following information: What are the results of impedance and attenuation measurements made on each of the proposed line sections, throughout the frequency band to be used?

3.5.2. Systems providing 12 carrier telephone circuits on symmetrical cable pairs (12 + 12) systems

(12 -f 12) systems on symmetrical cable pairs are used (without the need for laying a second cable) on either old deloaded cables, or (in special cases) on cables specially laid (these generally being short). Therefore, it is very unlikely that, in the international network, these systems will be used for long distances or will involve more than two countries. 108 SYSTEMS (1 2 + 12) ON CABLES

A A

Scheme No. 1

Channel No. 60 f t JSO f

A ^ A A Scheme No. 2

Channel No. 6 30 SU 6o m /os f

F ig u r e 22. — Line frequency spectra for international (12 + 12) cable systems

represents a group of 12 telephone channels, with ^ 1 virtual carrier frequencies spaced at 4 kc/s and in — » which the audio frequencies are upright in the different telephone channels

represents a group of 12 telephone channels, with virtual carrier frequencies spaced at 4 kc/s and in These symbols are provisional; graphical symbols have to be which the audio frequencies are inverted in the •standardised by agreement bet­ different telephone channels ween the C.C.I.F., the C.C.I.T. and the C.C.I.R. represents a pilot

| represents two pilots of which one or the other is T { transmitted

a) Frequency spectrum transmitted to line The C.C.I.F. recommends that the frequency spectrum transmitted to line should be in accordance with schemes Nos 1 or 2 of figure 22. Administrations or Private Operating Companies concerned in the establish­ ment of such an international system should agree to use either one or the other of the two schemes.

b) Line regulating pilots For (12 -f- 12) channel carrier telephone systems on symmetrical pairs where it is necessary to use pilots, the following frequencies are recommended: With scheme No. 1: 60 kc/s and 72 kc/s, With scheme No. 2: 54 kc/s and 60 kc/s or 30 kc/s and 84 kc/s. These pilots should be transmitted with an absolute power level of — 1.73 N (15 db) at a point of zero relative level. SYSTEMS (1 2 + 12) ON CABLES 1 0 9

Note. — Administrations which have agreed to use scheme No. 2 should agree to choose between the two groups of line regulating pilots shewn above. When the frequencies 30 kc/s and 84 kc/s are used, if a group is to be transferred to another carrier system Administrations concerned with the (12 + 12) system should take the necessary steps to ensure that this line regulating pilot does not interfere with the other system. Also, it is necessary to take steps so that the carrier leaks (30 or 84 kc/s) do not interfere with the regulation of the line. SECTION 4

General characteristics of the international carrier telephone systems on radio relay links

4.1. Use of radio relay systems for international telephone circuits

The international Telephone Consultative Committee carried unanimously the motion that to ease the radio frequency allocation programme whenever possible, telephone circuits between fixed points should be set up by means of metallic lines or radio relay systems using frequencies higher than about 30 Mc/s, and that when this is done, the objective should be to provide circuits with the transmission quality recommended by the C.C.I.F. for international metallic circuits.

4.2. Terminal equipments of radio relay systems incorporated in the general tele­ communication network

T he I nternational T elephone C onsultative C ommittee Considering that in Europe, and even in other parts of the world, there is a vast international telecommunication network (as well as national networks) which has been established in conformity with the recommendations of the C.C.I.F., particularly as far as the frequency spectrum of the telephone channels in the frequency band up to 4 Mc/s is concerned, and also as regards the essential technical characteristics of the terminal equipments of all the carrier systems, Considering further, that, particularly for the increasing introduction of “ rapid ” and semi-automatic telephone service, it will be necessary to increase appreciably, in the near future, the number of long distance national and international circuits, and that consequently, during the next few years, it will be necessary to install multi-channel telephone systems on radio links and to integrate these links into the general telecommunication network, and considering finally, that interconnection of the systems should be made easy and that the task of the telephone Administrations who will have to use and maintain these systems, should not be unnecessarily complicated, RADIO RELAY LINKS 111

carried unanimously the motion, that when technically possible and economically desirable, 1° in the case of radio relay systems with at least 60 telephone circuits, these systems should be arranged in such a way that at the point of interconnection with the general network, the telephone circuits are assembled in the basic group A (12 to 60 kc/s) or B (60 to 108 kc/s), in the basic supergroup (312 to 552 kc/s) (for the groups of 60 circuits), and that when the radio relay systems have 120 or more telephone circuits the supergroups should be assembled in the same way as is recom­ mended by the C.C.I.F. for carrier systems on coaxial pairs; 2° with radio relay systems having less than 60 telephone circuits and which are capable of transmitting groups of telephone circuits and not individual circuits, these groups should be identical to one of the basic groups A or B, recommended by the C.C.I.F. for carrier systems on lines; 3° in all cases, the terminal equipments of groups should conform to the essen­ tial clauses of the specification appearing in sub-section 3.1.2 above. Note. — With systems on radio relay systems having exactly 60 circuits, the telephone circuits should also be assembled in accordance with the frequency spec­ trum recommended by the C.C.I.F. for systems having 5 groups on symmetrical cable pairs. The possibility of using supergroup No. 1 (60 to 300 kc/s) on the 60 circuit radio relay systems, and of interconnecting the supergroup in this frequency band is also allowable.

4.3. Time division multiple radio relay systems

Whatever the respective positions of the disturbing channel, and the channel disturbed, and whatever the modulation percentage of the disturbing channel, the crosstalk ratio in a radio relay system with pulse position modulation, measured at a frequency of 800 c/s, should be not less than 7.5 N (65 db) for the sending and receiv­ ing terminals connected back to back (excluding the radio path and intermediate stations). The statistical effect not being applicable to these systems, it does not appear useful to make tests in which the disturbing signals are sent simultaneously on several channels. to

Absolute power level, in nepers Frequency in c/s

+ 0 .8 - UI FEUNY ICIS GENERAL — CIRCUITS FREQUENCY AUDIO ° 4 o,65 0,6 - 3oo U00 600 800 4000 1200 tUoo 15oo 1800 2000 2200 21100 8600 2800 3000 3 2 0 0 3U00 0 ,5 H - 1 0 I------1----1----1----1----1----1----1----1----1-----h—I----1—I----1--- K—I---- 1----1----1---- 1----1----*- o.U - 0.35 o,3 — 0.2 o j - 0 i W M M b o,t - W' o,2 -

0 . 3 - 1 o,\L- o,5 - o,6 -h

F ig u r e 23 — Graph. No. 4. — Allowable limits for the variation (as a function of frequency) of the relative power level at the output o f a frontier repeater (frontier side) of a four-wire international circuit, effectively transmitting the frequency band from 300 to 3 400 c/s SECTION 5

Audio frequency circuits

This section concerns audio frequency circuits which may be used as international circuits, i.e. either four wire cable circuits, or two-wire audio frequency circuits on open wire lines. The specifications which appear at the end of this section also apply to national trunk/toll circuits liable to be used for an international call.

5.1. General characteristics

5.1.1. Frequency band effectively transmitted

T he International T elephone C onsultative Committee, Considering, that it is desirable to have satisfactory transmission quality in the international telephone service, and to take advantage of the electro-acoustic progress in the design of subscribers’ telephone sets, Carried Unanimously the motion that the use of lines and equipments which limit appreciably the band of frequen­ cies effectively transmitted within the range 300 to 3400, should be avoided for inter­ national calls. The allowable limits for the variation (as a function of frequency) of the relative power level at the output of a frontier repeater, on a four-wire audio frequency circuit, transmitting effectively the frequency band from 300 to 3400 c/s, are shewn by graph No. 4 opposite (figure 23). Note. — Annex 19 of the Book of Annexes to Vol. Ill of the Green Book gives a summary of the methods used in various countries to improve the quality of old type audio frequency circuits.

5.1.2. Non-linear distortion Tests have been made by the C.C.I.F. Laboratory, the German Administration and the British Administration on the variation of articulation as a function of power at the output of the first repeater of a long trunk/toll audio circuit. The results of this comparison are shewn in figure 24 opposite, which has three curves; this comparison shews that, over the working range, the curves have nearly the same 114 AUDIO FREQUENCY CIRCUITS — GENERAL

German curve SFERT British curve Laboratory curve

-S o . —to o JO So 3o Power calculated at the output of the first repeater, expressed in decibels, with respect to 6 milliwatts

Figure 24. — Syllable articulation, as a function o f power calculated at the output o f the first repeater o f a long trunk/toll circuit AUDIO FREQUENCIES — OPEN-WIRE LINES 11 5 slope (though slight), but that for higher levels,, the articulation falls more rapidly for circuits with a higher cut-off frequency (curve of the C.C.I.F. Laboratory, using a lightly loaded circuit) than for circuits with a lower cut-off frequency (German curve). Also the shape of the curve of variations of articulation as a function of power does not depend on the number of valves or amplification stages in the repeater, over the working power range. (The C.C.I.F. Laboratory curve and that of the German Administration relates to a single valve repeater; the curve of the British Administration to two-valve repeaters). Finally, before the effects of non-linearity give rise to a noticeable reduction of articulation, there is a change of timbre (not taken into account by articulation tests). 5.1.3. Combination of international circuits

T he C onsultative International C onsultative Committee, Carried unanimously the motion, that, for reasons of stability, international telephone circuits should never be made joined together other than in complete sections, limited to two repeater stations. 5.1.4. Other characteristics

The recommendations of section 1.3 above are applicable.

5.2. Lines

5.2.1. Open-wire and mixed lines

Loading o f open-wire lines

T he C onsultative International T elephone C ommittee, Considering, that the loading of open-wire lines

1° makes the use of these lines difficult because of variation of insulation resistance and magnetization of the coils by lightning discharges;

2 ° makes difficult the use of repeaters on lines; 3° is incompatible with the use of these lines for carrier telephony; 4° makes the transmission of different audio frequencies too variable, introduces distortion, and as a result, reduces the articulation of the conversation, Carried unanimously the motion that telephone circuits on open-wire lines, fitted with repeaters, and used in international long distance calls, should not be loaded. 116 AUDIO FREQUENCIES — OPEN-WIRE LINES

Construction of open-wire lines

T he C onsultative International T elephone Committee, Considering that the setting up of long-distance international calls, in certain countries, requires the use of open-wire lines: that the best utilisation of these lines would be obtained by using the phantom circuits, and repeaters, followed by the installation of multi-channel carrier telephone systems: that, to ensure good performance from such lines, it is essential to achieve electric symmetry of the circuits, and also a uniform distribution of electrical constants throughout the length of a repeater section: that it is not possible to fix finally and for general use, the geometric or mecha­ nical configuration of the lines, the choice of these being a function not only of electrical factors, but also of economic factors varying with time and from one country to another, Carried unanimously the motion, that international telephone lines on open wires should satisfy the following mechanical requirements. 1 1° For the construction of long-distance international telephone fines, only conductors with a diameter not less than 3 millimeters and with sufficient mechanical strength to minimize breaks, should be used. 2° The strength of the pole fine should be adequate to carry the highest loading arising from storms, icing and snow.

Test points on international open-wire lines

T he International C onsultative T elephone C ommittee, Considering, that the localization of faults should be made by means of precision tests, avoid­ ing as far as possible, the cutting of the fines, and that the fitting of test points in many offices increases attenuation and destroys the homogenity of the fines, Carried unanimously the motion,

1° that the number of test points, which are sources of frequent interruptions, should be reduced on international open-wire fines, to the minimum compatible with local requirements:

2 ° that measuring stations, equipped with precision test gear, should be set up in exchanges situated on the route, about 200 kilometers apart. These exchanges will be called “ main test points ” and the portion of the circuit between two main test points, “ main section In a main section, the faults will be located by tests made with cooperation from test points on either side of the faults., The results of these tests will be given to the centres concerned.

*) Part “ B — electrical qualities ” of this recommendation ( Yellow Book, Vol. Ill bis, pages 49 to 52) has not been reproduced, as the C.C.I.F. is bringing it up to date. AUDIO FREQUENCIES — OPEN-WIRE LINES 11 7

3° that the conductor resistance and insulation resistance tests should be made regularly and at least each month, by the control stations or repeater stations on either side of frontiers and that the results of these tests should be exchanged between the services interested. Note. — If the circuit is fitted with repeaters, the repeater stations will be main test points. It is recommended that to avoid the use of long leading-in cables permanently in circuit between the route and a main test point, remote testing facilities be used.

Patrolling along international open-wire routes.

T he International C onsultative T elephone C ommittee, Considering, that it is desirable that open-wire international telephone lines should be frequent­ ly patrolled, in order to reduce faults as much as possible and to ensure rapid clearance, Carried unanimously the motion, that, when justified by the importance of the route, arrangements should be made for patrols along the routes as is already done in certain countries.

General conditions to be fulfilled by mixed lines

T he I nternational C onsultative T elephone C ommittee, Considering, that, on mixed lines i.e. lines with open-wire cable sections, it is difficult to have stable and effective operation of the repeaters: that there is always, at the junction of lines having different characteristics, reflection losses which reduce the overall efficiency of the circuit; that the insertion into telephone lines, of sections with different characteristics, even if extremely short (through tunnels, large towns etc.), is in the experience of certain countries represented in the International Consultative Telephone Committee, such as to interfere seriously with the development of long-distance telephony, because of the effect on the operation of repeaters and carrier telephone systems, and that it is therefore necessary to avoid the use of these mixed lines. that, nevertheless, in special cases the use of these mixed lines is unavoidable, and it is necessary to take special precautions. Carried unanimously the motion,

1° that it is desirable to avoid, whenever possible, mixed lines for long-distance international telephony,

2 ° that if it is impossible to avoid mixed lines, efforts should be made to reduce reflection effects in using, for example, suitable continuously-loaded or coil-loaded cable. The C.C.I.F. has set for study the Directives for the construction and loading 1 18 AUDIO FREQUENCIES — CABLES

of the cables inserted into open-wire lines, providing in particular an audio-frequency circuit which effectively transmits the frequency band from 300 to 3400 c/s ; 1 3° that, for a mixed line of any length, the equivalent and attenuation distortion should be as near as possible to those of a homogenous line.

5.2.2. Cables

Notes on the use o f different types o f loading

I. Having regard to the fact that in certain countries an extensive cable net­ work already exists, and to the conditions which must be satisfied when laying new loaded cables, it is not practicable to standardize in detail the characteristics such as loading coil spacing, conductor diameter etc., of cable circuits. The usual loading and corresponding conductor gauges offer sufficient variety to meet all practical conditions as regards spacing of repeater stations. Nevertheless, cases may arise where it is not possible to use these types of loading; care must then be taken to ensure that the general characteristics of the international telephone circuits to be set up conform to those in section 5 .1 above. When it is proposed to increase the gauge of the conductors, the C.C.I.F. points out that it is more difficult to achieve a uniform distribution of line constants with larger conductors. Hence, when it is necessary to increase the diameter of the conductors in order to reduce the attenuation of a particularly long repeater section, it may be necessary to choose a gauge such that the attenuation of the repeater section is less than its normal value. In this way it is possible to use the circuit with lower repeater gains, and to thus obtain the same stability, from the point of view of transmission, as with a more usual type of circuit.

II. To avoid reflection loss and also to ensure satisfactory operation of the repeaters, it is desirable to have uniformity of electrical characteristics throughout the repeater section. The cable pairs between adjacent repeaters in the same country should be absolutely uniform. However, for cable sections crossing a frontier between two countries it is sometimes possible to join two sections of cable made to different specifications; in fact, certain junctions of this sort, already made, are giving good results. Consequently, although uniformity is very desirable for the repeater section of an international cable crossing a frontier, the direct connection of two sections made to different specifications may be permitted, exceptionally, in certain cases and with the reservation that, for the section thus constituted, the conditions for matching of impedances described in the “ Impedance matching ” clause of Specifi­ cation A III below, entitled “ Essential clauses of a model specification for repeater sections of international loaded cables ” are met (see 1°) at the end of section 5.4.1

*) The old directives (Vol. Ill bis, pages 54 and 55) are not reproduced. AUDIO FREQUENCIES — REPEATERS 119 the note entitled: “ Different methods of cooperation between two Administrations or Private Telephone Companies for the construction of a loaded cable repeater section which crosses a frontier ” : 2° the section 5.3.2 below entitled “Repeaters to be inserted between two cables with different characteristics (ordinary telephony ”). The uniformity of repeater sections is of particular importance for circuits with 1.3 and 1.4 millimeter conductors; but, because for these circuits, the total capacity of the loading coil sections are very nearly the same for several common types of loading, there will be adequate uniformity if, in a complete repeater section, only one or other of the two kinds of loading coils is used, keeping respectively the caracteristics of the two types of loading as regards equipment for the cable sections to be connected.

5.3. Repeaters *)

5.3.1. General characteristics of repetears for four-wire circuits

Type. — Repeaters used on four wire circuits should amplify in one direction only and without noticeably affecting the quality of transmission for all frquencies which the pairs associated with the repeaters should transmit effectively, and for the maximum input power met with in practice. Amplification. — The gain of the repeater (or of the repeater with its equalizer) should vary with frequency, so as to adequately compensate for the distortion introduced by the line in the frequency band effectively transmitted (300 to 3400 c/s). Provision should be made for a gain regulating device which should preferably alter the gain in steps not exceeding 0.1 N (1 db). With very long circuits it may be necessary for the steps to be not greater than 0.05 N (0.5 db). The curves representing the gain/frequency characteristics should be parallel for all values of repeater gain, throughout the band of frequencies effectively trans­ mitted by the pair. The repeater should be such that, in the normal state of maintenance of the power supply, variations of the voltage of these supplies and the intensity of the current which they generate, should not produce a variation exceeding 0.03 E (0.26 db) in the gain of the repeater under service conditions. Impedance. — The impedance of the repeater, excluding line transformers, measured at the output and input terminals, and at a frequency of 800 c/s, should, n principle, have the value allowed by the C.C.I.F. for the impedance of international circuits.

*) The recommendation entitled “ Valves for repeaters ” ( Yellow Book, Vol. Ill bis, pages 64 and 65) has not been reproduced, as this is to be brought up to date by the C.C.I.F. 120 AUDIO FREQUENCIES — REPEATERS

The impedance of the repeater, excluding line transformers, will be approxima­ tely equal to that of the pair to which this repeater is to be connected, so that the modulus of the return current coefficient Z — W z + w should be not greater than 0.4 for the input impedance of the repeater, and 0.6 for the output impedance of the repeater, Z being the impedance of the line (including the line transformer), and W the impedance of the repeater; these two limits should not be exceeded at any frequency effectively transmitted. Monitoring. — Arrangements should be made which allow monitoring on circuits with an operator’s telephone in either or both directions, and, when necessary, talking on these circuits. When monitoring, the loss resulting from the connection of the monitoring instrument should not exceed 0.03 N (0.26 db). Crosstalk. — When repeaters are installed side.by side or one above the other, and with the repeaters energized, the results of crosstalk measurements made between the output terminals of the repeaters should be not less than the value corresponding to a crosstalk ratio of 8.5 N (74 db). For these measurements, the repeaters will be connected to impedances Z with a value equal to that of the impedance fixed for the international circuits, and the repeaters will be adjusted to their maximum gain. A suitable measuring device is shewn in figure 25. Non-linear distortion. — The harmonic distortion coefficient should not exceed 5 % for the frequency of 800 c/s and for a maximum power of 50 milliwatts in the case of a four-wire circuit, the supplies to the valves of the repeaters having their normal values and the grids of the valves being always negative with respect to cath­ odes ; the line transformers should be excluded from this measurement and the output of the repeaters during the test should be connected to a non-reactive 600-ohms resistance. (The limit of 5 % is provisional until allowable, limits for the non-linear distortion of a complete circuit have been fixed; this question is being studied by the C.C.I.F.) Further, the C.C.I.F. considers it would be desirable to fix provisionally, as follows, the maximum power measured at the output of a repeater: Repeaters for four-wire telephone circuits in c a b l e ...... 50 milliwatts Repeaters for voice-frequency teleg rap h y ...... 50 milliwatts This conditions has been fixed on the assupmtion that the relative power level, at the output of any repeater, should never exceed + 1.1 N (+ 10 db).

5.3.2. Repeaters to be used between two cables with different characteristics

Generally, these repeaters should fulfil the conditions given above for all four- wire repeaters. AUDIO FREQUENCIES .— REPEATERS 121

There are several methods for meeting these requirements for repeaters between four-wire circuits with different characteristics.

Disturbing Adjustable repeater attenuator

Disturbed repeater

F ig u r e 25

Cable type a Cable type b

AB — > ------A 0 , B — < 3 ------A - — B

F ig u r e 26

a) If the principle is accepted that the repeater should correct the attenuation of the preceding repeater section, throughout the frequency band to be effectively transmitted, strictly speaking one should use repeaters which correct the attenuation distortion in a different way in the two directions of transmission. The gain frequency characteristic of the repeater A ► B (see figure 26) should correspond to the attenuation frequency characteristic of cable type a, when on the .input side, it is terminated by Za , which is the impedance of cable a, and on the output side by Zb , which is the impedance of cable b. For the repeater B > A, it is exactly opposit. The application of this principle involves, when there are various types of cables in a country, the use of a relatively large number of different types of repeaters which should be specially designed for this purpose: b) If, for repeater A > B, a normal repeater of the type which is used for a network constructed entirely of cable type a, and similarly, for repeater B -----> A, a normal repeater suitable for cable b is used, the normal gain frequency charac­ teristic will not be obtained because, for example, the output of repeater A > B is terminated, by the impedance of cable type b, instead of by cable type a, and this produces an additional attenuation: Let: Za = the impedance of cable type a, Zb = the impedance of cable type b, Zv = the impedance of the repeater: 122 AUDIO FREQUENCIES — REPEATERS

k , = y - k 2 = -

The additional attenuation is:

K 2 (1 + K ,)2 a =y2 loge Kj. (1 + K2)2 This additional attenuation in practice is only a few hundredths of a neper and can therefore be ignored. There is thus need for a much smaller number of different types of repeater than the number indicated under a). c)- If the transmission characteristics of the existing types of cable are similar with respect to impedance and attenuation, it is not necessary to have a different correction for the two directions of transmission. In this case, it is practical to establish, from different attenuation curves of the existing types of cable, an average curve to which repeaters of the two directions should be designed. Because their attenuation frequency curves are different, it is necessary to distinguish between the circuits with medium loading and circuits with light loading. It is thus possible to reduce to two the number of “ junction ” type repeaters to be used. d) Another method is to use repeaters with an adjustable equalizer, which allows the use for each direction of transmission, of the most suitable gain frequency characteristic.

5.4. Specifications recommended by the International Telephone Consultative Committee, for audio-frequency cable circuits

I ntroduction

The International Telephone Consultative Committee recommends Admin­ istrations and Private Operating Companies to use the following specifications for the supply of audio-frequency telecommunication cables liable to be used in the international service (and also auxiliary equipment: loading coils, repeaters etc.). In these specifications, only the clauses relating to four-wire circuits apply to international telephone circuits on loaded cables. The complete specification applies to national trunk/toll circuits liable to enter into an international call, these being either two-wire or four-wire circuits. T h e International T e l e ph o n e C onsultative C om m ittee considers that the efficiency of such a-long distance cable (with several repeater stations), depends to a large extent on constructional details and the general arrangement of the system. An Administration or Private Operating Company might prefer to place with a firm, a contract for a complete system (cable and repeater) instead of having their own specialist engineers to assume the responsibility which results from acceptance SPECIFICATIONS — LOADED CABLES 123 of the parts of the system separately. The C.C.I.F. proposes to recommend to such an Administration or Private Operating Company to obtain from the contractor detailed information regarding the project and to obtain all the necessary guarantees to ensure strict adherence to the attached outline specification. The Administration or Private Operating Company should give to the Contractor information regarding the circuits necessary and estimated traffic. Certain Administrations and Private Operating Companies include in their specifications for underground cables details of methods of manufacture, laying etc. These are not included in the following specifications, because the International Telephone Consultative Committee considers it is better to leave the different Admin­ istrations and Private Operating Companies free to choose their own methods for manufacture and laying of cables, with the reservation that the complete circuits should satisfy the general conditions shewn in sections 1 . 1 . 1 and 1 . 1 . 2 above. (See also the Note entitled “ Different methods of cooperation between two Administrations or Private Operating Companies for the construction of a loaded cable section crossing a frontier ”).

5.4.1. Repeater section of a cable and its constituent parts

Specification A. I

Essential clauses o f a model specification of general application for the supply offactory lengths of loaded telecommunications cable. General. — The present specification lays down the electrical conditions to be met by factory lengths of loaded, paper-insulated, air-spaced, lead-covered telephone cable, for long-distance communications. These conditions are specified to ensure that cables will allow: 1° The use, if necessary, of phantom circuits; 2° The loading of the side circuits and if necessary of the phantom circuits; 3° The obtaining of satisfactory long-distance repeated circuits; 4° The obtaining of satisfactory long-distance programme circuits using screened circuits.

These conditions do not apply: 1° To cables with conductors having a diameter less than 0.8 mm (appro­ ximately 16 lb. per mile); 2° To groups of conductors having the same gauge but with less than twenty pairs; 3° When the total number of pairs is such that the arrangement of the conduc­ tors in the cable is necessarily unsymetrical, or such that the conductors of different gauge are placed in the same layer. Certain essential conditions relating to raw materials are specified. 124 SPECIFICATIONS — LOADED CABLES

Raw materials Copper Conductors. — Each conductor will consist of a wire of pure copper’ annealed, uniformly drawn, cylindrical, and of uniform quality and resistance, without cracks or other faults, having a conductivity at least equal to that which has been specified by the International Electrotechnical Commission, (Berlin, 1913) i.e. l/58th of an ohm per meter for standard annealed copper wire, having a cross section of one square mm at a temperature of 20° C. To correct for temperature, the temperature coefficient specified by the same Commission will be allowed; i.e. 0.00393 per degree C. for the temperature coefficient at a constant mass of standard annealed copper, at a temperature of 20°C. The diameter of the wires should not vary by more than =t 1.5 % of the nominal diameter. Joints made during manufacture.— When it is necessary to joint conductors during manufacture, a method fulfilling the following conditions will be used: The tensile strength of a length of conductor including a joint will be at least equal to 90% of that of an adjacent length, similar but without a joint. The resistance of a length of conductor not exceeding 15 centimeters in length (approximately six inches), and including a joint, should not exceed by more than 5 %, the resistance of an adjacent length, similar but without a joint. Twisted joints are not to be used. A flux containing acid may not be used. Insulating paper. — Insulating paper will be homogenous, of uniform thickness, with long fibres and as far as possible free from metallic particles; it should be prac­ tically free from resinous materials and from acid or alkaline substances, and should not contain substances which might have a harmful effect on the conductors or lead sheath. The insulating paper on the conductors should not tear when these conductors, taken from the finished cable are wrapped round a cylinder (or toroid), having a radius of curvature of 15 mm. (see the explanatory sketch of figure 27). A sample of the paper taken from the finished cable and having been exposed for two hours to an atmosphere of 65 % humidity should have a tensile strength at least equal to the weight of 5 kilometers (3.1 miles) of paper of the same type and dimensions. Materials used for the sheath and for the armouring o f cables. — The conditions to be met by these materials are specified separately for each cable. Pressure Tests. — The soundness of the lead sheath should be verified by pressure tests rather than immersion tests.

Electrical characteristics Resistance o f the conductors. — The resistance of any conductor in a factory length, measured with direct current, should not exceed by more than 4 % the value calculated for a rectilinear conductor having the nominal diameter of the conductor considered. SPECIFICATIONS — LOADED CABLES 1 2 5

Insulated wire wrapped round the ring

Interior diameter of the ring, slightly greater than that of the inner diameter of the sheath

F ig u r e 27. — Explanatory sketch

The average resistance for all the wires of a group of conductors of a given gauge should not exceed the nominal value defined above by more than 1 %. An allowance for lay will be made as follows:

Diameter (in millimeters) of the outside layer fromed by conductors of theTguage considered Allowance for lay Under 30 ...... 1.0% 30 to 4 0 ...... 1.6% ' 40 to 5 0 ...... 2.5% 50 to 6 0 ...... 3.7% 60 to 7 0 ...... 5.0% 70 to 80 ...... 7.0%

The resistance unbalance of two conductors of a same pair taken in any length of cable, measured with direct current, should not exceed by more than 1 % the loop resistance-of this pair. 1 26 SPECIFICATIONS — LOADED CABLES

The resistance unbalance of the two pairs of the same phantom group, the conductors of each pair being joined in parallel, in any length of cable, measured with direct current, should not exceed by more than 2 %, the loop resistance of the two pairs, the conductors of each pair being joined in parallel. Insulation resistance. — In a length of cable, the insulation resistance measured between a conductor and all other the conductors connected to the sheath and earth, should not be less than 1 0 , 0 0 0 megohms per kilometer (approximately 6200 megohms per mile), the potential difference used being from 100 volts to 600 volts. The reading should be made after electrification for one minute, the temperature being at least equal to 15° C (approximately 60° F.) Dielectric strength. — If specially asked for, the cables should be constructed in such a way that the insulation of any length of cable is capable of withstanding a potential difference, specified in each particular case but not exceeding 2 0 0 0 volts r.m.s., applied for at least 2 seconds between all the conductors (bunched) and the earthed sheath. The test will be made with alternating current of 50 c/s. The test voltage should not exceed by more than 10% the maximum value of a sinusoidal voltage having the same r.m.s. value. The test could also be made with direct current (see annex 20 of the Book of Annexes of Vol. Ill of the Green Book entitled “ dielectric strength tests ”). Effective capacity measured with alternating current. — The effective capacity of a pair is the capacity measured between the two conductors of this pair, all the other conductors being connected to the lead sheath; the nominal value of this capacity for each cable will be specified. The effective capacity of the phantom circuit of a phantom group, is the capacity measured between the two pairs of this phantom group, each pair being short cir­ cuited, all the other conductors of the cable being connected to the lead sheath. By definition, the nominal capacity of the phantom circuit is equal to 1.6 times the nominal capacity of the pair (multiple twin or Dieselhorst-Martin cables). The test will be made with alternating current at room temperature. No temperature correction will be applied. In case of dispute, the results obtained with a alternating current having a frequency of 800 c/s and a temperature of at least 15° C (approximately 60° F) will be considered as final. In each length of cable, the average capacity of all the pairs of the same gauge will be in accordance with the conditions imposed by the Administrations and Private Operating Companies concerned: a tolerance will however be allowed on each factory length. For programme circuits, a tolerance of effective capacity of ± 12% from the nominal value will be allowed. In each factory length, the average of the effective capacities of the phantom circuits of each group of the same gauge will not differ by more than ± 5 % from the value obtained by multiplying the average value of the capacities of the pairs of this group by 1 .6 . SPECIFICATIONS — LOADED CABLES 1 27

This factor is valid for multiple twin cables. For star quad cables, this factor is higher; its value should be specially fixed in each particular case. The effective capacity of each side circuit and of each ,phantom circuit will be measured on at least 1 0 % of the factory lengths. The “ difference of capacity ” of a side circuit or of a phantom circuit in any length of cable should not exceed the following values in a group of conductors of the same guage: Average . Maximum “ Difference of capacity ” means the difference between, the capacity of any circuit of a group and the average capacity of all similar circuits of this group, in the same factory length. This difference is expressed as a percentage of the average value. , Note. — For each group of side and phantom circuits in the cable, the average capacity of the different loading sections within the same repeater section, should not differ by more than ± 2 % from the average value of the capacity for the group of circuits considered, for the whole of the loading sections. In the case where such a degree of regularity could not be obtained directly by factory methods, if there are differences higher than the limits indicated, it is recommended that factory lengths should be allocated, in each section within the same repeater .section, so that the capacities of the different loading sections satisfy the condition above. Leakance. — The average leakance of the side and phantom circuits will be measured on a small percentage of the factory lengths, with an alternating current having a frequency of 800 c/s. v " The average leakage constant for each type of circuit and for factory length tested, should not exceed 25. This constant is equal to the ratio of the average leakance at the average capacity measured with alternating current. Its value could also be expressed by the ratio G leakance co C capacitance which should not exceed 0.005. Capacity Unbalance. — The capacity unbalances measured with alternating current at a frequency of about 800 c/s. on cable lengths of 230 meters (approximately 750 feet), should not exceed the values shewn below, each group of circuits of a same gauge being considered separately. These values are suitable for pairs in loaded cables to be used at audio frequencies. 128 SPECIFICATIONS — LOADED CABLES

Allowable limits for capacity unbalances (in [jljjlF for strengths of 230 meters)

I. Ordinary telephone circuits Av. Max. side-side . . . 40 150 phantom-side 75 375 In all quads side-earth . . 150 600 phantom-earth 300 1200 Capacity unbalances between circuits side circuit . . 150 600 in the outer layer and earth phantom circuit 300 1200 Between circuits situated in adjacent pair-pair *) . 40 170 quads in a same layer or between phantom-pair *) 40 170 a quad in the first layer and a phantom-phantom 40 170 quad: in the centre ...... Between four-wire go and return pairs, (This does not apply to adjacent quads)' ...... ». 20 II. Programme Circuits i • * sid e-sid e...... 150 a) Within-a screened quad (multiple phantom-side ...... 375 twin or star quad): ...... side-earthed screen ...... 600 phantom-earthed screen *) 1200 [3) Screened pairs . pair-earthed screen ...... 600 pair-earth ; ...... 600 y) Unscreened pairs pair-pair in adjacent quads . . 170 pair-phantom in adjacent quads 170

The measurement of the above unbalances involves a large number of tests and it is desirable to limit them. This limiting may best be done by making tests in the cases marked with an asterisk * in the table above, only on 2 % of the factory lengths of an order, with a minimum of two lengths. In exceptional cases, Admin­ istrations and Private Operating Companies concerned could request tests on a greater number of lengths; the factory lengths on which these measurements are made will be selected by Administrations and Private Operating Companies. If a length does not comply with the above specification for unbalance, this length will be rejected; the Administration or Private Operating Company could request measurements on the other factory lengths. In the case of cables intended to be worked four-wire, average capacity un­ balances between “ go ” and return pairs will be measured on one or more factory lengths, and the average unbalance for a factory length of 230 meters should not exceed three [jljjiF. In each factory length of cable, for a length other than 230 meters, the capacity unbalances, measured with alternating current for the various diameters of conduc­ tors, should not exceed the values obtained by applying the following rules: SPECIFICATIONS — LOADED CABLES 1 2 9

a) Between circuits: s id e -s id e ...... p a ir - p a ir ...... average values phantom-pair...... phantom-phantom ...... multiply the'values given in the above table by the square root of the ratio of the length to 230 meters. b) Between circuits: s id e -s id e ...... p a ir - p a ir ...... maximum values phantom-pair...... phantom-phantom ...... phantom-side...... average and side-earth...... maximum phantom-earth...... values multiply the values given in the above table by the ratio of the length to 230 meters. This rule does not apply to lenths of less than 100 meters (approximately 110 .yards), to which the values for factory lengths of 1 0 0 meters, calculated by the above rules, will apply. Notes. — 1. Definitions of capacity unbalances are given below. 2. When sheath capacity unbalances for pairs and phantoms in the outer layer are specified, it is not necessary to specify for these circuits limits for capacity unbalances to earth.

Definitions o f unbalances

If, in a short section of telephone pair (e.g. in a factory length of cable), the electrical characteristics of this part, (between wires or between each wire and earth) are irregular, there is said to be an unbalance at this point. Thus, there can be resistance unbalances, leakage unbalances, self or mutual inductance unbalances, and capacity unbalances. In cable pairs, it is the capacity unbalances which are important as regards crosstalk and inducted noise. Telephone cable conductors are laid up to form quads as shewn in figure 28.- With different conductors of one or several quads, side and phantom circuits (simple and double) are formed, using the arrangements shewn diagramatically in figure 29 opposite. The different capacity unbalances of importance in a factory length of cable are defined as follows:— The capacity unbalance of a side circuit of a quad, with respect to the other side of the same quad, is the value of the capacity which, when connected between a wire of a pair and a wire of the other pair of this quad, corrects the unbalance. The capacity unbalance of a phantom circuit, with respect to any one of the side circuits of the same quad, is the value of the capacity which when connected between a wire of this pair and the two short-circuited wires of the other pair of this quad, corrects the unbalance. SPECIFICATIONS — LOADED CABLES

Q u o u tte I)-M ( Oil ^kese^m sl-M adiii iw u t)

Qucude U)oi& (/Uaj\cm cbd) 2 u c u tte , (Quad) & (Jai/ies ttiM ees jevi U tile (cyucud fixur)

\

F ig u r e 28 CIRCUIT REEL OU < CIRCUIT COM BIN AN T [(SIDE ClFfcUlT)

1° ( CIRCUIT FANTOME SIMPLE OU PCFCTOS LAE CABLES LOADED —SPECIFICATIONS CIRCUIT COMBINE> SIMPLE (PHANTOM CIRCUIT) go

CIRCUIT FANTOME CIRCUIT FANTOME OU DOUBLE OU CIRCUIT COMBINE / CIRCUIT C O M B IN E ym * PHANTOM CIRCUIT) l (DOU0^ NroM )

3° CIRCUIT FANTOME QUADRUPLE OU CIRCUIT COMBINE >■! ^QUADRUPLEQUADRUPLE PHANTOM)

F ig u r e 29 1 3 2 ' SPECIFICATIONS — LOADED CABLES

The capacity unbalance of a pair, with respect to a pair of a different quad, is the value of the capacity which, when connected between a wire of one pair and a wire of the other pair, corrects this unbalance. The capacity unbalance of a phantom circuit, with respect to either of the pairs of another quad, is the value of the capacity which, when connected between one of the pairs of the phantom circuit and of the wires of the pair in question, correct this unbalance. The capacity unbalance between two phantom circuits is the value of the capa­ city which when connected between a pair of the first phantom circuit and a pair of the second phantom circuit, corrects this unbalance. The capacity unbalance of a side circuit with respect to earth, is the value of the capacity which, when connected between one of the wires of the pair and all the others conductors of the cable joined to the earthed sheath corrects this unbalance, the two other wires of the quad being joined to the middle point of the proportion arms. The capacity unbalance of a phantom circuit, with respect to earth, is the value of the capacity which, when connected between one of the two pairs of the phantom circuit and all the conductors of all the other quads of the cable joined to the earthed sheath, corrects this unbalance. The sheath unbalance of a side circuit (or pair) in a quad in the outer layer is the value of the capacity which when connected between one of the wires of the pair and the earthed sheath, corrects this unbalance, all conductors in other pairs being at the same potential as the two conductors of the circuit under test. The sheath unbalance of a phantom circuit formed by a quad in the outer layer is the value of the capacity which, when connected between one of the pairs of the phantom circuit and the sheath, corrects this unbalance, the conductors of all quads in the cable being connected together at the same potential as the wires of the quad under test.

Specification A. II

Essential clauses of a model specification of general application for the supply of loading coils for loaded telecommunication cables. The loading coils should be suitable for loading two side circuits and the phan­ tom circuit. The coils used will be assembled so as to form a loading unit such that the introduction of this unit into a quad loads the two side circuits and the phantom circuit. The electrical conditions specified below apply to the side and phantom circuits of such a loading unit. Magnetic materials used will be of the compressed powdered iron type, or of other material having equally satisfactory characteristics. Loading cases. — The coils will be assembled in suitable protective cases, which will be hermetically sealed. The cases should be watertight and be able to be buried in humid soil without deteriorating. SPECIFICATIONS — LOADED CABLES 133

Suitable arrangements for connecting the cases to the main cable will be provided.

Magnetic stability. — The magnetic stability of the material of which the cores are made should be such that the change in the inductance of a coil is not more than ± 2 % after passing through the winding corresponding to one wire, of a direct current with a value from zero to two amperes. The maximum direct current should be applied for about five seconds. The measurement will be made five minutes after the current ceases to be applied. This test spoils the coils and should only be applied to samples.

Inductance. — The inductance measured with a current of one milliampere, at a frequency of 1800 c/s, should be equal to the nominal value with the following tolerances: 1) For coils with an inductance not less than 22 millihenrys ± 1-5% 2) For coils with an inductance not less than 10 millihenrys and less than 2 2 millihenrys ...... ± 2 .0 % 3) For coils of an inductance less than 10 millihenrys . . , . ± 3 .0 %

Resistance. — The difference between the effective resistance and the direct current resistance of the loading coils for side and phantom circuits, measured on a loading unit, should not exceed the following value, in order to avoid excessive attenuation distortion: 1) For telephone circuits used only for voice frequencies 125 ohms per henry 2) For old-type programme circuits * 180 ohms per henry

Effective resistance. — The effective resistance of the loading coils for side and phantom circuits, measured as'a loading unit, with a current of 1 milliampere, should .not exceed the following value: 1) For telephone circuits used only to transmit audio frequencies, (ordinary telephony): 200 ohms per henry at a frequency of 3400 c/s 2) Old type music circuits: 300 ohms per henry at a frequency of 6400 c/s The additional resistance due to hysteresis h, measured at 800 c/s and expressed in ohms per milliampere per henry, should not exceed the values shewn below: 1) For two-wire circuits not exceeding 200 kilometers: h = 24 {/ L ohms/mA x H;

*) Circuits specially set up for programme transmissions and which meet the requirements of section 1 “ Old-type transmission Circuits ”, of the 3rd part of this work. 1 3 4 SPECIFICATIONS — LOADED CABLES

2) For two-wire circuits, 200 kilometers or more, as well as four-wire circuits used for audio frequency transmission only:

h = 12 \/ L ohms inA x H;

3) For old type music circuits *):

h — 6 \/ L ohms/mA x H. In these formulae, L is the inductance of the coil, expressed in henrys.

Crosstalk. — The crosstalk for the loading coils in cases, will be measured with alternating current of 800 c/s and with a current of not less than five milli- amperes. This test will be made under the following conditions: The terminals of the loading coils will be closed with non-reactive resistances approximately equal to the characteristic impedance of the circuit in which these coils are to be connected. The crosstalk attenuation between the different circuits in the same loading coil case should not be less than the values shewn below: Side-side in the same loading unit Side-phantom in the same loading unit Side-side in different loading units 10 N (87 db) Side-phantom in different loading units phantom-phantom in different loading units

Insulation resistance. — Insulation resistance measured between any winding of a loading unit and all the other line windings (in the same loading unit and in all the other loading units), and the case, will not be less than 15,000 megohms. This test will be made with a potential difference not less than 1 0 0 volts and not greater than 500 volts, the temperature being not less than 15° C (approximately 60° F).

Dielectric strength. — The insulation between any two line windings will be capable of withstanding a potential difference with an r.m.s. value of 500 volts. This test will be made with alternating current at a frequency not less than 50 c/s, the potential difference being applied suddenly. Further, the insulation between any line winding and the case will be capable of withstanding a potential difference of 2 , 0 0 0 volts r.m.s. applied for two minutes. The maximum value of the test voltage should never exceed by more than 10% the maximum value of a sinusoidal voltage'having the same r.m.s. value.

Capacity unbalance to earth. — The difference of capacity to earth of the loading coils of two pairs of the same quad should not exceed a value fixed provisionally at 1 0 0 fx(i.F.

*) Programme circuits which meet the requirements of section 1 “ Old type Programme Circuits ”, of the 3rd part of this work. SPECIFICATIONS — LOADED CABLES 13 5

Inductance unbalance. — The difference in inductance of the loading coils of the two pairs of the same quad — measured on the phantom circuit — should not, provisionally, exceed 0.25 % of the inductance of the phantom circuit.

Resistance unbalance. — The difference of resistance, of the loading coils of the two pairs of the same quad, measured with direct current and measured on the phantom circuit,, should not, provisionally, exceed 0 . 2 0 ohms.

Specification A. Ill

Essential clauses of a model specification for repeater sections of loaded telecommu- cation tables

General. — The present specification lays down the principal electrical condi­ tions which the repeater sections of the loaded cables should meet after laying, assuming that the cable lengths and loading coils meet the appropriate specifications. The clauses of this specification have been revised to permit the use of phantom circuits in these cables and obtain good-quality transmission on long-distance commu­ nication circuits with two or four-wire repeaters.. The clauses below apply equally to four-wire and two-wire circuits, excepting where shewn in the text.

Resistance unbalance. — In any cable section between repeaters, the difference between the resistance of two conductors of any pair, measured with direct current, should not exceed three ohms for conductors having a diameter not exceeding one millimeter, and two ohms for larger conductors. The difference between the resistances of the two pairs of any quad, measured with direct current, should not exceed three ohms for conductors having a diameter not exceeding one millimeter or two ohms for larger conductors. The difference between the resistances of two pairs of any quad, the conductors of each pair being in parallel, should not exceed four ohms for the conductors having a diameter not exceeding one millimeter, or three ohms for larger conductors.

Insulation resistance. — The insulation resistance measured at the end of the cable (excluding internal cabling) and between any conductor and all the other conductors connected to the earthed sheath, should not be less than 1 0 , 0 0 0 megohms per kilometer, this insulation resistance being measured with a potential difference not less than 300 volts and not more than 600 volts, the readings being made after electrification for one minute at a temperature of 15° C.

Uniformity of the average capacities of the loading sections. — For each group of side and phantom circuits of the cable, the average capacities of the different loading sections within the same repeater section should not differ by more than 2 % from the average value of the capacity measured for the group of circuits considered, for all of the loading sections. 1 3 6 SPECIFICATIONS — LOADED CABLES

Impedance regularity. — The relation between the impedance Zct of any side or phantom circuit and the impedance. Zeq of the corresponding balance, calculated from measurements of the average constant of the circuit should meet the following conditions after measurement of the real and imaginery parts of the impedance Zct and Zeq; the differences of these real and imaginary parts are expressed as a percentage dT and dx of the impedance Zeq of the balance. If dr and dx are plotted as carte­ sian coordinates of a point, this point should, for all circuits and for any frequency in the frequency band to be transmitted effectively, be situated within a circle having a radius of 9 %. Also points corresponding to 90 % of the circuits, and at all the preceding frequencies should be situated within a circle having a radius of 7%. The values of the regularity return loss corresponding respectively to the values above, 9 % and 7 % are 3.1 N. (27 db) and 3.4 N (30 db).

Crosstalk. — The table below shews the maximum allowable crosstalk between any circuits in accordance with the above specifications, measured on a repeater section. These values are those which should be measured at the ends of the cable and only include crosstalk due to the cable and loading coils. These numbers do not cover crosstalk arising from terminal'transformers, protectors, repeater station cabling or equipment or from the cable terminating rack. The crosstalk will be measured by voice tests or by using one of the objective methods of measuring with alternating current recommended by the C.C.I.F. (see Booke of Annexes to Vol. Ill of the Green Book). For these measurements the disturbing and disturbed circuits will be terminated with impedances equal respectively to the image impedances of the disturbed and disturbing circuit. The following minimum figures are provisionally recommended for far-end and near-end crosstalk: Near-end or far-end crosstalk between two-wire circuits in the same quad or in different quads: 7 N (61 db). Near-end crosstalk between four-wire pairs in quads transmitting in opposite directions: 7.5 N (65 db). Near-end or far-end crosstalk between a two-wire circuit and a four-wire pair, or between a two-wire circuit and an unscreened programme circuit:

a) With the two-wire circuit being the disturbing circuit, 7.5 N (65 db);

b) With the two-wire circuit being the disturbed circuit: 7.0 N (61 db). Far-end crosstalk between four-wire pairs transmitting in the same direction: 7.5 N (65 db). SPECIFICATIONS — LOADED CABLES 13 7

Near-end or far-end crosstalk between an unscreened programme circuit and a four-wire pair: 7.5 N (65 db). Near-end or far-end crosstalk between a screened programme circuit and any telephone pair: a) With the telephone pair being the disturbing pair; 9.5 N (82 db); b) With the telephone pair being the disturbed pair: 7.5 N (65 db). Near-end or far-end crosstalk between screened programme circuits: 9.5 N (82 db).

Notes. — 1) Loading spacing. — The nominal loading coil spacing in a repeater section will be equal to the theoretical value within ± 2%. The actual loading coil spacing measured along a repeater section may differ by 1 0 meters from the nominal spacing.

2) Cut-offfrequency. — The cut-off frequency of the different systems of loading applied to side and phantom circuits will be determined from the following formula:

1 ~ 7T [/L C where f0 = the cut-off frequency in cycles per second: L = the loading coil inductance in henrys: G = the effective capacity of the cable pair between loading coils, in farads. The above formula being intended for the classification of different types of loading for administrative purposes, there is little advantage in complicating this formula by making it more precise by taking into account the inductance of the cable pair between loading coils. When greater accuracy is needed, it is necessary to determine by precise calcula­ tions, the “ attenuation-frequency ” and “ impedance-frequency ” characteristics allowing for all the parameters of the cable and loading coils. The first paragraph of annex 21 of the Book of Annexes to Volume III of the Green Book' gives some information on this question; in the continuation of this Annex a simple formula is given for determinating the cut-off frequency of a loaded cable which is somewhat more accurate than the formula

1 'f° ~ 7i | / l c to be used where greater accuracy is necessary.

3) Velocity of propagation. — The nominal velocity is calculated by the formula: 138 SPECIFICATIONS — LOADED CABLES

v “ |/ E ? v = nominal velocity in kilometers per second; s — loading spacing in kilometers; L and C being the constants as defined aboce.

4) Characteristic impedance. — The characteristic impedance of the side and phantom pairs is calculated from the following formula:

where Zc = characteristic impedance (L and C being the constants defined above). 5) Attenuation constant. — The attenuation constant will be deduced from measurements made on a complete repeater section. If the repeater is not exactly in the middle of a loading section, the pair will be built out to a half section.

REMARK

Different methods of cooperation between two Administrations or Private Operating Companies for the construction of a loaded cable section crossing a frontier

Given the difficulties which arise from the existence in certain countries, of large networks and conditions which have necessarily to be met in the establishment of new cable projects, it is not practicable to standardize detailed characteristics of the make-up of cable circuits such as: loading coil spacing, conductor diameter, pairs etc. (See Specification A. Ill below). Nevertheless, care must be taken to ensure that the general characteristics of the inter­ national telephone circuits to be set up, should be as indicated above. Where it is proposed to increase the gauge of the conductors, the C.C.I.F. points out that it is more difficult to achieve a uniform distribution of the line constants with heavy conductors. Thus, when it is necessary to increase the diameter of the conductors, in order to reduce the attenuation of a particularly long repeater section, it may be necessary to select a gauge such that the attenuation of the repeater section is less than its nominal value. In this way it is possible to use the circuit with the lower gains and obtain the same stability from the point of view of transmission as for a more usual type of circuit. To ensure the satisfactory operation of the repeaters as well as to avoid reflection losses, it is desirable to have a uniform distribution of electrical constants throughout the repeater section. The cable lines should be absolutely between adjacent repeaters situated within the same country: and the construction of the cables should be such that the circuits are perfectly balanced and the electrical constants of the circuits uniformly distributed. It is mentioned in Specification A. Ill above that if the uniformity of the factory lengths is not sufficient to give adequate regularity in the loading sections of a repeater section, this regularity must be achieved as far as possible by a suitable allocation of factory lengths within the loading section. Precise rules have been given in this Specification A. Ill on the question of loading coil spacing. If, because of local conditions, the loading coil spacing is not uniform, special arrangements should be made. SPECIFICATIONS — LOADED CABLES 1 3 9

For cable sections crossing a frontier, it may be necessary in certain cases to connect two cable sections not having the same specification; in fact, certain connections of the kind already carried out are giving good results. Consequently, although uniformity is very desirable for the repeater section of an inter­ national cable crossing a frontier, the direct connection of two sections made lo different specifications may be permitted exceptionally, in certain cases and with the reservation that, for the section thus constituted, the conditions for matching of impedances described in the clause “ Impedance matching ” of Specification A. Ill below, entitled ‘ Essential clauses of a model specification for repeater sections of international loaded cables (see section 5.3.2 above entitled Repeaters to be inserted between two cables with different characteristics ”. The uniformity of repeater sections is of particular importance for circuits with 1.3 and 1.4 millimeter conductors; but because, for these circuits, the total capacity of the loading coils sections is very nearly the same for several common types of loading, there will be adequate uniformity if, in a complete repeater section, only one or other of the two kinds of loading coils is used, keeping respectively the characteristics of the two types of loading as regards the equipment for the cable sections to be connected. If it should happen that different types of loading are used by each of the countries concerned with a repeater section of an' international loaded cable crossing a frontier, or that the methods of balancing are different (balancing by crosses or by condensors), two cases have to be considered: (a) where two countries have adopted the same type of loading for their national network; (b) where two countries have adopted different types of loading for their national network:

In case (a), the most satisfactory procedure technically and economically is to obtain the whole of the repeater section from the same contractor: it is always necessary to do this when there is less than a quarter of the length of the section in one of the countries, and this method is strongly to be recommended for all cases. An agreement could however be made if necessary, so that the contract concluded by each country with the single supplier should be on the same technical basis. In countries where the manufacture of the telephone networks by the national industry is considered as being of the first importance, this has the disadvantage that one of the coun­ tries has to accept that part of the cable on its territory should be of foreign origin. This disadvantage however, seems of little importance because there are probably several •cables connecting the two countries who could take it in turns to supply. Such arrangements for the supply of cable sections are already in current practice when submarine cables are laid between two countries. The advantages of this method are:— 1° A better quality of transmission for the whole cable, because the different parts of this are more uniform; 2° A greater economy in manufacture; 3° The contractor will give to the two Administrations or Private Operating .Com­ panies guarantees on the characteristics of the whole section. If the two countries do not follow the above recommendations, and each construct the cable up to the frontier, it is then necessary to take special measures to ensure uniformity of construction; in particular the capacity and inductance standards used for measurements in the cable and loading coil factories must be compared, and above all, uniform spacing of the loading coils must be preserved. When this is done,, each contractor will naturally only guarantee the portion he has supplied, and the admissible variations of the electrical characteristics can give rise to a slight irregularity at the junction point. The magnitude of this irregularity will be variable and due mainly to differences in machines, to the extent of cooperation between manufacturers and to the position of the joint with respect to the ends of the repeater section. 140, SPECIFICATIONS — LOADED CABLES

In case (b) the use of the same method of loading for the whole section must be re­ commended. The best solution is, as has been said above, to obtain the whole of the repe­ ater section from the same contractor. If however, this solution is not possible on the grounds of national economy, the country with the shorter length of repeater section should adopt, exceptionally, the essential characteristics allowed in the other country for the pairs concerned; diameter of wires, capacity per kilometer, inductance of loading coils. If necessary however, it is in order for four-wire pairs to have ditferent gauge conductors on either side of the frontier. Where two countries have different types of loading for their national networks, and where each country constructs its portion of the repeater section to a common specification, this will cause one country to introduce in its network a portion of repeater section differing from its normal type. The resulting maintenance and construction difficulties are however not serious. The manufacturers are sufficiently masters of their technique not to find serious difficulty in making a cable having a capacity differing a little from the standardized capacity, and coils with an inductance differing from normal values. As far as maintenance is concerned, it should be noted that a minimum reserve of cable and loading coils generally permits maintenance over a period of years. Finally, it should be noted that in the case of urgent necessity, it is possible to repair a cable with a short length of cable of a different type and a loading coil case may be removed temporarily. In all cases, it is desirable that in each country, the last joint before the frontier should be in a manhole with easy access to allow localization of a fault to one or other of the coun­ tries. The use of non-loaded cable sections may offer technical and economic advantages over loaded cables in certain special cases, e.g. for certain submarine cables; but non-loaded cable sections have only been used up to the present in international audio telephony on. a limited scale; in the absence of experience it is not possible to recommend at present the general use of non-loaded cables for the international telephone service. Nevertheless, in certain special cases, each of which should be specially studied, (for example, when it is a question of submarine cables), it is possible to use, for international telephony sections Of non-loaded cable, providing the international circuits to be set up have general charac­ teristics in accordance with the recommendations above.

5.4.2. Terminal equipments and intermediate repeaters

Specification B. I

Essential clauses of a specification for the supply of line transformers

This specifications covers line transformers to be used on audio-frequency repeated circuits. General clauses

1°) The transformer, ratio should be such that, when the line transformer is connected to circuits for which it has been designed, the impedance measured at the ends of the office windings should be in accordance with the recommendation of the C.C.I.F. entitled “ Impedance of international and trunk/toll circuits ” (see Section 1.2.7 above); '• 2°) The composite attenuation of a normal line transformer (a transformer with a turn ratio less than 3), measured with a power of from 1 to 50 milliwatts, SPECIFICATIONS — LINE TRANSFORMERS 141 should not exceed 0.08 N (0.7 db) for any frequency of the audio-frequency band effectively transmitted. 3°) The transformers should be balanced as regards inductance, resistance and capacity, so that the crosstalk attenuation between the windings of side circuits and phantom circuits should be greater than 9.0 N (78 db) when these windings are terminated with balanced non-inductive resistances, representing the lines. The arrangement of the different line transformers on the same rack should be such that the crosstalk attenuation between transformers of the different side and phantom combinations should be greater than 12 N (104 db). 4°) The insulation resistance between any two windings or between any winding and the case (or screen if used) should not be less than 500 megohms when measured with 100 volts direct current. The insulation between the line winding and the office winding and also between any winding and case (or screen if used), should withstand 500 volts alternating current 50 c/s. Where there is parallelism between telephone lines and high-voltage power lines, the insulation between the two windings and between the line winding and case (or screen) alternating current — 50 c/s. The insulation between the office winding and the. case (or screen) should withstand 500 volts r.m.s. alternating current— 50 c/s. 5°) The properties of the line transformers should not be changed appreciably by direct currents or alternating currents met with in practice.

Special clauses

6 °) Transformers intended to be used in the line and balance circuits of a two-wire repeater In two-wire repeaters, when a transformer is included on the line side and another transformer on the balance side, these two transformers should be selected in pairs so that the impedances Zx and Z 2 of the windings measured at the repeater terminals at any frequency within the frequency band effectively transmitted, satisfy the con­ dition :

Z. — Zo Z, + z2 < 0,02 under all conditions of current temperature etc., met with in practice. When making the above measurements, the line and balance are replaced by resistances equal to the modulus of their nominal impedance. 1 4 2 SPECIFICATIONS — LINE TRANSFORMERS

7°) Transformers designed to transmit 16 to 50 c/s signalling current. For these frequencies, a line transformer is characterized by its power ratio, which may be measured by using one of the methods described in the Book of Annexes to Vol. Ill of the Green Book 2nd part, section 1.1.3. To compare transformers the power ratio is measured with the “ line ” side of the transformer closed with pure resistance of 2000 ohms, by applying 45 ± 3 volts to the “ office ” side of the transformer. The power ratio should not be less than 55 %. In the above recommendation, it is assumed that the transformer ratio is at 1:1. If the transformation ratio n is some other value, the resistance of 2000 ohms used to close the transformer should be replaced by a resistance equal to 2 0 0 0 ohms.

Specification B. II

Essential clauses of a typical specification for the provision o f audio-frequency repeaters.

1. Repeaters for two-wire circuits General. — On two-wire circuits, on open wire or in cable, reversible repeaters having two balances which balance separately the two sides of the telephone circuit only should be used. These repeaters should not sing, that is to say oscillation should not occur for the maximum gain, when the “ line ” and “ balance ” terminals corresponding to one direction are closed with non-reactive resistances having a value equal to the impedance specified for the repeater, the “ line ” and “ balance ” terminals for the other direction being open or short-circuited, or vice versa. The two resistances should be connected directly to the terminals of the differential transformer. In the case where the repeaters include adjustable equalization this condition should be met for all possible settings of the equalization. Amplification. — The repeaters should ensure a faithful reproduction of speech, that is the repeater gains should be such that the circuit effectively transmits frequencies from 300 to 3400 c/s. To achieve this result either repeaters amplifying all the frequencies by the same amount associated as necessary with independant equalizers or repeaters constructed to give the desired correction themselves may be used. In the case where the repeaters compensate for the distortion introduced by the line, the gain of the repeaters should vary with frequency over the frequency band that the circuit should effectively transmit. For higher frequencies the gain should fall off in such a way as to reduce to zero in the neighbourhood of the cut SPECIFICATIONS — 2-W IRE REPEATERS 143 off frequency of the circuit; for lower frequencies the gain should not exceed the values required for an exact equalization of the circuit. Gain adjustment for the repeater should be provided preferably, by steps not exceeding 0 . 1 nepers or 1 decibel. The gain frequency characteristic curves of the repeater should, for all settings of repeater gain, be parallel over the band of frequencies which the cable effectively transmits. The equipment should be designed so that with the power supplies maintained in their normal condition the maximum change in repeater gain caused by power supply voltage variations should not exceed 0.03 nepers (0.26 decibels).

Impedance. — The impedance of the repeater including line transformers should be approximately equal to that of the line in which the repeater is used such that the modulus of return current coefficient:

Z — W z + w does not exceed 0 . 2 in the case a circuit with medium loading and 0 . 1 in the case of a lightly loaded circuit at any frequency in the band of frequencies effectively trans­ mitted, Z being the characteristic impedance of the circuit and W the impedance of the repeater measured as follows: The impedance of the repeater is measured in its working conditions including the two balances, but suppressing retroactive effects. For example, to measure the impedance at the “ X ” side, of the repeater the circuit on the “ Y ” side is replaced by its balance; the balance on the “ X ” side must be equivalent to the circuit on the “ X ” side, which is replaced by the measuring instrument; the potentiometer in the X-Y direction is left in its normal working position and transmission is sup­ pressed in the Y-X direction without altering the impedance.

Monitoring. — Arrangements should be provided for monitoring on the circuits with an operator’s telephone set, in either or both directions and for speaking on the circuits when necessary. When monitoring takes place on a repeater in service the insertion loss of the monitoring equipment should not exceed 0.03 nepers (0.26 decibels).

Crosstalk. — When repeaters are installed side by side or one above the other with power connected under operating conditions, the measured crosstalk between the output terminals of the repeaters should be greater or equal to a crosstalk ratio of 8 nepers or 70 decibels. It should be understood that for these measurements the repeaters will be connected to impedances (Z) having a value equal to that fixed for international circuits and that the repeaters will be adjusted to their maximum gain. A suitable measuring arrangement is shown in figure 30. 14 4 SPECIFICATIONS — 2-W IRE REPEATERS

Disturbing Adjustable repeater attenuator

Oscillator Detector

Disturbed repeater

F ig u r e 30

Notes. — 1. The gain in the direction not being measured should be suppressed 2. The line transformers and cabling connecting them to repeater bay are not included in the tests; it is necessary that the measuring equipment should be arranged so that the repeaters are in their operating condition, particularly so .far as their symmetry is concerned. 3. Crosstalk is measured in accordance with the methods recommended by the C.C.I.F. (see Book of annexes to Volume III of the Green Book, 2nd part, section 1.9.1).

Non-linear Distortion. — The total harmonic distortion should not exceed 5 % for a frequency 800 c/s and for a maximum power of 2 0 milliwatts in the case of a two-wire circuit, the power supplies for the repeater valves having their normal value and the control grids being kept at a negative potential relative to the cathodes; the line transformers do not form part of the measuring circuit and the repeater output is closed with a pure resistance of 600 ohms during the test. (The figure of 5% is provisional; it is valid until permissible limits can be fixed for the non-linear distortion of a complete circuit; this question is being studied by the C.C.IF.). On the other hand the C.C.I.F. consider it desirable to fix provisionally, for the different cases, the maximum power measured at the output of a repeater as follows: Repeaters for telephone circuits

two-wire in c a b l e ...... 2 0 milliwatts two-wire (open wire) . . . 2 0 milliwatts

2. Repeaters for four-wire circuits General. — Repeaters used for four-wire circuits should amplify in one direction without appreciably affecting the quality of transmission for all frequencies which the circuits associated with the repeaters should effectively transmit and for the maxi­ mum input power in practice. Amplification. — The gain given by the repeater (or by the repeater and its associated equalizer) should vary with frequency so as to compensate sufficiently the distortion introduced by the line in the band of frequencues effectively trans­ mitted (300-3400 c/s. SPECIFICATIONS — 4-W IRE REPEATERS 14 5

Arrangements should be provided to adjust the gain of the repeaters preferably in steps not exceeding 0.1 nepers or 1 decibel. In the case of very long circuits, it may be necessary to provide adjustment in steps not exceeding 0.05 nepers or 0.5 decibels. The gain frequency characteristic curves of the repeater should be parallel for all adjustments of the repeater in the band of frequencies effectively transmitted by the circuit. The design of the repeater should be such that with the power supplies maintained in their normal condition the maximum change in repeater gain caused by power supply voltage variations should not exceed 0.03 nepers (0.26 decibels).

Impedance. — The impedance of the repeater, excluding line transformers, measured at 800 c/s at the imput and output terminals, should in principle have the value allowed by the C.C.I.F. for the impedance of international circuits. The impedance of the repeater, excluding line transformers, will approximately equal that of the circuit in which the repeater operats such that the modulus of return current coefficient:

Z — W z + w does not exceed 0.4 for the input impedance of the repeater and 0.6 for the output impedance, Z being the line impedance (including line transformer) and W the repeater impedance; these two figures must not be exceeded for any frequency effectively transmitted.

Monitoring. — Arrangements should be provided for monitoring on the circuits with an operator’s telephone set, in either or both directions and for speaking on the circuits when necessary. When monitoring takes place on a repeater in service the insertion loss of the monitoring equipment should not exceed 0.03 nepers (0.26 decibels).

Crosstalk. — When the repeaters are installed side by side or one above the other with power connected under operating conditions, the measured crosstalk between the output terminals of the repeaters should be greater or equal to a cross­ talk ratio of 8.5 nepers or 74 decibels. It should be understood that for these measurements the repeaters will be terminated with impedances (Z) having a uniform value equal to that fixed for international circuits and that the repeaters will be adjusted to their maximum gain. A suitable measuring equipment is shown in figure 31.

10 1 4 6 SPECIFICATIONS — 4 - WIRE REPEATERS

Disturbing Adjustable repeater attenuator

Disturbed repeater

F ig u r e 31

Non-linear distortion. — The total harmonic distortion should not exceed 5% for a frequency of 800 c/s and for a maximum power of 50 milliwatts in the case of a four-wire circuit, the power supplies for the repeater valves having their normal value and the control grids being kept at a negative potential relative to the cathodes; the line transformers not forming part of the measuring circuit and the repeater output closed with a pure resistance of 600 ohms during the test. (The figure of 5% is provisional; it is valid until permissible limits can be fixed for the non-linear distortion of a complete circuit; this question is being studied by the C.C.I.F.) On the other hand the C.C.I.F. consider it desirable to fix provisionally, for the different cases, the maximum power measured at the output of the repeater as follows: Repeaters for four-wire telephone circuits in cable .... 50 milliwatts Repeaters for voice-frequency telegraph s y ste m s...... 50 milliwatts This condition has been fixed on the assumption that relative power level at the output of any repeater will never exceed + 1 . 1 nepers or + 1 0 decibels.

Specification B. Ill

. Essential clauses for a specification for the provision of four-wire terminating sets 1. This specification is applicable to four-wire terminating sets incorporating differential transformers. 2. ' The minimum value of the composite attenuation of the terminating unit,' measured between the “ input ” and “ output ” terminals of the four-wire circuit, closed with 600 ohm resistances (the “ line ” and “ balance ” termihals being them­ selves connected to 600 ohm resistances), at any frequency in the band of frequencies effectively transmitted must be at least 7 nepers or 61 decibels; for the measurement the condensers of the termination should be short-circuited; if the condensers are not accessible the limit of 7 nepers is reduced to 6 nepers or 52 decibels. 3. The composite attenuation of the terminating unit in the direction from the four-wire to the two-wire side and vice-versa should not exceed 0.55 nepers or 4.8 decibels, including the attenuation of any filter or similar arrangements. In the case SPECIFICATIONS — 4-W IRE TERMINATING SETS 14 7 where the terminating unit and the filter are two distinct items, the limit for the attenuation of the terminating unit is reduced to 0.5 nepers or 4.5 decibels. 4. The impedance, measured at the output of the terminating unit at the point where the circuit will be connected, should have the value recommended by the C.C.I.F. for the impedance of international circuits and repeaters, when the three other outlets are closed with pure resistances having the same value as the nominal impedance of the lines or repeaters with which the terminating unit will be used. It is desirable that these impedances should be as far as possible independant of frequency.

Specification B. IV

Essential clauses for a specification for the provision of echo suppressors. Characteristic times: — The significance of operating time, hang-over time of partial closing of an echo suppressor is shown in figure 32. The operating time and hang-over time should be adjustable to give for any particular case the values given in the note which follows according to the particular echo conditions of the circuit under consideration. In particular the hang over time tb2o should be adjustable to give one of the three values 50, 150 or 250 milliseconds as desired.

Attenuation introduced into return channel

F ig u r e 32 Characteristic times of a continuous-action type of echo suppressor 1 48 SPECIFICATIONS — ECHO SUPPRESSORS

A limitation for the time of partial closing may be given after a study of the opera­ tion of echo suppressors by various Administrations and Private Operating Companies. Insertion loss. — The insertion' loss introduced in the speech circuit by the echo suppressor should not vary by more than 0.06 nepers (0.52 decibels) for any fre­ quency effectively transmitted by the circuit. Crosstalk and stability of telephone transmission. — The installation of the echo suppressor should not appreciably increase the crosstalk between the different circuits or reduce the stability of the circuit. Operating level. — To avoid false operation of the echo suppressor, the ope­ rating level (related to zero relative level) of a terminal echo suppressor at its most sensitive frequency should be a maximum of — 2.5 nepers or — 22 decibels and a minimum of — 3.5 nepers or — 30 decibels. Similarly to avoid the false operation of intermediate echo suppressors due to noise the operating level (related to zero relative level) should be at the minimum — 3.5 nepers or — 30 decibels. Variation of sensitivity as a function of frequency. — It is desirable to design the echo suppressor so that the sensitivity varies with frequency to reduce the risks of false operation. The frequency for which the level of local operation is smallest should be between 700 and 1200 c/s.

Local sensitivity (nepers)

. 3,5

. 3 A . 8.5 / *

A - 1 a/ \B rruuv A -1 5 , \D rruuc C/

9too .1 2 0 0

F ig u r e 33

If the sensitivity characteristic of the echo suppressor at various frequencies is plotted with frequency in c/s on the abscissa and local sensitivity in nepers as ordi­ nates (figure 33) and if AB are the intercepts on the curve of a fine drawn parallel with the frequency axis 1 neper below the point of maximum sensitivity and CD the intercepts of a parallel line 1.5 nepers below this point: SPECIFICATIONS — ECHO SUPPRESSORS 1 4 9

the segment AB should correspond to a frequency difference equal or greater than 500 c/s; the segment CD should correspond to a frequency difference equal or greater than 1 0 0 0 c/s. In addition it is desirable that the line parallel with the neper axis passing through the point of maximum sensitivity should divide the segments AB and CD into two parts for which the part having the greatest value in c/s should be the one situated on the side of the highest frequencies. In other words it is desirable that the slope of the curve of variation with fre­ quency of local operating levels of the echo suppressor should be greater for the frequencies lower than the frequency corresponding to the minimum level of local operation (maximum sensitivity) than the slope of the curve for the frequencies above the frequency corresponding to the minimum level of local operation.

NOTE

Information on the subject of calculating operating time and optimum hang-over time for an echo suppressor connected to a telephone circuit is given in section 1.2.2 above.

Specification B. V

Essential clauses for a specification for the provision of power supplies for repeaters 1. As a result of the necessity to ensure an uninterrupted service on interna­ tional circuits it is essential to provide power supply installations in such a way that interruptions to the normal supplies can be made good. ' 2. It is necessary that the whole power supply installation should be sufficiently free from disturbances. If repeater stations are provided with direct current power supply instal­ lations (batteries, rectifiers) supplied by a different contractor from the one who supplies the repeaters, the Administration concerned should make arrangements with the two contractors to ensure that this requirement is met. 3. The installation should be arranged so that variations in power supply voltages (heater supplies, anode supplies and grid priming supplies) do not exceed ± 2 % of their nominal value. It is recommended that automatic regulation equip­ ment be provided for this purpose.

Specification B VI

Indications for the preparation o f a specification for the provision of thermionic valves for repeaters (for ordinary telephony or for programme transmissions. The C.C.I.F. is at present studying the bringing up to date of the text under this title on pages 254 to 256 of Volume III bis of the Yellow Book. 1 5 0 SPECIFICATIONS — STATION CABLING

Specification B. VII

Essential clauses for a specification for the provision o f cabling in audio-frequency repeater stations 1. Cabling between the cable head and the protectors, where they exist, or bet­ ween the cable head and the line transformers where there are no protectors: Minimum breakdown voltage: 2000 volts (effective voltage applied for at least 2 seconds between all conductors in parallel and the sheath connected to earth). The test should be made with a 50 c/s alternating current. Insulation resistance: 600 megohms x kilometres, between a conductor and all other conductors connected together and to earth and to the metal cable sheath if provided. 2. Cabling between protectors, where they exist, or line transformers where there are no protectors and the repeater bays: Minimum breakdown voltage: 500 volts (see above for the testing conditions) Insulation 100 megohms x kilometre (see above for the testing conditions) 3. Crosstalk: for the whole of the cabling between the cable head and the repeater bay, it is recommended that a crosstalk attenuation should be obtained at least equal to 8.5 nepers or 74 decibels between cabling for two-wire circuits or for four-wire circuits in the same direction of transmission and 9.5 nepers or 83 decibels between cabling for four-wire circuits in opposite directions * There is no need to distinguish, from the crosstalk point of view, between circuits which will or will not be provided with repeaters in the station considered.

* The C.C.I.F. is at present studying if these two limits could be replaced by a single limit expressed in the form of a signal-to-crosstalk ratio. SECOND PART

UTILISATION OF INTERNATIONAL TELEPHONE CIRCUITS FOR TELEGRAPHY OR PHOTOTELEGRAPHY COEXISTANCE OF TELEGRAPHY AND TELEPHONY

SECTION 1

Use of carrier-current telephone channels for voice- frequency telegraphy

Essential characteristics for a carrier-current telephone channel required to carry 24 voice-frequency telegraph channels each operating at 50 bauds

(a) Transmitted power and noise

Telephone Administrations are committed to provide throughout the European network, for the telegraph services, telephone channels each capable of providing ah 18-channel voice-frequency telegraph system each channel operating at 50 bauds. In the carrier current lines of the future telephone Administrations will be able to provide the telegraph services with telephone channels permitting the use of 24 voice-frequency telegraph channels (each capable of 50 bauds) on condition that the power of the telegraph signal on each channel, during the transmission of a continuous marking signal, equals 9 microwatts at a point of zero relative level. This precaution is essential to avoid the overload of amplifiers and modulators on carrier systems. As a result of this restriction of the transmitted telegraph power the voice-frequency telegraph system will be able to use effectively the band of frequencies between 360 and 3240 c/s. The power of 9 microwatts corresponds to

5 milliwatts

(24) 2

This figure could be revised if a statistical study of the peaks of telegraph power (per telephone channel carrying 24 telegraph channels) show it to be desirable. 1 5 2 Y. F. TELEGRAPHY ON CARRIER TELEPHONE CHANNELS

It may happen that a telephone channels gives a relatively high level of noise, in which case the telegraph service must abandon the use of 24 telegraph channels on such a telephone channel and limit itself to 18 channels only. In such a case the maximum permissible power for each telegraph channel is 5 milliwatts — 7 7 - 5 ------= about 15 microwatts 18 *

Note. — The power per telegraph channel should not in any case ever exceed 5 milliwatts

1 2 2 or about 35 microwatts, calculated for a 12-channel telegraph system in accordance with the principles indicated above.

(b) Attenuation distortion In the above-mentioned band (360 to 3240 c/s) the variation of equivalent with frequency of a carrier telephone channel should in the worst case be within the limits of graph No. 1 of figure 34 which will be adopted for telephone working. For the majority of carrier telephone channels used for voice-frequency tele­ graphy, it is recommended that in general graph No. 5 of figure 35 should be used in all cases where a circuit intended to provide 24 telegraph channels is set up on a single group link * for either a normal or reserve circuit for voice-frequency telegraphy.

Note. — Telephone channels 1 and 12 of a group may show greater attenuation distortion than the other channels of the group. It is recommended that the use of these channels 1 and 1 2 for voice-frequency telegraphy should be avoided.

(c) Phase distortion Practical experience obtained up to the present shows that it is not necessary to introduce a supplementary clause relative to the slope of the “ group delay-fre- quency ” characteristic in the C.C.I.F. specifications for carrier systems, even considering the case where a telegraph link consists of telephone channels of several carrier systems connected in tandem. It may happen that under adverse conditions some telephone channels of such a connection are of insufficient quality to provide 24 telegraph channels. In such .a case a better combination of telephone channels must be chosen for the telegraph service and this better combination will always be possible. By way of information, annex 4 of the Book of Annexes of Volume III of the Green Book gives the result of a calculation made by the French Telephone Administration.

* A group link is a transmission path of defined band width (48 kc/s) connecting two group distribution frames or equivalent points. It extends from the point where the group is first assembled to the point where it is dispersed to channels. The term normally covers both directions of trans­ mission. Y. F. TELEGRAPHY ON CARRIER TELEPHONE CHANNELS 153

(d) Frequency stability It has already been foreseen that in future carrier systems the virtual carrier frequencies will be stable to about ± 2 c/s based on the requirements of voice-fre­ quency telegraphy (see sub section 3.1.2 above). N '////, *i.o . m * o,g . t * o,8 *o,f * 0,6 . * o,S. m i * o ,tt.

+ o,3 _ 0.85. + o.8

* 0 .1 uw i 3oo 6oo 8oo looo fiooo 8lloo 3ooo idoo f

- 0.25

F ig u r e 34. Graph No. 1. — Variation with frequency o f the equivalent in terminal service, relative to the value measured at 800 c/s (international circuit transmitting the band of frequencies 300 to 3400 c/s) N *4,0 . *0,g. + 0.8 . *o,f. * 0,6 . * o,5 . * o.bS, ,* o.H- % * 0.35. m e * o,3 * o,85. + 0,8. * 0,1 . 0 U 90 ;18HO 1 1— — 1------1— - 0.1 . 3oo S00 800 1000 8000 Shoo 3 0 0 0 38oo 3t w o f - 0,8. - o ,8 5

F ig u r e 35 Graph No. 5. — Variation with frequency of the equivalent in terminal service relative to the value measured at 800 c/s (circuit routed on a single group link to provide 24 voice-frequency telegraph channels) f = frequency c/s N = variation of equivalent (Nepers) Note. — The curve of variations of equivalent with frequency should lie within the non-hatched 1 5 4 V. F. TELEGRAPHY ON CARRIER TELEPHONE CHANNELS

So far as the stability of the frequencies provided by the carrier frequency generators for voice-frequency telegraphy is concerned, the C.C.I.F. has recommended that the frequencies should not differ by more than 3 c/s from the nominal value when the telegraph channels are routed on a telephone circuit which is not made up exclusively of ordinary audio-frequency telephone sections. (e) Crosstalk The near-end signal, crosstalk ratio between the two directions of transmission of a telephone circuit using a duplex voice-frequency telegraph system must be at least 4 nepers or 35 decibels. * Taking into consideration the crosstalk introduced by equipment, a near-end signal crosstalk ratio of 35 decibels can in general be guaranteed between the two directions of any telephone circuit for the “ nominal maximum circuit on coaxial pairs ” if a ratio of 50 decibels is obtained between the two directions of transmission for each channel “ modulator—demodulator ”, a value specified by certain Admin­ istrations. Any telephone circuit on modern carrier systems in cable could then be used for duplex voice-frequency telegraphy.

Arrangements for setting-up and changeover for a voice-frequency telegraph circuit (a) The make-up of a four-wire circuit for voice-frequency telegraphy differs from that of a telephone circuit by the absence of terminating units, signalling equipment and echo suppressors. A circuit required to provide 24 voice-frequency telegraph channels on a carrier telephone channel should preferably be routed on a telephone channel on a single group link only.

A I — _ t 1 n,^-o,U N 1 n'f^ + Q,k N

(Qutcujk(l) Q

ns rig

© utcuit(8)

F ig u r e 36 A and B = points where — 1° Voice-frequency telegraph circuit (1) is connected to the telegraph equipment, — 2° Changeover takes place from voice-frequency telegraph circuit (1) to the traffic tele­ phone circuit (2) used as a reserve for circuit (1) T = “ Transmit ” voice-frequency telegraph equipment R = “ Receive ” voice-frequency telegraph equipment.

* Subject to the agreement of the C.C.I.T. V. F. TELEGRAPHY — RESERVE CIRCUITS 1 5 5

(b) Points A and B (figure 36) where the changeover between the voice- frequency telegraph circuit and its reserve circuit takes place (and which are conven­ tionally regarded as the origin and extremity of the four-wire circuit used for voice- frequency telegraphy) should be at the same relative level for the two circuits, the levels being determined from the level diagram of the telephone circuit. The relative level at point A must not exceed — 0.4 nepers. The relative level at point B must be at least + 0.4 nepers.

(c) The relative power level at the point at the receiving end where changeover between the voice-frequency circuit and its reserve circuit takes place, must be as constant as possible with time. Furthermore any interruption in the circuit even for a very short duration spoils the quality of the telegraph transmission.

Reserve circuits for voice-frequency telegraphy

The International Telephone Xonsultative Committee Considering that there is a case for standardizing certain of the arrangements for replacing faulty 4-wire circuits used for voice-frequency telegraph systems; that whilst it seems unnecessary for the details of the changeover arrangements to be the same in all countries it will be of interest to agree on the general require­ ments to be met; that the use of telephone channels in carrier systems for voice-frequency telegraphy will become general in the future; that it will be of interest that the curves giving the relative power level differences, as a function of frequency, between the origin and extremity of the normal voice- frequency telegraph circuit and its reserve circuit do not differ for any frequency by more than 0 . 2 nepers, so that when the telegraph system is changed over to its reserve circuit there is no variation in level to cause excessive distortion for the voice- frequency telegraph system especially for the higher and lower frequencies of the transmitted band; that it is not possible in the present state of the telephone network to guarantee a limitation of this type. In fact the voice-frequency circuit and its reserve circuit have generally a different make-up and often one is an audio circuit whilst the other is a channel in a-carrier system. While it is possible to make small changes to the “ equivalent-frequency ” curve of an audio circuit it is difficult to modify the curve for a carrier channel since it depends primarily on the characteristics of the filter of the carrier system;

(a) that it is recommended

( 1 ) that for each circuit used for a voice-frequency telegraph system a telephone circuit should be nominated as a reserve circuit; 1 5 6 V. F. TELEGRAPHY — RESERVE CIRCUITS

(2 ) that this reserve circuit should if possible have a different routing to the normal voice-frequency telegraph circuit; (3) that the maintenance measurements made on the reserve telephone circuit should be the same as on the normal voice-frequency telegraph circuit and consequently special arrangements should be made for the maintenance of these circuits;

'(b) that it is desirable that the changeover from the normal voice-frequency telegraph circuit to its reserve circuit should be made as rapidly as possible in view of the more general use in the future of automatic switching for telegraph channels;

(c) To this effect it is recommended

( 1 ) that the normal voice-frequency telegraph circuit and the reserve circuit are routed via the same switching panel at the terminal stations concerned

(2 ) that the changeover should take place at two points of the same relative power level for the normal voice-frequency telegraph circuit and the reserve circuit; (3) that the procedure to be adopted for changeover from the normal voice-frequency telegraph circuit to the reserve circuit and vice versa should be determined by agreement between the two Administrations or Private Operating Companies concerned;

(d) that if faults affect simultaneously the normal voice-frequency telegraph circuit and the reserve circuit, it is recommended that the Administrations or Private Operating Companies concerned quickly consult each other and agree on immediate temporary arrangements to remedy the situation;

(e) that it is desirable in view of the distortion which could occur on certain voice-frequency telegraph channels when the changeover from the voice-frequency telegraph circuit to its reserve circuit takes place, that the curves giving the relative power level differences, as a function of frequency, between the origin and extremity of the normal and reserve circuits should be as similar as possible. SECTION 2

Private telegraph transmission on a rented international circuit with alternative private telephone service

(1) The frequency of 1500 c/s is recommended for private telegraph transmission between subscribers permanently connected via a rented international circuit. (2) The permissible power for a continuously transmitted telegraph marking signal is 0.3 milliwatt (equivalent to an absolute power level of — 0 . 6 nepers or about — 5 decibels at a point of zero relative level). It is recommended that measurements be made to ensure that this limit will not be exceeded at the time any international circuit is rented which may later be used for such telegraph transmissions. It is the responsability of the Administrations or Private Operating Companies concerned to take any necessary precautions to ensure that such telegraph trans­ missions do not disturb their interior telephone service. These precautions may involve a limitation of the transmitted telegraph power or of the length of time telegraph transmission is used or of the type of telegraph transmissions. (These supplementary facilities given to the users may be charged for if necessary). To enable a phototelegraph office to transmit pictures simultaneously to several receiving points it is necessary to ensure that the level does not fall below the limits fixed for an international telephone circuit at any point in the phototelegraph links concerned. If it is not possible to compensate for the loss caused by branching the telephone circuit at intermediate stations by increasing the sensitivity of the receiver phototelegraph equipment it will be necessary to insert supplementary amplifiers at the intermediate branching points. (3) Voice-frequency signalling units on rented telephone circuits used for private telegraph transmission between two subscribers premises permanently connected together should be immune operation from telegraph signals. It has been noticed that a certain type of existing signalling units is affected by these telegraph signals, but arrangements can be made to improve such units without appreciable difficulties f or the frequency chosen. (4) It appears that the maximum limit of 250 milliseconds adopted for the hang-over time for echo suppressors on international circuits (see section 1 . 2 . 2 of the 1 st part above) is not long enough to cause suppression (even partial) of teleprinter answer-back signals. SECTION 3

Phototelegraph transmission (provisional recommendation)

The International Telephone Consultative Committee, unanimously recommends that for telegraph transmission of still pictures over telephone circuits (photo­ telegraphy) the following conditions should be observed: Circuits to be used. — Phototelegraph transmissions cannot be allowed on circuits normally used for the automatic telephone service, unless there is no risk of dis­ turbance to the international semi-automatic signalling. Hovewer in the case of a circuit normally used for semi-automatic service rented on a yearly basis for photo­ telegraph transmissions this risk can be avoided by removing the signalling equipment from the circuit. On the other hand only telephone circuits for which the coupling by reaction does not exceed the permitted value (see later) can be used for photo­ telegraph transmission. For this reason two-wire circuits cannot be used in practice for picture telegraph transmissions. In the normal four-wire arrangement the circuit can only be used at any one time in one or the other direction of transmission; in this case it is usually necessary to use an echo suppressor to suppress reactive coupling effects. If simultaneous transmission is required in the two directions, or if during picture transmission in one direction it is required to use the other direction for the transmission of service information between the two subscribers, the terminating

* A “ phototelegraph office ” is defined here as any installation for the transmission or reception of pictures. PHOTOTELEGRAPHY 159 units and echo suppressors may be disconnected by agreement between the Admin­ istrations and Private Operating Companies concerned.

1. Phototelegraph transmissions using amplitude modulation

Equivalent and levels (a) The equivalent at 800 c/s between international centres must not exceed 1.3 nepers (11.3 decibels). (b) The equivalent at 800 c/s between phototelegraph offices must not exceed 3.3 nepers (28.7 decibels). (c) The equivalent must remain as constant as possible during the transmission of pictures. Sudden changes of even 0.1 nepers or 0.9 decibels will be perceptible on the pictures transmitted. It is also necessary to avoid all breaks in the circuit however short they may be. For this reason the greatest attention must be given to measurements made on the repeaters or lines and to battery change-overs. To avoid faults it is desirable that the exchange equipment at international centres should be taken out of circuit when the circuit is extended to the phototelegraph offices. Special precautions must be taken to ensure that no intermodulation is caused by the line or amplifier even if such intermodulation is inaudible. (d) Provisionally the absolute power level corresponding to the maximum amplitude of the photo telegraph signal should be adjusted to 0 nepers ( 0 decibels) related to a point of zero relative level on the level diagram of the telephone circuit.

Attenuation distortion. — The attenuation distortion between phototelegraph offices must not exceed 1.0 nepers or 8.7 decibels over the band frequencies effectively transmitted for the picture transmission. Since a distortion of 1.0 nepers or 8.7 decibels has already been allowed for the telephone circuit itself it will therefore be necessary when required to equalize the lines connecting the phototelegraph office to the telephone exchange.

Phase distortion. — The C.C.I.F. provisionally recommends the following for either audio or carrier-type international circuits: If tm is the group delay for the nominal lower limit of the band of frequencies transmitted, tu is the group delay for the nominal upper limit of the band of frequencies transmitted tmin is the minimum group delay in the band of frequencies transmitted then we should have

tm ~ tm in 1 0 m S tyi Anin 5 mS Whilst for a single modern-type carrier system (providing at least 12 channels) (and in particular for channels in the middle of the basic group of such a system) the phase distortion will be relatively small, it cannot be guaranteed that the phase PHOTOTELEGRAPHY

' Add 1000 microseconds

F ig u r e 37. — Variation with frequency, relative to the value at 2000 c/s, o f the group delay o f a typical telephone channel on a type L carrier system (corresponding to the mean o f channels 1—12 for type A2 channel banks) f = frequency (c/s) |j.S = group delay (microseconds) PHOTOTELEGRAPHY 161 distortion will always be as good as the limits given above (or necessarily that it can be improved) for the existing or future international circuits conforming to the C.C.I.F. specifications. In fact the presence of filters (either in terminal equipment or as group or supergroup transfer filters from one system to another) results in appreciable increases in the phase distortion. It would not be economically justifiable to make a more severe specification (in respect of phase distortion) than the present recommendation purely in view of the occasional use of a few circuits for high-speed phototelegraph transmission. Annexes 1 to 4 which follow give information on the phase distortion which can be met in practice. Naturally in the case of a circuit rented on a yearly basis specially for photo­ telegraph transmission it will be possible to choose by agreement between Admin­ istrations a circuit which satisfies the above limits for phase distortion.

Crosstalk and other noise. —• Crosstalk and noise will not be troublesome if the difference between the absolute power level of the disturbing current and the absolute power level corresponding to the maximum amplitude of the phototelegraph signals is at least 4 nepers (34.7 decibels).

Stability. — The stability of the circuit must be at least 2 nepers or 17.4 decibels. If echo suppressors are used in the links a stability of 0.5 nepers or 4.3 decibels will be sufficient.

Precaution. — It is desirable that circuits used for picture transmission should be specially marked at the terminal exchanges and in the intermediate repeater stations; in addition special instructions should be given to staff in the exchanges and repeater stations so that they will not go on the line when a picture transmission is taking place.

2. Phototelegraph transmissions using frequency modulation

Recommended transmitted power. — In the case of a frequency-modulated phototelegraph transmission on a long-distance telephone circuit it is recommended that the absolute power level of the phototelegraph signal should normally be adjusted to — 10 decibels (— 1.15 nepers) at a point of zero relative level taken from the level diagram of the telephone circuit, and always kept at this value.

ANNEX 1

Phase distortion on telephone channels of coaxial carrier systems in the United States of America The curve of figure 37 shows typical values for the variation with frequency of the group delay, relative to the value at 2000 c/s for telephone channels in the type L coaxial system. They are applicable in practice to any length of circuit on a single carrier system. It should be realised that phase distortion is not an important characteristic for telephone circuits and that the attached curve will not necessarily be met for future types of carrier systems.

li 162 PHOTOTELEGRAPHY

ANNEX 2

Phase distortion introduced by various carrier equipment used in Great Britain

1. The group delay-frequency characteristic of a telephone channel routed on a modern type of carrier system, which effectively transmits the audio frequency band 300- 3400 c/s, is determined mainly by the group delay characteristics of the channel translating equipment (which translates the audio-frequency band to basic group and vice versa); table 1 which follows gives the characteristic obtained on a channel translating equipment of the type used by the British Administration.

Table 1 “ Group delay frequency ” characteristic of a channel translating equipment of the type used by the British Administration

Frequency (c/s) 300 500 800 1 200 1 800 2 400 2 800 3 100 3 400

Group delay (milliseconds) . . . 3.5 2.1 1.4 1.1 1.0 1.1 1.3 1.7 2.8

Note 1. — The equipment translates from the audio-frequency band to basic group B (60-108 kc/s) and back again.

Note 2. — The characteristic is that actually measured on an equipment and is given to show the order of values obtained. The characteristic does not represent a mean value neither does it give the maximum values that would be obtained, or a specification limit.

2. The characteristic of the circuit, as determined by the number of channel translating equipments through which it passes may be modified to an appreciable extent by other equipment in the circuit and particularly by the frequency band occupied by the circuit in passing through these equipments.

3. A typical installation of equipment used by the British Administration showed the general characteristics given in paragraphs 3.1 to 3.5. The more important differences in group delay usually had the characteristic of a positive or negative slope over the frequency band corresponding to 300-3400 c/s on the channel. 3.1. Equipments which translate from one basic group of 12 circuits to another (e.g. basic group B to A or group A to B) or from basic group to the line frequency range (e.g. the case of carrier systems on symmetrical pairs) and equipments which translate from basic group to basic supergroup and from basic supergroup to the line frequency range, may introduce a difference in group delay of the order of 50 microseconds in the worst channels and 1 0 microseconds in the majority of channels for each pair of modulations or demodulations. 3.2. Each filter passing a group of 12 circuits and having a sharp cut-off (e.g. 24 channel filter for symmetrical pairs or through group filter) may introduce a difference in group delay of the order of 1 millisecond on the worst channel and 2 0 microseconds on most channels. 3.3. Each filter selecting a supergroup of 60 circuits and having a sharp cut off may introduce a difference in group delay of the order of 50 microseconds for the worst channel and 1 0 microseconds on most channels. PHOTOTELEGRAPHY 163

F ig u r e 38 (Netherlands — Constructor A) Curveu = channel No. 1 Curve b = channel No. 2

500 iOOO 1500 SOOO 8500 3000 3500 ffOOO f

F ig u r e 39 (Netherlands — Contractor B). — Curve applicable to all channels o f a group f = frequency c/s mS = group delay (milliseconds) 1 6 4 PHOTOTELEGRAPHY

3.4. 24-channel line links on symmetrical pairs comprising cable, amplifiers and equalisers may introduce a difference in group delay of the order of 60 microseconds per 1 0 0 miles (about 160 km) for the worst channels and 1 0 microseconds in most other channels. 3.5. Coaxial line links comprising cable, amplifiers and equalisers introduce a rela­ tively small difference in group delay (less than 1 0 microseconds per 1 0 0 miles) except in supergroup 1 (60-300 kc/s) where a difference of up to about 500 microseconds per 100 miles may occur. 3.6. The difference in group delay on a complete circuit will be determined by the translating equipment, the filters and lines over which it is routed and no standard charac­ teristics can be predicted. Thus for the nominal maximum coaxial circuit the group delay- frequency characteristic will be similar to the characteristic of table 1 above but could have approximately three times the magnitude (e.g. if all three pairs of modulations had the characteristics of table 1 the overall group delay relative to the minimum value which occurs at about 1800 c/s, would rise to about 7.5 milliseconds at 300 c/s and 5.5 milliseconds at 3400 c/s) and this may be modified by a positive or negative “ slope ” or a combination of both, of up to possibly a few milliseconds. In addition there may be a random variation of the order of a few hundred milliseconds.

ANNEX 3

Phase distortion on channels of carrier systems in the Netherlands This distortion is shown in the curves of figures 38 and 39 which follow. Figure 38 refers to the group delay with equipment provided by contractor A. Curve “ a ” refers to channel 1 and curve “ b ” to channel 12 of a basic group. Figure 39 refers to the group delay with equipment provided by contractor B. In the latter case all the channels of the group are about the same.

ANNEX 4

Phase distortion in telephone channels of symmetrical pair carrier current systems in cables (with 5 basic groups or 12 + 12 type) used by the Federal German Administration Figure 40 shows the variation of group delay on a telephone channel of the new German carrier systems for long-distance traffic (the V60 system providing 60 telephone circuits on symmetrical pairs), relative to the minimum group delay. The curves of propagation time (valid also for the edge channels) measured on terminal equipment of different types are contained within the shaded area; for the mean curve shown by the dotted line the variation of group delay is 2.4 milliseconds at the lower end and 1.25 milliseconds at the upper end of the transmitted frequency band of 300-3400 c/s. Case of a nominal maximum circuit. — If the arrangement of the nominal maximum circuit on coaxial pairs is applied to symmetrical pair carrier circuits, phase distortion on channels 2 to 1 0 of a group is produced only by the filters of the channel modulating equipment. The phase distortions produced by the triple modulation and demodulation of the channel amount to about three quarters of the permissible tolerance given by the present C.C.I.F. recommendations. The difference between the group delay at the frequency under consideration and the minimum group delay must not exceed 10 milliseconds at 300 c/s and 5 milliseconds at 3400 c/s. PHOTOTELEGRAPHY 16 5

Case where groups or supergroups are transferred from one system to another. — The Fe­ deral German Administration does not consider that it would be useful further to reduce the limits fixed up to the present by the C.C.I.F., because for the edge channels 1 and 12 of a group additional phase distortion can be produced by group or supergroup modulating equipment or by the transfer of groups without modulation or demodulation. In the same way for carrier systems on two-wire lines with different frequency bands in the two directions of transmission (for example the German Z 12 N) system, additional phase distortion is pro­ duced in the edge channel adjacent to the frequency band eliminated by the directional filters which in the case of a large number of intermediate repeaters can result in the per­ missible tolerance for the propagation time being exceeded. The phase distortion shown in figure 40 which follows refers to telephone channels which have only one receive channel modulating equipment. Several group and supergroup modulations are permissible. However with filters having a large rise in attenuation in the stop band of frequencies (for example through group filters for the 60 to 108 kc/s band) a small increase can occur in the phase distortion of the edge channels 1 and 1 2 of the group. Case of circuits permanently used for phototelegraphy. — If it is arranged that telephone circuits used for phototelegraphy include only one channel transmitting and receiving equipment and are not edge channels of a group, a very small phase distortion can be obtained, all the more so since the phototelegraphy uses only a part of the carrier channel where the phase distortion is proportionately smaller (much less than 1 millisecond). It is therefore not necessary to fix tolerances. In the case of permanent phototelegraph links it will be possible to meet this arrangement in the majority of cases. 771 |S

F ig u r e 40 (Federal German Republic). — Variation o f group delay on a telephone channel o f Y 60 systems, relative to the minimum group delay f = frequency (c/s) mS = group delay (milliseconds) Band of frequencies transmitted 300-3400 c/s SECTION 4

Coexistance of telegraphy and telephony at audio frequencies

T h e I nternational T e l e ph o n e C onsultative C om m ittee Considering that the present techniques enabling telephone and telegraph traffic to be carried in the same cable, either by separate conductors, or on the same conductor; that by these arrangements and taking the precautions indicated below, the telephone circuits, including phantom circuits, are not in practice affected by the telegraphy, either in their electrical characteristics or as concerns their use in traffic; that even when the cable is subject to disturbances from power installations (in particular power lines for alternating current electric railways) a telephone and telegraph service can be obtained free from faults by the use of proved arrangements; that on the other hand the simultaneous use of a long-distance cable for international telephony and telegraphy is to be recommended for economic reasons. Unanimously recommends that from now the simultaneous operation of international telephone and tele­ graph links in the same cable is permitted in principle, either using separate conduc­ tors, or using the same conductor, on condition that the arrangements made for telegraphy do not disturb present or future telephone traffic; that provisionally, in the present state of technology, installations for simulta­ neous or coexistant telegraphy and telephony should satisfy the following conditions:

Simultaneous telegraphy and telephony on the same conductors In order not to prejudice the transmission quality of the telephone circuit, the following conditions must be met:

A. Sub-audio telegraphy (Nil) *.

* The Vlth Plenary Meeting of the C.C.I.T. (Brussels 1948) considered that there was no point in continuing the study of questions concerning sub-audio telegraphy and that they should be abandoned. SIMULTANEOUS TELEGRAPHY AND TELEPHONY 1 6 7

B . SUPER-AUDIO TELEGRAPHY

This arrangement (described in pages 232 to 234 of Volume I bis of the White Book of the C.C.I.F.) provides only one telegraph channel in addition to the telephone channel. It can only be used on lightly loaded or unloaded circuits but is not applicable in the case of carrier operation. In this case Administrations and Private Operating Companies will generally be able by mutual agreement to provide some other arrangement which will provide more than one telegraph channel in addition to the audio telephone channel. The use of super audio telegraphy when necessary must not in any case spoil the transmission quality of the adjacent telephone channel and in particular must not limit the band of frequencies to be effectively transmitted for good speech reproduction (300 to 3400 c/s).

C . S imultaneous t e l e g r a p h y b y p h a n t o m s o r d o u b l e p h a n t o m s

(using for example two quads in multiple twin cables or two quads in star quad cables). 1. The e.m.f. applied by the telegraph transmitting equipment into the cable circuit must not exceed 50 volts. 2. When the terminals of the transmit telegraph equipment are closed with 30 ohms in place of the cable the current in this resistance must not exceed 50 milli- amperes. This limit is increased to 100 milliamperes if the cable is equipped with loading coils having iron dust cores or with other equally satisfactory material. 3. The disturbing noise produced in a telephone circuit by the telegraph equipment must not exceed a value corresponding to a psophometric e.m.f. of 1 millivolt on a 600 ohm circuit at a point of -1.0 neper or -8.7 decibels relative level. To achieve this, it is recommended that low pass filters should be inserted in all telegraph circuits using direct current. It is possible that this limit may be reduced in the case where the telephone circuit is already subject to appreciable disturbance from a neighbouring power line. *. 4. Equipment for simultaneous telegraphy must not introduce unbalance (relative to earth) on the telephone circuits. (Recommendations concerning the protection of telecommunication lines against the harmful effects of industrial electricity lines, 1937 edition, 1938 revision, pages 7 and 8 ). 5. The increases in crosstalk produced by equipment for simultaneous tele­ graphy on telephone circuits must not exceed a value corresponding to a reduction in near end crosstalk attenuation of 0.5 neper (4.34 decibels).

6 . Side circuits of phantom or double phantom circuits used for telegraphy must not be used for programme relays.

* The question of the maximum value of psophometric voltage permissible at the end of an international circuit and of the contributions from various noise sources, is being studied by the C.C.I.F. 168 VOICE FREQUENCY TELEGRAPHY — AUDIO FREQUENCIES

Coexistant telegraphy and telephony on separate conductors 1. The case where telegraphy uses loaded conductors which may later be used for telephony: The conditions given above under C, 1,2, and 3 should be met. 2. The case where unloaded conductors are used for telegraphy: The conditions given above under C, 3 only must be met.

Voice-frequency telegraphy at audio frequencies

The maximum instantaneous voltage produced by the simultaneous application of the frequencies used for the telegraph signals on any circuit, must not exceed that of a sinusoidal signal of 5 milliwatts at a zero relative level point deduced from the level diagram of the telephone circuit. The voice-frequency telegraph sending equipment will in the majority of cases not be connected at the origin of the telephone circuit but at a point where the relative power level h is different from 0. The power of the signal corresponding to the maximum instantaneous voltage allowed at this point is then: Pmax = 5 e2h milliwatts, (h in nepers) The maximum voltage when Z is the impedance of the circuit is then:

Umax = [5.10- 3 e2h Z] 3* volts. In multi-channel voice frequency telegraphy with n frequencies, this voltage Umax will certainly not be exceeded if the effective voltage Uf for any of the frequencies does not exceed the nth part of Umax

1 Uf ~ Umax n If in place of the relative power level h we introduce the relative voltage level hv then the first relation becomes

K = h + log. and

Uf = — [ehv] (/3 volts n Measurements are made when sending each frequency in turn on the circuit by means of a continuous marking signal. Each signal generator is adjusted so that the voltage indicated above is sent for each frequency. The measurement of voltage which is made at the input of the circuit used for multi-channel voice-frequency telegraphy may be made with any suitable voltmeter. If normal transmission measuring equipment is used calibrated in absolute levels, the value of level which must not be exceeded for a system of n channels becomes: Uf hmes = loge = K + 0 . 8 — loge n nepers VOICE FREQUENCY TELEGRAPHY — AUDIO FREQUENCIES 1 6 9

If the absolute voltage level at the input of the circuit used for voice-frequency telegraphy is increased to hv = 0.7 nepers or 6 .1 decibels for example, the following corrections should be made to the measured values: 3-charinel system hmes — 0.7 + 0.8 — loge 3 = + 0.4 nepers or + 3.5 decibels 6 -channel system hmes = 0.7 + 0.8 — loge 6 = — 0.3 nepers or — 2.6 decibels 12-channel system Ames = 0.7 + 0.8 — loge 12 = 1.0 nepers or — 8.7 decibels It is considered unnecessary to check the voltages or powers when the circuit is in use.

REMARK Conditions to be met for circuits used for voice frequency telegraphy

1. For .voice-frequency telegraphy four-wire circuits should preferably be used. The type of loading to be used depends on the number of carrier frequencies to be transmitted; for example for systems with not more than 1 2 channels medium loading will be satisfactory, even for long-distance transmission; whereas for 18 instead of 1 2 channels lighter loading will be required. With two-wire circuits two-directional operation (Duplex) will not be possible, because the circuits cannot be balanced sufficiently accurately to avoid mutual interference between the two directions of transmission. However if only the low frequencies are used for one direction of transmission and the high frequencies for the other direction a two-wire circuit may be used for voice-frequency telegraphy.

2 . The make-up of a four-wire circuit used for voice-frequency telegraphy differt from that of a telephone circuit by the absence of terminating units, signalling equipmen. and echo suppressors. Points A and B (see figure 36) where the changeover of the voice-frequency telegraph circuit and its reserve circuit takes place (and which are conventionally regarded as the origin and extremity of the four-wire circuit used for-voice frequency telegraphy) should be at the same relative level for the two circuits, the levels being determined from the level diagram of the telephone circuit.

N

+o,8

+ o,6 + o,S * o,H % + o,3 e + o,8 Hz A) -o ,8 r<9 ; V77777777777777777777777777777777777777777777777, - o ,k .

- o,6 .

F ig u r e 41 Graph No. 6. — Limits for the variation with frequency, relative to the value at 800 c/s, of the relative power levels (in nepers) between the origin and extremity o f a circuit used for voice frequency telegraphy (telephone circuit using the band 300-2600 c/s) 1 7 0 TELEGRAPHY AND TELEPHONY — AUDIO FREQUENCIES N

+o,SS. < //////////////////////////////////////////////////, + 0,5. -<8> Hz +o,3& ♦ 0 .8 . VA m m z

W'- //, i © © i | «oOJ

F ig u r e 4 2 Graph No. 7. — Limits for the absolute power level (in nepers), jOr maintenance tests, at the output of a frontier repeater (frontier side) for an international circuit using the band 300- 2600 cjs used for voice frequency telegraphy (when a power corresponding to 1 milliwatt at a point of zero relative level, deduced from the level diagram o f the telephone circuit, is sent at the origin of the voice-frequency telegraph circuit

The relative level at point A must not exceed —0.4 neper. The relative level at point B must be at least +0.4 nepers.

(a ) Graph No. 6 (figure 41) shows the variations with frequency of the difference between the relative power levels at the origin and extremity of the circuit (points A and B) relative to the nominal value at 800 c/s. (b ) The limits allowed for the relative power level at the output of a frontier repeater are the same as those allowed for a 4-wire telephone circuit, when sending a power at the origin of the telegraph circuit, corresponding to 1 milliwatt at a point of zero relative level determined from the level diagram of the telephone circuit. These limits are shown on graph No. 7 (figure 42). It does not appear necessary to fix particular limits for the variations with frequency of the level measured at the output of the frontier repeater since these may be calculated easily from the limits allowed for the relative power level. (c) The relative power level at the point at the receiving end where the changeover between the voice-frequency telegraph circuit and its reserve circuit takes place must be as constant as possible with time. Furthermore any interruption in the circuit, even for a very short duration, spoils the quality of the telegraph transmission. Great care must therefore be taken when measurements are made on circuits and repeaters, when changing over batteries etc. To draw the attention of the staff to this matter, it is desirable for circuits used for voice-frequency telegraphy to be specially marked at the terminals and in the intermediate repeater stations. (d) It is desirable to make special arrangements to avoid any intermodulation on the circuits and in the repeaters. Such intermodulation may in particular be caused by variation in battery voltages or by the connection of equipment for sub audio telegraphy to the cable pairs. PART 3

INTERNATIONAL PROGRAMME TRANSMISSIONS

Use of international telephone cables for relays of broadcast programme

T h e I nternational T el e ph o n e C onsultative C om m ittee, Considering that sound broadcasting has become an important part of social life: that experience has shown that the exchange of broadcast programmes on a large scale is best affected by employing long distance cable circuits capable of transmitting effectively all the frequencies essential for good reproduction of music since such circuits are not subject to atmospheric disturbances which frequently affect radio transmissions; that relays of broadcast programmes by means of international cable circuits present, from the economic viewpoint, the advantages of allowing the transmission from a number of transmitting stations of a programme of excellent artistic quality and consequently expensive, or a discussion or a conference of wide general interest; Makes the following unanimous recommendation that it is desirable that relays of broadcast programmes are effected by “ pro­ gramme circuits ” in international cables: A distinction is made between: 1° “ Circuits of the old type ” intended to transmit programmes to broad­ casting stations when it is not desired to reproduce faithfully all the essential charac­ teristics of the various musical instruments: the characteristics of these circuits are given in section 1 below. 2° “ Normal circuits for broadcast programmes ” intended to transmit program­ mes to broadcasting stations when it is desired to reproduce faithfully the essential characteristics of the instruments; the characteristics of these circuits are given in section 2 below. SECTION 1

Old-type programme circuits

Technical responsibilities during an international broadcast programme. Defi­ nition of the constituent parts of an international programme connection. — In order to apportion responsibility during the transmission of a broadcast programme there is occasion to distinguish (see figure 43 below): (a) the Broadcast Authority which is the source of the programme (studio or outside broadcast point or programme switching centre) and which, in the figure is at some distance from the repeater station at Edinburgh; (b) the outgoing local line, which connects the broadcast authority to the first repeater station. (c) “ the (long-distance) international programme fine ” consisting in principle of a chain of national and international programme circuits, the national circuits being of the same type as if they were international circuits. In the figure this long distance line is “ EDINBURGH—MESTRE ”, and consists of the national circuit EDINBURGH—LONDON, the international circuit LONDON—PARIS, the na­ tional circuit PARIS—LYON, the international circuit LYON—TORINO and the national circuits TORINO—MILANO and MILANO—MESTRE; (d) the incoming local line, which connects the last repeater station to the receiving broadcast authority; (e) the receiving broadcast authority for which the programme is intended and which in the figure is at VENEZIA some distance from MESTRE. The assembly of the long-distance international programme circuit and the local lines constitute the “ international programme link The international programme line is, in all cases, the sole responsibility of the Telephone Administrations. The local lines may be the responsibility of either the Telephone Administration, the Broadcast Authority or the two together according to arrangements in each country.

Definitions of the origin and extremity of an international circuit. — The “ origin ” of an international circuit is considered as the output of the first amplifier and the extremity as the output of the last amplifier of the circuit. In the case of figure 43 the direct circuit London-Paris, for example, is comprised between the points C and D. In the case of a circuit on a carrier system for programme transmissions, the origin of the circuit is the input of the modulating equipment and the distant terminal is the output of the demodulating equipment. Broadcasting Broad­ authority Terminal Outgoing Transit Incoming casting receiving the repeater country country country authority Local programme station transmit­ line ting the programme EDINBURGH LONDON PARIS LYON TORINO MILANO B KC I KD I KF

BBC RAI

cSuGjMe (a^tcm de jdistam e) zaxjio^Monicj/ue uAtem aim tafe ( <£ntem m ticm a£ p/tocj/zam wie £UtzJ

<=£iaisovi zaduyj/iRtynuj/uje. wk/uM hxnAaf& (eUdj&amUcmjafp/tocpzcuMme HwM)

Volume meter or peak meter. Equalizer. 0 m

F ig u r e 43. — Diagram of an international programme link 1 7 4 PROGRAMME TRANSMISSIONS — OLD-TYPE CIRCUITS

Hypsogramme o f an international programme circuit. — In the case of a programme circuit on a special pair, the hypsogramme of the circuit should be arranged in such a way that it is possible to use the maximum power that an amplifier can transmit without distortion when a power of 8 milliwatts [(corresponding to an ab­ solute level of + 1.04 neper (+ 9 decibels)] is applied to a point of zero relative level of the circuit. With these conditions, the nominal value of the relative level at the output of each amplifier of the international programme circuit is fixed provisionally at + 0.7 nepers at + 6 decibels. In effect is has been recommended that the amplifiers of the programme circuit should be able to transmit a maximum power of 50 milliwatts (corresponding to an absolute level of + 1 7 decibels). On the other hand, as stated above, the power applied at a point of zero relative level of the circuit may attain 8 milliwatts (absolute level of power + 1.04 nepers (+ 9 decibels). Thus the relative level of power at the output of an amplifier will be+17 — 9 = + 8 decibels or + 0.9 nepers; but if the maximum variation of this level as a function of time of ± 0 . 2 neper or ± 2 decibels is taken into account the nominal relative level at the output of the amplifier is 0.7 nepers or + 6 decibels.

Band o f frequencies effectively transmitted. — In the case when an old-type programme circuit is used, the band of frequencies effectively transmitted by the complete link relaying the transmitted programme should be from 50 to 6400 c/s at least. For a frequency to be considered as effectively transmitted the equivalent at this frequency should not exceed the equivalent at 800 c/s by more than 0.5 neper or 4.3 decibels.

Attenuation distortion of the international circuit. — The limits allowed for the variation with frequency of the relative level at the output of an amplifier are given on the graphs Nos. 8 and 9 figures 44 and 45. The curve of the relative level at the output of an amplifier should lie in the unshaded area. The limits of this area may be displaced by a movement of the whole upwards or downwards by an amomnt equal to the difference between the actual relative level at 800 c/s and 0.7 neper or 6.1 decibels (if this relative level differs from 0.7 neper or 6.1 decibels) but less than the allowable tolerance of 0.2 neper or 1.7 decibels for a non-frontier station or 0.1 neper or 0.9 decibels for a frontier station. The relative level is equal to the absolute voltage level measured at the point considered when a voltage, constant at all frequencies, is applied at the origin of the circuit. If the output impedance of the first amplifier is not negligible compared with that of the line then an electromotive force, constant at all frequencies, may be applied to the line by means of a generator having an internal impedance equal to the nominal output impedance of the first amplifier.

Phase distortion. — The phase distortion of the international circuit (or chain of interconnected circuits) used for programme transmissions should be such that the difference between the time of propagation does not exceed the following values: PROGRAMME TRANSMISSIONS — OLD-TYPE CIRCUITS 17 5

non-frontier station

0,9 0,8 ------ky------o,6 0,5 ^ / zzz/ z/// z z z z /// a V /7 /////////////; o,tf y z z /z z z z f , o,3

o, 2 I W zzzzz 7 %z , VZZZ/ 0,1 'V i V 1 i l so wo Zoo Uoo 8oo 1600 32do 5000 6koo Frequency c/s

F ig u r e 44 Graph No. 8 (non frontier station). — Allowable limits for the variations, as a function o f frequency, o f the relative voltage level on an old-type programme circuit.

non-frontier station

0,8 O’? o,6 v /zw //////////////, 0,5 // :_____ v* W 7 7 7 7 7 7 OM

50 too BOO Uoo 8oo 1600 3800 6U00 Frequency c/s

F ig u r e 45 (Graph No. 9 (frontier station). — Allowable limits for the variation, as a function of frequency, of the relative voltage level on an old-type programme circuit

— between the group propagation time at 50 c/s and the group propagation time at 800 c/s, 70 milliseconds; — between the group propagation time at 6400 c/s and the group propagation time at 6400 c/s, 10 milliseconds. Note. — The “ group propagation time ” in question is the differential with respect to the angular velocity of the phase change (of the circuit or chain of circuits) for the frequency / (the angular velocity co is the product of the frequency 2n). 1 76 PROGRAMME TRANSMISSIONS — OLD-TYPE CIRCUITS

This “ group propagation time ” is the time taken to travel along the circuit (or chain of circuits) of the peak of the envelope of a group of two sinusoidal waves of angular frequency very close

Noise. — (a) psophometric voltage. — The psophometric voltage measured at the extremity of the circuit (point where the nominal relative level has been fixed at + 0.7 neper) with the psophometer for music circuits, * with a high impedance input, the circuit being closed at its extremity by a pure resistance of 600 ohms and at the origin by a pure resistance equal to the modulus of the nominal impedance of the first amplifier should not exceed 6.2 millivolts for cable circuits and 15.6 millivolts for open wire lines. This results in the ratio between the “ maximum voltage ** and the psopho­ metric voltage (circuit noise and crosstalk) being at least 710/1 (6.55 nepers or 57 decibels) for cable circuits and at least 283/1 (5.65 nepers or 49 decibels) for open wire lines. (b) Unweighted voltage. — The voltage due to noise measured objectively without a weighting network at the extremity of the international circuit at the point Pn should not exceed 62 millivolts for cable circuits and 156 millivolts for open wire lines. This measurement should cover the frequency band from 30 to 20,000 c/s: it is useful to make sure that there is no danger of saturation or parasitic modulation.

Note. — In fixing these values the dynamic ratio has to be taken as 4.6 neper (40 decibels).

Intelligible crosstalk. — Provisional form: (a) The near or far-end crosstalk ratio (for the voice) between two interna­ tional programme circuits of the old type or between one such circuit and all other circuits for programme relays, should be at least 8.5 nepers or 74 decibels for cable circuits and at least 7.0 nepers or 61 decibels for open wire lines. (b) The near or far-end crosstalk ratio (for the voice) between a telephone circuit disturbing circuit) and an international programme circuit of the old type (disturbed circuit) should be at least 8.5 nepers or 74 decibels for cable circuits and at least 7.0 nepers or 61 decibels for open wire lines. (c) The near or far-end crosstalk ratio (for the voice) between an international programme circuit of the old type (disturbing circuit) and a telephone circuit (dis­ turbed circuit) should be at least 6.7 nepers or 58 decibels for cable circuits and at least 5.4 nepers or 47 decibels for open wire lines.

Non-linear distortion. — Provisionally the harmonic margin measured at the extremity of the international circuit, when a sinusoidal wave at any frequency within the band of frequencies transmitted is applied at the origin of the circuit (where the nominal relative level is 0.7 nepers) at a power of 4 X 8 = 32 milliwatts, should

* The psophometer should be equipped with a special weighting network, the characteristics of which are given in Volume IV of the Green Book, section 3.2.2 paragraph B. ** The “ maximum voltage ” at a point in a circuit is the RMS voltage at this point when a sinusoidal wave with a power of 8 milliwatts is applied at a point of zero relative level of the inter­ national circuit. PROGRAMME TRANSMISSIONS — OLD-TYPE CIRCUITS 177 be at least equal to 2.3 nepers (20 decibels). The Broadcast Authorities have indi­ cated that the effects of non-linear distortion are already perceptible in a broadcast transmission when the harmonic margin is 3.2 nepers or 28 decibels.

Line-up and supervision of the international programme link

It is assumed that the international connection corresponds to that shown in figure 43 above. It is also assumed that the various circuits to be connected to form the international connection are permanent circuits which are regularly maintained. Measurements to be made before the “ regulating period ” which precedes a programme transmission. — The local lines should be adjusted in such a way that when they are connected to the long-distance international programme line the hypsogramme of the international programme link shall be met. * For example, in figure 43 the station EDINBURGH effects the equalization and line-up for the local line from the British Broadcasting Corporation (B.B.C.) Measurements to be made during the regulating period which precedes a programme transmission. — The C.C.I.F. recommends the use of the line-up method called “ constant voltage ”. **) After the connection of the various circuits to form the international programme link (conforming to the hypsogramme of these circuits) it is sufficient to verify, by means of a hypsograph or by measurements at discreet frequencies that the relative voltage at the distant incoming repeater station has the correct value at the following frequencies:— — for an old-type c irc u it...... 50,800 and 6,400 c/s. Also, and only if the control stations asks, a measurement of the psophometric noise is made at the distant incoming repeater station. These preliminary adjustments having been made the local lines are connected to the long distance international programme circuit at the terminal repeater stations. This is the end of the “ line up period ” and the commencement of the “ preparatory period ” which corresponds to the instant when the complete connection is placed at the disposal of the Broadcast Authorities. The latter then preceeds to measure and adjust as necessary. Measurements to be made by the Broadcast Authorities during the “ preparatory period ”. — After the Broadcast Authority has taken possession of the international

* According to the definition of the hypsogramme of an international circuit it follows that a sine wave of maximum amplitude equal to the peak voltage (at a specified point) transmitted by the studio has a nominal absolute voltage level of 1,04 neper (9 db) at a point of zero relative level on the long-distance international programme circuit. ** If certain Administrations or Private Operating Companies have programme amplifiers which are not suitable for use of line-up by the constant voltage method there is no objection to using the method of equalization of constant electromotive force—even though it may cause incon­ venience from the point of view of maintenance—provided that these Administrations and Private Operating Companies take the necessary arrangements at frontier stations to make the change from the constant electromotive force method to the constant voltage method recommended by the C.C.I.F. Nevertheless hew amplifiers to be installed on programme circuits should be designed with a view to equalization on a constant voltage method.

12 178 PROGRAMME TRANSMISSIONS — OLD-TYPE CIRCUITS connection, it makes measurements on the complete link in the band of frequencies effectively transmitted, from the point where the programme is picked up to the point where the programme is received. It is desirable to recommend to the Broadcast Authorities when making mea­ surements, to apply at the origin of the international programme link (point A of figure 43 above) a sine wave of which the maximum amplitude should be 9 decibels or 1 neper below that of the peak voltage (i.e. the instantaneous voltage maximum that should never be exceeded at this point in the course of a programme transmission) and if necessary verify that the nominal output voltage level at each repeater is T- 6 decibels or + 0.7 neper i.e. the absolute level of zero voltage at a point of zero relative level of the international programme link. It is not necessary to re-adjust the output level of intermediate repeaters since these have already been set during the line up period.

Note. — The numerical values given above ensure that during the programme transmission the peak voltage at a point of zero relative level will not exceed the amplitude of a sine wave having a power of 8 milliwatts. The reasons for which during this final line-up a voltage 9 decibels or 1 neper below the peak voltage is applied at point A are: (a) It is not desirable to overload the terminal equipment of carrier systems by transmitting continuously a test signal corresponding to the peak voltage which is only attained momentarily during the transmission of the programme proper. (b) If Administrations and Private Operating Companies make their initial and maintenance measurements with a nominal absolute voltage of + 6 decibels or + 0.7 neper at the repeater output, it is useful to use the same voltage, if it is ne­ cessary to check during the preparatory period.

Supervision. — At the end of the preparatory period the transmission of the programme proper commences and is monitored at the studio, in the repeater stations and at the transmitter. This supervision is made with a speech level meter. One of the instruments, of which the characteristics are summarized in the table below, may be used. Annexes 23 and 24 of the Book of Annexes to Volume III of the Green Book. set out the advantages and disadvantages of the instruments which are of two types of which the most representative are:— — a speech level meter of which integration and return to zero times are appro­ ximately the same and of the order of 165 milliseconds;

— a peak indicator of which the integration time is very short (less than 2 0 milliseconds) and of which the return to zero time is rather long (of the order of 2 seconds). , Annex 25 of the Book of Annexes to Volume III of the Green Book describes the modulation factor meter used by the Bristol Administration and the British Broadcasting Corporation in agreement. Since are is not a simple and univocal correlation between simultaneous readings of the two very different types of apparatus for all types, of programmes, it is desirable Principal characteristics o f the various instruments used for monitoring the volume or peaks during telephone conversations or programme transmissions

Rectifier Time to reach Integration time Time to return to zero Type of instrument characteristic 99 % of final (milliseconds) (value and definition) CIRCUITS OLD-TYPE — TRANSMISSIONS PROGRAMME (Note 1) reading

“ Speech Voltmeter ” British type 3 (S.V.3) iden­ tical to the speech power meter of the A.R.E.A.N. 2 230 100 (approx.) equal to the integration time

V.U. meter. (United States of America) 1.0 to 1.4 300 165 (approx.) equal to the integration time

Speech power meter of the type “ Volume indica­ 2 around 200 equal to the integration time tor ” S.F.E.R.T.-C.C.I.F. 400 to 650

Peak indicator for programme transmissions 1 around 12 4 3 seconds for the pointer to fall 26 db. used by the British Broadcasting Corporation (B.B.C. Peak Programme meter)

Maximum amplitude indicator used by the Fede­ 1 around 80 5 (approx.) 1 or 2 seconds from 100% to 10% of ral German Republic (type U. 21) the reading in the permanent state.

*) The number given in the column is the index n in the formula. [V(output) = V« (input)] applicable for each half cycle. **) The “ integration time ” has been defined by the C.C.I.F. as the “ minimum period during which a sinusoidal voltage should be applied to the instrument so that the pointer attains to within 0.2 neper or nearly 2 db the deflection which would be obtained if the voltage were applied indefinitely ”. A logarithmic ratio of 2 db corresponds to a percentage of 79.5% and a ratio of 0.2 neper to a percentage of 82 %.

SO< 1 18 0 PROGRAMME TRANSMISSIONS — OLD-TYPE CIRCUITS

that the Broadcast Authority at the studio and the Telephone Administration pro­ viding the programme circuit should use the same apparatus in order to speak the same language. In general the Telephone Administration and the Broadcast Authority of a country agree to use the same type of apparatus. It is desirable to reduce to a minimum the number of different types of instruments and to discourage the intro­ duction of new types which only differ in detail from those already in service. During the programme transmission one should monitor at the point A (output . of the last amplifier under the control of the Broadcast Authority sending the programme) to ensure that the “ peak voltage ” (taking into account the type of programme) adopted during the line-up of the complete link is not exceeded. There is occasion to recall that the variation of the musical sounds from an orchestra is of the order of 60 to 70 decibels whilst the specification for circuits for programme transmission is based on a variation of around 40 decibels. A compression of the “ dynamic ratio ” of the programme is therefore necessary at the output from the studio before passing it over the programme circuit.

Lines \ Special pairs for sound broadcasting. — See the specifications Al, A ll and Alll in the first part of the present volume section 5.4.1.

Amplifiers

Type. — The amplifier should be equipped with valves and the frequencies effectively transmitted should extend at least from 50 c/s up to a frequency equal to 0.7 of the cut-off frequency, but for loaded cable this should be below 10,000 c/s. For open wire lines the repeater should be capable of transmitting effectively the frequencies up to 1 0 , 0 0 0 c/s. Amplification. — The amplifier and its associated equipment should compensate as much as possible for the attenuation of the preceding repeater section. Thus, in the band of frequencies effectively transmitted, the gain should rise with frequency in such a manner that the distortion produced by the line is cancelled. The gain should be adjustable in steps not greater than 0.1 neper. In all the band of frequencies effectively transmitted the amplification curves (as a function of frequency) should be parallel for all positions of the gain regulator. When the power supply is regularly maintained the voltage variations at the supply terminals of the amplifier should not cause changes of gain exceeding 0.03 neper (0.26 decibel). PROGRAMME TRANSMISSIONS — OLD-TYPE CIRCUITS 181

Impedance. — The relation between the input and output impedance of the amplifier and the line to which it is connected should be such as to avoid detrimental reflections as much as possible. It might be an advantage to make the output impedance low in relation to the line impedance in order to reduce the harmonic distortion co-efficient of the amplifier and to simplify the connection of amplifiers and lines of different impedance. In this case the input impedance of the amplifier should be matched as far as possible to the impedance of the line to avoid detrimental reflections. Monitoring arrangements. — A monitoring facility should be provided before and after the amplifier to allow supervision of the programme. This facility should have a high impedance so as not to reduce the gain (insertion loss) by more than 0.03 neper (0.26 decibel). Crosstalk. — The crosstalk ratio measured in conditions of service should be at least equal to 10 neper or 87 decibels between two amplifiers used for programme transmissions or between these amplifiers and a repeater used for ordinary telephony. When these measurements are made the amplifier should be terminated with impedance corresponding to those of the lines used in service. For general conditions for the measurement of crosstalk see the Book of Annexes to Volume III of the Green Book, 2nd part, section 1.9.1. Output Power. — The maximum undistorted output power of the amplifier should be approximately 50 milliwatts. Absence of noise. — The absolute level of disturbing noise should be at least 8.65 neper or 75 decibels below the absolute level of the “ maximum voltage” which corresponds to a psophometric voltage approximately equal to one five thousand six hundredth part of the “ maximum voltage ”. Non-linear distortion. — The harmonic margin at maximum power should be at least 3.2 neper or 28 decibels at any frequency in the band of frequencies effectively transmitted. Furthermore the gain should not vary more than 0.1 neper or 0.9 decibel at all frequencies effectively transmitted when the output power is varied from 1 to 50 milliwatts. SECTION 2

Normal circuits for programme transmissions

For the transmission of a high quality musical programme or of speech in different languages it is not desirable to use a programme circuit of the old type. On the other hand the wide frequency band available on modern high velocity lines enables the provision normally of programme circuits of improved quality. These “ normal circuits for programme transmissions ” should satisfy the following conditions:

Technical responsibilities during an international broadcast programme. Defi­ nition of the constituent parts of an international programme connection. — In order to apportion responsibility during the transmission of a broadcast programme there is occasion to distinguish (see figure 43 above):— (a) the Broadcast Authority which is the source of the programme (studio or outside broadcast point or programme switching centre) and which, in the figure is at some distance from the repeater station at Edinburgh; (b) the outgoing local line, which connects the broadcast authority to the first repeater station. (c) “ the (long-distance) international programme line ” consisting in principle of a chain of national and international programme circuits, the national circuits being of the same .type as if they were international circuits. In the figure this long distance line is “ EDINBURGH-MESTRE ”, and consists of the national circuit EDINBURGH-LONDON, the international circuit LONDON—PARIS, the na­ tional circuit PARIS-LYON, the international circuit LYON-TORINO and the national circuits TORINO-MILANO and MILANO-MESTRE; (d) the incoming local line, which connects the last repeater station to the receiving broadcast authority; (e) the receiving broadcast authority for which the programme is intended and which in the figure is at VENEZIA some distance from MESTRE. The assembly of the long-distance international programme circuit and the local lines constitute the “ international programme link ”. The international programme line is, in all cases, the sole responsibility of the Telephone Administrations. The local lines may be the responsibility of either the Telephone Administration, the Broadcast Authority or the two together according to arrangements in each country. PROGRAMME TRANSMISSIONS — NORMAL CIRCUITS 183

Definitions of the Origin and the Extremity of an international circuit. — The ■ “ origin ” of the circuit is at the output of the first repeater and the “extremity” is at the output of the last repeater. In the case of figure 43 the circuit London-Paris for example, will comprise the parts between the points Cand D. In the case of a circuit for programme transmissions on a carrier system the origin of the circuit is the input of the modulating equipment and the extremity is the output of the demodulating equipment.

Provisional general characteristics of normal type circuits for programme transmissions

The provisional general characteristics given below apply to normal type programme circuits whatever the constitution of such circuits; a length of 1 0 0 0 kilometres has been taken. *

Hypsogramme of an international programme circuit. — In the' case of a pro­ gramme circuit (normal type) on a special pair, or on the phantom of non loaded pairs, the hypsogramme of the circuit should be arranged in such a way that it is possible to use the maximum power that an amplifier can transmit without dis­ tortion when a power of 8 milliwatts [corresponding to an absolute level of + 1.04 neper ( + 9 decibels)] is applied to a point of zero relative level of the circuit. With these conditions, the nominal value of the relative level at the output of each amplifier of the international programme circuit is fixed provisionally at + 0.7 neper at + 6 decibels. In effect it has been recommended that the amplifiers of the programme circuit should be able to transmit a maximum power of 50 milliwatts (corresponding to an absolute level of + 17 decibels). On the other hand, as stated above, the power applied at a point of zero relative level of the circuit may attain 8 milliwatts [absolute level of power + 1.04 nepers (+ 9 decibels)]; thus the relative level of power at the output of an amplifier will b e + 17 — 9 = + 8 decibels or + 0.9 neper; but if the maximum variation of this level as a function of time of ± 0 . 2 neper or ± 2 decibels is taken into account the nominal relative level at the output of the amplifier is 0.7 neper or + 6 decibels. If the international programme circuit (normal type) is routed over a 12-circuit group of a carrier system it is provisionally recommended that the “ point of zero relative level of the international programme circuit ” should coincide with the the point of zero relative level as deduced from the hypsogramme of the telephone channels of this 1 2 -circuit group. Nevertheless it might be advantageous to be able to adjust the equipment to a maximum difference of + 0.35 nepers ( ± 3 decibels) between the relative level for programme transmission and for telephone transmission in order to obtain the best balance with regard to noise and intermodulation. In order to facilitate interconnection, the nominal value of the relative level at the points P1 and Pn is also provisionally fixed at + 0.7 nepers or + 6 decibels in the case of a circuit routed over a carrier system.

The revision of these limits, for a longer circuit, is being studied by the C.C.I.F. 1 8 4 PROGRAMME TRANSMISSIONS — NORMAL CIRCUITS

Band of frequencies effectively transmitted. — In the case, of a normal type programme circuit, the band of frequencies effectively transmitted by the complete link should extend from 50 to 10,000 c/s at least. For a frequency to be considered as effectively transmitted the 'equivalent at this frequency should not be greater than the equivalent at 800 c/s by more then 0.5 neper or 4.3 decibels. Attenuation distortion o f the international circuit. — Graph No. 10 of figure 46 shows the limits allowed for the variation, as a function of frequency (relative to the value measured at 800 c/s/ of the absolute level of voltage measured at the end of the circuit (output of last repeater) when a voltage, constant at all frequencies is applied at the origin of the circuit (output of the first repeater). The absolute level of voltage measured in this condition is equal to the relative voltage at the extremity of the circuit. If the output impedance of the first amplifier is not negli­ gible compared to that of the fine then an electromotive force, constant at all fre­ quencies, may be applied to the line by means of a generator having an internal impedance equal to the nominal output impedance of the first amplifier.

N + 0.8

50 h10 8tio 8 oo 6000 10000 jex i » 85 0 0 ■f - o,t ■

- 0,8

- o,3.

-

- 0 , 5 1 / 6 z ./

F ig u r e 46 Graph No. 10. — Variation, as a function o f frequency, o f the relative voltage level at the extremity (output o f last amplifier o f the normal type programme circuit) f = frequency (c/s) N = variation of level (nepers)

Phase distortion. — The index of the phase distortion (or difference between the group propagation time t (f) for the frequency considered and for the frequency corresponding to the minimum group propagation time) should not exceed the values in the table below.

*10 ooo tm{n ...... less than 8 milliseconds tioo rm in less than 2 0 milliseconds 50 tm in less than 80 milliseconds PROGRAMME TRANSMISSIONS — NORMAL CIRCUITS 185

Note. — These values have been fixed from experience acquired with audio circuits. Better values may be obtained with normal type programme circuits routed on carrier systems if the number of modulations and demodulations is not excessive.

Noise. — (a) Psophometric voltage. — The psophometric voltage measured at the extremity of the international circuit (point where the nominal relative level has been fixed at + 0.7 nepers) .with the psophometer for music circuits, * with a high impedance input, the circuit being closed at its extremity by a pure resistance of 600 ohms and the origin by a pure resistance equal to the modulus of the nominal impedance of the first amplifier, should not exceed 6 . 2 millivolts for cable circuits and 15.6 millivolts for open wire lines. This results in the ratio between the “ maximum voltage ** and the psophometric voltage (circuit noise and crosstalk) being at least 710/1 (6.55 nepers or 57 decibels) for cable circuits and at least 283 /1 (5.65 nepers or 49 decibels) for open wire lines. (h) Unweighted voltage. — The voltage due to noise measured objectively without a weighting network at the extremity of the international circuit should not exceed 62 millivolts for cable circuits and 156 millivolts for open wire lines. This measurement should cover the frequency band from 30 to 20,000 c/s: it is useful to make sure that there is no danger of saturation or parasitic modulation.

Note. — In fixing these values the dynamic ratio has to be taken as 4.6 neper (40 decibels).

Intelligible crosstalk. — Provisional form: (a) The near or far-end crosstalk ratio (for the voice) between two international programme circuits of the normal type or between one such circuit and all other circuits for programme relays, should be at least 8.5 nepers or 74 decibels for cable circuits and at least 7.0 nepers or 61 decibels for open wire lines. (b) The near or far-end crosstalk ratio (for the voice) between a telephone circuit (disturbing circuit) and an international programme circuit of the normal type (disturbed circuit) should be at least 8.5 nepers or*74 decibels for cable circuits and at least 7.0 nepers or 61 decibels for open wire lines. (c) The near or far-end crosstalk ratio (for the voice) between an international- programme circuit of the normal type (disturbing circuit) and a telephone circuit (disturbed circuit) should be at least 6.7 nepers or 58 decibels for cable circuits and at least 5.4 nepers or 47 decibels for open wire lines. The C.G.I.F. draws the attention of Administrations to the fact that when it is desired to use simultaneously, in the two directions, programme circuits routed on carrier systems and using the same position in the frequency spectrum (this being

* The psophometer should be equipped with a special weighting network, the characteristics of which are given in Volume IV of the Green Book, section 3.2.2 paragraph B. ** The “ maximum voltage ” at a point in a circuit is the R.M.S. voltage at this point when a sinusoidal wave with a power of 8 milliwatts is applied at a point of zero relative level of the inter­ national circuit. 18 6 PROGRAMME TRANSMISSIONS — NORMAL CIRCUITS

the most economical arrangement) special precautions may have to be taken to obtain the above crosstalk limits between go and return programme circuits. This is due to the crosstalk which may be produced in the terminal modulating equipments and in the line equipment.

Variation of relative level as a function of time. — In addition to satisfying the limits for attenuation distortion, the relative level at 800 c/s at the extremity of the international circuit should not vary during the course of a transmission from its nominal value more than ± 0 . 2 nepers (or ± 2 decibels). Furthermore, for programme circuits routed on special pairs or on phantom circuits of unloaded cable, the level at the output of the frontier amplifier measured at 800 c/s should not vary during the course of a transmission from its nominal value more than ± 0 . 1 neper (or ± 1 decibel).

Non-linear distortion, Provisionally the harmonic margin measured at the extremity of the international circuit, when a sinuosidal wave at any frequency within the band of frequencies transmitted is applied at the origin of this circuit (where the nominal relative level is 0.7 nepers) at a power of 4 x 8 = 32 milliwatts, should be at least equal to 2.3 nepers (20 decibels) (provisional limit fixed from expe­ rience obtained with programme circuit of the old type). The Broadcast Authorities have indicated that the effects of non linear-distortion are already perceptible in a broadcast transmission when the attenuation of the harmonic distortion is 3.2 neper or 28 decibels.

Note. — In the case of programme circuits routed on carrier systems, this non­ linear distortion is almost entirely due to the terminal equipments and it is possible, in the future, that a higher value for the harmonic margin than the limit of 2.3 nepers (given only as an indication) may be obtained.

Line-up and supervision of the international programme link

It is assumed that the international connection corresponds to that shown in figure 43 above. It is also assumed that the various circuits to be connected to form the international connection are permanent circuits which are regularly maintained.

Measurements to be made before the “ regulating period ” which precedes a programme transmission. — The local lines should be adjusted in such a way that when they are connected to the long-distance international programme line the hypsogramme of the international programme link shall be met. *) For example, in figure 43 the station EDINBURGH effects the equalization and line-up for the local line from the British Broadcasting Corporation (B.B.C.).

*) According to the definition of the hypsogramme of an international circuit it follows that a sine wave of maximum amplitude equal to the peak voltage (at a specified point) transmitted by the studio has a nominal absolute voltage level of 1.04 nepers (9 db) at a point of zero relative level on the long-distance international programme circuit. PROGRAMME TRANSMISSIONS — NORMAL CIRCUITS 18 7

Measurements to be made during the regulating period which precedes a programme transmission. — The C.C.I.F. recommends the use of the line-up method called “ constant voltage ”. ** After the connection of the various circuits to form the international programme link (conforming to the hypsogramme of these circuits) it is sufficient to verify, by means of a hypsograph or by measurements as discreet frequencies that the relative voltage at the distant incoming repeater station has the correct value at the following frequencies:— — for a normal type circu it...... 50,800 and 10,000 c/s. Also, and only if the control stations asks, a measurement of the psophometric noise is made at the distant incoming repeater station. These preliminary adjustments having been made, the local lines are connected to the long-distance international programme circuit at the terminal repeater stations. This is the end of the “ line-up period ” and the commencement of the “ preparatory period ” which corresponds to the instant when the complete connection is placed at the disposal of the Broadcast Authorities. The latter then proceed to measure and adjust as necessary.

Measurements to be made by the Broadcast Authorities during the “ preparatory period ”. — After the Broadcast Authority has taken possession of the international connection, it makes measurements on the complete link in the band of frequencies effectively transmitted, from the point where the programme is picked up to the point where the programme is received. It is desirable to recommend to the Broadcast Authorities when making mea­ surements, to apply at the origin of the international programme link (point A of figure 43 above a sine wave of which the maximum amplitude should be 9 decibels or 1 neper below that of the peak voltage (i.e. the instantaneous voltage maximum that should never be exceeded at this point in the course of a programme transmission) and if necessary verify that the nominal output voltage level at each repeater is + 6 decibels or + 0.7 nepers i.e. the absolute leyel of zero voltage at a point of zero relative level of the international programme link. It is not necessary to re-adjust the output level of intermediate repeaters since these have already been set during the line up period.

Note. — The numerical values given above ensure that during the programme transmission the peak voltage at a point of zero relative level will not exceed the amplitude of a sine wave having a power of 8 milliwatts. The reasons for which during this final line-up a Voltage 9 decibels or 1 neper below the peak voltage is applied at point A. are:

** If certain Administrations or Private Operating Companies have programme amplifiers which are not suitable for use of line-up by the constant voltage method there is no objection to using the method of equalization of constant electromotive force—even though it may cause incon­ venience from the point of view of maintenance providing that these Administrations and Private Operating Companies take the necessary arrangements at frontier stations to make the change from the constant electromotive force method to the constant voltage method recommended by the C.C.I.F. Nevertheless new amplifiers to be installed on programme circuits should be designed with a view to equalization on a constant voltage method. oo 0 0

Principal characteristics o f the various instruments used for monitoring the volume or peaks during telephone conversations or programme transmissions

Rectifier Time to reach Integration time Type of instrument * characteristic 99 % of final (milliseconds) Time to return to zero (Note 1) reading ** (value and definition) CIRCUITS NORMAL — TRANSMISSIONS PROGRAMME

“ Speech Voltmeter ” British type 3 (S.V.3) iden­ tical to the speech power meter of the A.R.E.A.N. 2 230 100 (approx.) equal to the integration time

V.U. meter. (United States of America) 1.0 to 1.4 300 165 (approx.) equal to the integration time

Speech power meter of the type “ Volume indica­ 2 around 200 equal to the integration time tor ” S.F.E.R.T.-C.C.I.F. 400 to 650 •

Peak indicator for programme transmissions 1 around 12 4 3 seconds for the pointer to fall 26 db. used by the British Broadcasting Corporation (B.B.C. Peak Programme meter)

Maximum amplitude indicator used by the Fede­ 1 around 80 5 (approx.) 1 or 2 seconds from 100% to 10% of ral German Republic (type U. 21) the reading in the permanent state.

* The number given in the column is the index n in the formula. [V. (output) = Ve (input)[ applicable for each half cycle. ** The “ integration time ” has been defined by the C.C.I.F. as the “ minimum period during which a sinusoidal voltage should be applied to the instrument so that the pointer attains to within 0.2 nepers or nearly 2 db the deflection which would be obtained if the voltage were applied indefinitely ”. A logarithmicratio of 2 dbcorresponds to a percentage of 79.5% and a ratio of 0.2 nepers to a percentage of 82 %. PROGRAMME TRANSMISSIONS — NORMAL CIRCUITS 18 9

(a) It is not desirable to overload the terminal equipment of carrier systems by transmitting continuously a test signal corresponding to the peak voltage which is only attained momentarily during the transmission of the programme proper. (b) If Administrations and Private Operating Companies make their initial and maintenance measurements with a nominal absolute voltage of + 6 decibels or + 0.7 neper at the repeater output, is useful to use the same voltage, if it is necess­ ary to check during the preparatory period,

Supervision. — At the end of the preparatory period the transmission of the programme proper commences and is monitored at the studio, in the repeater stations and at the transmitter. This supervision is made with a speech level meter. One of the instruments, of which the characterisitics are summarised in the table below, may be used. Annexes 23 and 24 of the Book of Annexes to Volume III of the Green Book set out the advantages and disadvantages of the instruments which are of two types of which the most representative are:— — a speech level meter of which integration and return to zero times are appro­ ximately the same and of the order of 165 milliseconds;

— a peak indicator of which the integration time is very short (less than 2 0 milliseconds) and of which the return to zero time is rather long (of the order of 2 seconds. Annex 25 of the Book of Annexes to Volume III of the Green Book describes the modulation factor meter used by the British Administration and the British Broadcasting Corporation in agreement. Since there is not a simple and univocal correlation between simultaneous readings of the two very different types of apparatus for all types of programmes, it is desirable that the Broadcast Authority at the studio and the Telephone Administration provid­ ing the programme circuit should use the same apparatus in order to speak the same language. In general the Telephone Administration and the Broadcast Authority of a country agree to use the same type of apparatus. It is desirable to reduce to a mini­ mum the number of different types of instruments and to discourage the introduction of new types which only differ in detail from those already in service. During the programme transmission one should monitor at the point A (output of the last amplifier under the control of the Broadcast Authority sending the pro­ gramme) to ensure that the “ peak voltage ” (taking into account the type of pro­ gramme) adopted during the line-up of the complete link is not exceeded. There is occasion to recall that variation of the musical sounds from an orchestra is of the order of 60 to 70 decibels whilst the specification for circuits for programme transmission is based on a variation of around 40 decibels. A compression of the “ dynamic ratio ” of the programme is therefore necessary at the output from the studio; before passing it over the programme circuit. 19 0 PROGRAMME TRANSMISSIONS — NORMAL CIRCUITS

Lines

Normal type programme circuits may be provided in wide band cables by the following method:

Special pairs for sound broadcasting. — If a broadcast programme is to be distri­ buted to a number of intermediate points along the line (normally carrying carrier telephone systems) it might be necessary to use a pair of conductors under a special screen for broadcast programmes; but in that case it may be preferable to transmit the broadcast programme over the carrier system itself or on the phantom of the unloaded pairs. It is recalled, regarding this, that interstice pairs in a coaxial cable are principally intended for the maintenance and supervision of the telephone carrier system routed over the coaxial pairs.

Normal circuits for broadcast transmission routed over channels of a telephone carrier system in cable. — It is recommended to use the frequency band corresponding to three telephone channels of a carrier system to form a normal type programme circuit. One such assembly of three channels may be used in this manner in the basic 1 2 -circuit group. Taking account of the arrangements already made in the equipment of different countries, which may not be changed without serious economic inconvenience, two positions may be used for this assembly of three channels to provide broadcast transmission in each of the basic groups A and B.

Basic Group A Position I frequency band used: 24 - 36 kc/s virtual carrier frequency: 24 kc/s Position II frequency band used: 44 - 56 kc/s virtual carrier frequency: 44 kc/s

. Basic group B Position I frequency band used: 84 - 96 kc/s virtual carrier frequency: 96 kc/s Position II frequency band used: 64 - 76 kc/s virtual carrier frequency: 76 kc/s The choice of one of these positions will be made by direct agreement between interested Administrations, but if there is disagreement position I is preferred. With the limit that has been recommended above for the “ peak voltage ” transmitted by one such assembly of three channels, these assemblies (used for broadcast transmissions) may be placed in any basic groups (or in all the basic groups) of a supergroup (or in all supergroups) of a carrier system on coaxial pairs. The C.C.I.F. has not limited the possible positions (in the basic supergroup) of the groups over which will be routed “ normal type programme circuits ” but it might be said that the basic groups which appear most appropriate (in the super­ PROGRAMME TRANSMISSIONS — NORMAL CIRCUITS 191 group) for the programme circuits are the groups No. 2, 3 and 4. These groups are subject to less attenuation distortion (produced by certain filters in the supergroup) than groups Nos 1 and 5 at the edges. The supergroups most appropriate in which to place the programme circuits are those which are transmitted on the coaxial cable with the lowest carrier frequencies because the frequency deviation (due to instability of the frequency generators) on the channels of these groups will be proportionally lower than the deviation on channels placed in supergroups transmitted at a high frequency. Supergroup No. 2 (the basic supergroup) has the additional advantage of being subject to one stage of modulation less than the other supergroups. In the case of a carrier system on symmetrical pairs, it may be necessary specially to choose the group of the system and the pairs to be used in order that the conditions concerning crosstalk for the complete music circuit will be satisfied. Use of phantom circuits on unloaded symmetrical pairs equipped with carrier systems. — Recent experience has shown that the phantom circuits on symmetrical pairs in cables equipped with carrier systems may allow the transmission (following the definition above) from 50 c/s to 10,000 c/s. These circuits have the advantage of enabling simple derivations at various repeater stations of the carrier system, thus allowing the distribution of radio programmes of the picking up of a supplementary programme at various points along the line. Use of the band of frequencies below 12 kcjs. — The use of phantom circuits (see above paragraph) naturally necessitates that a multiple twin or a star quad cable is available. If a pair cable is not available, a possible solution would be to place the programme transmission in the frequency band below 1 2 kc/s, i.e. below the frequency band used for the telephone carrier channels; but this solution entails difficulties as regards the filters or when a crosstalk balancing frame exists, PAGE INTENTIONALLY LEFT BLANK

PAGE LAISSEE EN BLANC INTENTIONNELLEMENT 4TH PART

INTERNATIONAL TELEVISION TRANSMISSIONS ON LINES

I. General characteristics of an international circuit on metallic lines for television transmissions (provisional recommendation of the C.C.I.F. applicable to all standards of television in Europe)

The C.C.I.F. propose to adopt for studies concerning international television transmissions in cable the nominal maximum circuit described below. The numerical data in the present recommendation is being studied to decide if it would apply satisfactorily to the nominal maximum circuit.

Definitions

Origin and extremity of the television circuit The most general television circuit is represented schematically by figure 47 below. In specifying the transmission characteristics two cases are considered: — the long distance international line BC, — the international television circuit AD. (a) The long distance line BC. — The long distance line should be considered individually as regards the establishment and periodic maintenance of the line, the responsibility for which is wholly that of telephone Administrations. The charac­ teristics of this line are verified by steady state measurements.

Remark. — The points B and C may be situated either in the premises of the Telephone Administration or in those of the Television Administration. The choice is left, on the national level, to the Telephone and Television Administrations of a country. In all cases and whatever the conventions agreed between the telephone Admi­ nistration and the Television Administration the points B and C are video points.

13 1 9 4 TELEVISION TRANSMISSIONS

(b) The television circuit AD. — The specifications of the television circuit are necessary for the end-to-end tests preceding a television transmission. ^ These tests will be made in the transient state by means of test waveforms which have yet to be determined by agreement between the C.CJ.F. and the C.C.I.R. Studio or Long- distance Studio, switching National National switching international centre or radio junction line junction line centre line transmitter

Transmitting International Receiving equipment television circuit equipment

F ig u r e 47

Nominal maximum circuits General. — There is occasion to consider the three “ nominal maximum circuits for television transmission ” enumerated below:

1 ° the homogeneous nominal maximum circuit on coaxial pair which has been defined by the C.C.I.F. (see below),

2 ° the homogeneous nominal maximum circuit on radio relay systems which will be defined by the C.C.I.R. 3° a mixed nominal maximum circuit (consisting of sections of coaxial pair and radio relay systems) which will be studied later by a working party of the C.C.I.F. (3rd. and 5th S.G.) with representatives of the C.C.I.R. before the next Plenary Assembly of the C.C.I.R. (Warsaw 1956). It is understood that each of the three nominal maximum circuits corresponds to the “ long-distance international line ” BC, and does not include the junction lines (AB or CD) nor television standards convertors. 1. Nominal maximum circuit for television transmissions on coaxial pair. — The nominal maximum circuit for television transmissions on coaxial cable as defined by the C.G.I.F. conforms to the scheme shown in figure 48. The principal characteristics are:— TELEVISION TRANSMISSIONS 19 5 length 2500 kilometres (or 1600 miles) with 4 video junction points: the extremities Band C of the “ long-distance line ” and two intermediate points (which are reduced to video frequency) MM’.

8500 'km { d600_wd&fi)_ ___ i I i M M' C-I ! B i L- o 0 O 1 O modulating equipment from video frequency (g) demodulating equipment to video frequency

F ig u r e 48

2. Nominal maximum circuit for television transmissions on radio relay links. —- Study Group XI of the C.C.I.R. (Brussels 1955) has recommended for radio relay systems a nominal maximum circuit 2500 kilometres long also with two intermediate points at which the signal is reduced to video frequency.

Provisional recommendations for television transmissions in black and white Taking account of the above definitions the C.C.I.F. makes the following pro­ visional recommendations for the general characteristics of the long distance interna­ tional line BC. These characteristics apply to the transmission of waveforms irrespective of the standards of television (to 405, 625 and 819 lines) used in Europe and described in Report No. 35 of the C.C.I.R. (Documents of the Vllth Plenary Assembly London 1950 Volume 1 page 420).

A — I n p u t -a n d o u t p u t im p e d a n c e

Specification for the long distance line BC This specification should consist of two distinct parts. — line from end-to-end, — interconnection at international frontiers. The part of this specification concerning the line from end-to-end should in particular include the conditions concerning reflections at the points B and C expressed in the form of a return loss in the band of frequencies transmitted at these points. The impedances to be taken into consideration are, on the one hand, those of of the transmitting and receiving equipments and, on the other hand, the charac­ teristic impedances of the junction cables to the studio or the radio transmitter or to a switching centre. The specifications of the impedance at the points B and C should, in particular, be such as to avoid disputes in the choice of junction cable with the studio, the radio transmitter or the switching centre. 196 TELEVISION TRANSMISSIONS

In all cases the return loss should be equal to or greater than 20 decibels for the whole band of frequencies which should be effctively transmitted. (This is equivalent to stating that the modulus of reflection coefficient should be less than or equal to 0.1). If the junction lines AB and CD are short, the return loss at the points B and C (the transmitting and receiving equipments being assumed to be pure resistances of 75 ohms), should be equal to or greater than 20 decibels in the band of frequencies transmitted.

Note. — At the ends A and D of the television circuit AD it is advantageous to fix the impedance conditions. This will be especially useful when circuits are switched at this point. These conditions should be expressed in the form of return loss in the frequency band at these points. The impedances to be taken into consid­ eration are: in the case of switching: — the characteristic impedances presented by the junction cables to the switching point; in the case of terminal use; — the impedances of the junction cables and those of the studio apparatus which is connected to these cables. In all these cases the return loss should be equal to or greater than 20 decibels. Nevertheless: (a) if the junction lines AB and CD are short, the return loss at points A and D will be expressed relative to 75 ohms. (b) If, in addition, the exploitation is terminal it is only necessary to define the impedance of the circuit seen from A. In this last case the conditions to fulfil from the point of view of matching of impedances should be the following:— The input and output terminals are of the coaxial type i.e. unbalanced with respect to earth. Input impedance (impedance of circuit at A): 75 ± 2.5 ohms. The tolerance of 2.5 ohms should be interpreted as defining the upper limit of the modulus of the difference between the impedance actually existing and the nominal value of this impedance.

Characteristics of the video signal at the “ video junction points ” of the origin and distant end of the long-distance international television line as regards the specification of this line

The “ video junction point ” is the name for all the points in the television circuit at which the video signal appears, e.g.: (a) the junction point between two different systems such as a coaxial cable and a radio relay system; (b) the point where a monitor may be connected; (c) the point of junction of the television circuit and the studio apparatus; TELEVISION TRANSMISSIONS 19 7

At these points the video signal should present the characteristics given in paragraphs B, C and D below.

B. P o l a r it y a n d d .c . c o m p o n e n t

The polarity of the signal will be positive, i.e. such that the passage from black to white entails a rise, in the algebraic sense, of the potential at the terminal not at earth potential (see figure 49). V

F ig u r e 49

V = difference in potential between the terminal (not at earth potential) of the input impedance (or output) and the ground (difference of potential positive in an upward direction).

The useful d.c. component (i.e. the component which is associated with the average brightness of the image) may or may not be present in the video source and has not to be transmitted' over a transmission chain or delivered at the output. All the non-useful d.c. component accompanying the signal and without any relation to it (arising for example from the d.c. supply current to valves) should be such as not to dissipate more than 0.5 watts in the input impedance. (This specifi­ cation is intended to indicate the electrical power that the nominal input resistance of 75 ohms should be capable of normally carrying with direct current). When this resistance is disconnected, the voltage should not exceed 60 volts direct. (In order to limit the voltage to a value which should never be dangerous 4to personnel.)

C. A m p l it u d e o f s ig n a l

The C.C.I.F. urgently requests the C.C.I.R. to standardize the value of 1 volt for each of the systems of 405, 625 and 918 lines used in Europe. This will enable all difficulties on the subject to be avoided at “ video junction points ”.

D. P ic t u r e - t o - synchronizing p u l s e r a t io

The C.C.I.F. urgently requests the C.C.I.R. to standardise the same nominal value for this ratio for all the systems of 405, 625 and 819 lines used in Europe. 198 TELEVISION TRANSMISSIONS

E. NO n -l in e a r DISTORTION C.C.I.R. Study Group XI (Brussels, 1955) has expressed the wish that for the complete television circuit between studio and radio transmitter the conditions should be as follows:— 1. For the vision signal. — In annexes 1 to 4 below, are given the tolerances allowed and the method of measurement used by the Administrations of the following countries: France, Federal German Republic, United Kingdom, Switzerland. 2. For the synchronizing signal. — At the output of the long-distance circuit, the tolerances of the peak-to-peak amplitude of the synchronizing signal are from + 1 0 % to — 30% of the nominal value of this amplitude for slow variations. For rapid variations, and notably if these variations are associated with the picture content, the tolerances are certainly more strict, but it has not yet been possible to specify them. Annex 4 submitted by the Swiss Administration gives on example of these tolerances and the method of measurement used. The C.C.I.F. is studying the conditions which might be imposed on the long­ distance international line BC from the point of view of non-linear distortion in order to agree as much as possible to the desires of the C.CJ.R. given above. Nevertheless it is evident that the Telephone Administrations may not accept any responsibility for the distortion produced in a standards convertor situated in the permises of the Broadcast Authority through which the complete television circuit passes (between studio and radio transmitter).

F. V a r ia t io n of attenuation a s a f u n c t io n o f time C.C.I.R. Study Group XI (Brussels, 1955) has proposed the following require­ ments for the television circuit AD. Until there are better methods for the measurement of the gain stability of a circuit for television transmissions the following method and tolerance will be allowed. The peak-to-peak amplitude of the total signal at the input of the circuit being adjusted initially to a value of 1 ± 5 % volts and maintained constant, the variations of the peak-to-peak amplitude of the total signal at the output should not exceed the following values. 1° short period (e.g. 1 second): ± 0.3 db

2 ° medium-period variations (e.g. 1 hour): ± 1 db 3° long-period variations (e.g. 1 month): — if the circuit is not supervised, i.e. not adjusted each time it is used the gain should not deviate more than ± 2 decibels from its nominal value; — if the circuit is permanently supervised the tolerances under (2 ) above will apply i.e. the gain should be readjusted each time it deviates more than ± 1 decibel from its nominal value. The C.C.I.F. is studying the arrangements to be used and the measurements to take on the long-distance line BC for television transmissions in order to be able to satisfy the requirements stated by the C.C.I.R. TELEVISION TRANSMISSIONS 1 9 9

G. SlGNAL-TO-NOISE RATIO FOR THE LONG-DISTANCE LINE After taking note of the requirements of the C.C.I.R. for the “ television circuit ” the C.C.I.F. considers that the values given below might be recommended for the “ long-distance line BC ” (between video points): The values given below refer to the decibel ratio of the: peak-to-peak amplitude of the vision signal (not including the synchronizing signal) R.M.S. value * of the noise in the band O — f c in the case of continuous random noise,/c being the upper limit of the video spectrum of the system considered, and the ratio of the: peak to peak amplitude of vision signal (not including the synchronising signal) peak to peak amplitude of noise in the case of recurring noise and discontinuous random noise. The table below gives, for different systems, the ratio corresponding to the following types of noise: (a) continuous random noise of a uniform power spectrum as a function of frequency; (a') continuous random noise of a power spectrum rising with frequency at the rate of 6 db per octave; (b) recurrent noise.

405-line system (a) 50 db 1 r«v42dbK‘ = 3M<* (b) 30 db at 50 c/s 45 db at 100 c/s 55 db from 1 kc/s to 1 Mc/s and decreasing linearly from 1 to 3 Mc/s to 25 db at 3 Mc/s.

625-line system and Belgian system o f 819 lines (a) 48 db 1 /c = 5 Mc/s if the HF channel is 7 Mc/s and

(o’) 41 db j/c = 6 Mc/s if the HF channel is 8 Mc/s (b) 30 db at 50 c/s 45 db at 100 c/s 50 db from 1 kc/s to mc/s and decreasing linearly from 1 to 3 Mc/s to 25 db at 3 Mc/s.

* It is recommended to specify the continuous random noise by its R.M.S. value rather than the half peak-to-peak value defined in the draft of the report of the Sub Committee XI-D of the C.C.I.R. (London, 1953, Volume I, page 270). There are certain difficulties in relating the peak-to- peak amplitude to the R.M.S. value. 200 TELEVISION TRANSMISSIONS

819-line system

(a) * 1 (a') 40 db j = 1 0 (b) 45 db from 50 c/s to 1 Mc/s and decreasing linearly from 1 to 6 Mc/s to 20 db at 6 Mc/s and 20 db from 6 to 10 Mc/s.

For all systems Discontinuous random noise: ratio of 25 db for very short impulsive noise and for low repetition frequency. For long impulsive noise and high-repetition frequency it is not yet possible to specify the required ratio between the signal and noise. Crosstalk. — The crosstalk introduced in the television transmission by parasitic signals causes disturbances on the picture which are either recurrent or continuously random according to whether the crosstalk is produced by a carrier system or an ordinary telephone communication at audio frequency. Nevertheless, even this last type of communication is liable to cause recurrent disturbances when carrying the calling or clearing signal. Thus it is recommended, for the logarithmic ratio between the vision signal and the parasitic signal of the crosstalk, provisionally and only to fix the order of magnitude, to have the same limit which has been fixed above for the logarithmic ratio between the vision signal and recurring noise.** Difference between the propogation time of the image and the sound. — The difference between the minimum group propagation time of the sound signal and the components of the video signal between towns connected by the television transmission system considered should be not greater than 0 . 1 seconds. Note. — The question does not arise if the sound is transmitted on the same channel as the video signal. If a separate channel is used for the sound, distinct from the television channel, but having a high propogation velocity, the limit of 0 . 1 seconds is so great that it practically imposes no restriction on television transmissions even over very great distances. As regards television transmissions over short or medium distance which are only likely to be effected in the near future, the limit of 0 . 1 seconds will likewise allow the use for the sound of relative low-velocity channels.

* Noise of type (a) only occurs in transmission over coaxial cables; no value is proposed as this mode of transmission is not at present used for the 819-line system. ** The British Administration has proposed for the 405-line standard a different recommend­ ation, “ The British Administration has not undertaken tests on the effects of crosstalk between two circuits for television transmissions, but they consider that a good guide is given in the article by A. D. Fowler entitled “ Observer reaction to video crosstalk” Journal o f the Society of Motion Picture and Television Engineers, pages 416-424, November, 1951. In this article, it is proposed to admit a minimum crosstalk ratio of 58 decibels in the case where the crosstalk currents have a flat characteristic (without distortion): other values are proposed for crosstalk currents of other types. ” TELEVISION TRANSMISSIONS 201

H. P h a se a n d A m p l it u d e characteristics At the present time the steady-state specifications are still necessary for the division of responsibility between Administrations. Figures 51 to 53 below give the characteristics proposed provisionally on a steady-state basis for the various standards of television used in Europe.

405-line system See figure 50.

. 625-line system. (a) I f the HF channel is 7 Mcjs See figure 51. (b) I f the HF channel is 8 Mc/s identical to (a) except that the relative values at the frequency of 5000 kc/s are in this case relative to 6000 kc/s.

819-line system See figure 52. As regards transitory-state specifications a note by the Administrations of France and Great Britain is reproduced for information in annex 26 of the Book of Annexes to Volume III of the Green Book.

Notes. — 1. Certain characteristics described in paragraph H have only been intended for a circuit of 1 0 0 0 km without intermediate video junction points. The C.C.I.R. requests the C.C.I.F. to consider whether all these characteristics may be accepted for the “ nominal maximum circuit ” of 2500 km with two in­ termediate junction points. 2. The C.C.I.R. draws the attention of the C.C.I.F. to the fact that the speci­ fications concerning the amplitude and phase characteristics as a function of frequency do not ensure, ipso facto, the correct transmission of very low frequencies necessary to obtain a satisfactory image. To verify if the transmission of these very low frequencies is satisfactory it is advisable to use a special test signal consisting of a waveform of nominal amplitude in the form of a 50 c/s square wave superimposed on the normal synchronizing and suppression signal. This waveform is applied at the input of the circuit and at the output the height of the two ends of the square wave should not exceed some percentage of the peak-to- peak amplitude of the total signal. 3. These steady-state specifications although useful, if not indispensable, for the calculation of the elements of the television circuit do not entirely ensure satisfact­ ory transmission from the transient viewpoint which is of primary importance in television. The C.C.I.F. has also, therefore, made proposals concerning the form of test signal for the transitory state. 202 TELEVISION TRANSMISSIONS

The C.C.I.R. is not yet in a position to give its views on these propositions and the distortions that might be tolerated at the output from the circuit; but it draws attention to its interest and is continuing the study of this problem which it is urgently desired to complete.

Attenuation (db)

(a) Variation of attenuation as a function of frequency relative to the value at 100 kc/s.

Group propagation time (microseconds)

+ 0,5. '2 2 ^

+ 0,85. + 0,15.

•800 8000 8500 8800 -0,15. -0,85.

- 0,5.

Notes. — 1. The characteristic. “ group propagation time-frequency ” should not present rapid variations between the limits of this graph.

2. Furthermore, from 30 c/s to 200 kc/s the caracteristic “ phase change-frequency ” should not deviate by more than ± 6° from a straight line having an ordinate at zero or at a multiple of 360°.

3. The group propagation time is measured with the test frequency modulated by 100 kc/s.

(b) Variation of group propagation time as a function of frequency relative to 200 kc/s.

F ig u r e 50. — Amplitude and phase characteristics proposed provisionally for a television circuit (405-line standard) o f 800 km, consisting of 4 sections, with 3 intermediate video points. TELEVISION TRANSMISSIONS 2 0 3

Attenuation (db)

Group propagation time (microseconds)

Notes. — 1. Furthermore, in each interval of 100 kc/s the variation should not exceed 0.1 microseconds. The limits indicated for the group propagation time may appear slightly high but for the present it is not possible to fix tighter limits.

2. On the other hand from 30 c/s to 200 kc/s the “ phase change-frequency ” characteristic should not depart by more than ± 6 % from a straight line having an ordinate at the origin or at a complete multiple of 360°.

(b) Variation of group propagation time as a function of frequency relative to the value at 200 kc/s

F ig u r e 51. — Amplitude and phase characteristics proposed provisionally for television circuits (625-line standard) 204

Attenuation (db) EEIIN TRANSMISSIONS TELEVISION

Group propagation time (microseconds)

F ig u r e 52. Amplitude and phase characteristics proposed provisionally for television circuits (819-line standard). TELEVISION TRANSMISSIONS 2 0 5

ANNEX 1

Specification of non-linear distortion used by the French Administration Two distinct specifications are proposed for defining the signal distortion caused by non-linearity in equipment. The first concerns the “ vision ” part of the signal and refers to the variation of the slope of the input-output amplitude characteristic. The second concerns the “ synchronizing ” signal and refers to the amplitude of this signal at the output. In practice the measurements relative to these two specifications are made simult­ aneously according to the following method:— Two signals represented by the figure 53 are applied simultaneously to the input of the circuit White ______level (nominal) o ,f V WVVWWVW (VWV ------White H level Black 0,3 V level I 1 F . -J JZ

1st Waveform 2nd waveform

F ig u r e 53

— a grey level adjustable between 0.1 V and 0.8 V; — a sinusoidal or square modulation superimposed on the grey level of peak-to-peak amplitude 0.2 Y and frequency of the order of 500 kc/s; — a synchronizing signal of amplitude 0.3 V. In the first of the waveforms the duration of the grey level is equal to the line period and in the second it is equal to around 1 / 8 of this period. The equivalent of the transmission being regulated such that the nominal voltage at the input corresponds to the nominal voltage at the output, the height h is varied and the peak-to-peak amplitude of the modulation imposed on the grey is measured as well as the peak-to-peak amplitude of the synchronizing signal at the output. The results of these measurements should satisfy the following conditions:—

(a) when h is varied from 0 . 1 to 0 . 6 V the amplitude of the modulation should not be less than 0.75 of its maximum value;

(b) when h is varied from 0 . 1 to 0 . 8 V the former tolerance is reduced to 0 . 6 of its maximum value. (Condition of slight overload on the circuit); (c) when h is varied for 0.1 to 0.8 Y the amplitude of the synchronizing signal should remain equal to 0.3 + 0.03 V and —0.09 V. Remark. — The values above are given as an indication of the values in practice on a 2500 km circuit. 2 0 6 TELEVISION TRANSMISSIONS

ANNEX 2

Method of measuring non-linear distortion used by the Federal German Republic 1. At the video input of the circuit a television waveform (see figure 54) is applied consisting of the line synchronising pulses and a saw-toothed voltage of a quarter line duration followed by a constant white level up to the end of the line. A sinusoidal voltage of frequency 4 mc/s for example and peak-to-peak amplitude of 10% of the total voltage is superimposed on the saw-toothed voltage. (The exact values of the saw-toothed voltage are given in the figure; they are chosen such that the peaks of the teeth are at black level on one side and white level on the other). At the output of the circuit a 4 Mc/s band pass filter is used. Then, by means of an oscilloscope the slope of the amplitude—amplitude curve is measured (see figure 55).

'ffcrfts

H o tk . 1,0

0,35 o,3

To verify if the slope thus measured is sufficiently independent of the value of the d.c. component, the waveform is modified as shown in figure 54, i.e. the saw-toothed wave­ form is moved from one end of the line to the other. For the tolerance of the slope a ratio of ±10% is accepted. A precision of ± 3 % has been obtained with this method and is considered acceptable. TELEVISION TRANSMISSIONS 2 0 7

2. As regards the distortion of the synchronizing pulse the following method is proposed: When the waveforms corresponding either to figure 54 or figure 55 are applied the voltage of the synchronizing pulses should not differ more than +10% or —30% from the value measured if the synchronizing pulses alone were transmitted.

(3eiA ie

1&

F i g u r e 56

ANNEX 3

United Kingdom of Great Britain and Northern Ireland Non-linear distortion The following procedure may be adopted to measure non-linear distortion: (i) Determine the gain of the circuit and adjust the synchronizing pulse within its tolerances. (ii) Add to the synchronizing pulse a narrow white bar (10% of the line period) and increase its amplitude by steps of 20% from 0 to 100% of the total amplitude of the signal at the input, measure the corresponding amplitude of the bar at the output. (iii) Repeat the same procedure with a bar occupying all the line width available, the amplitudes of the bar and synchronizing signal being measured at the output for each input level. Then for all amplitudes of the wide bar and the narrow bar the ratio: amplitude of bar at the output/amplitude of the bar at the input should lie between 0.9 and 1.1 times the gain of the circuit (the nominal gain being 1) and the amplitude of the synchronising signal at the output should not vary more than + 10 to —30% of the amplitude measured when the synchronizing pulse alone is transmitted. For rapid measurement, one may, if it is desired, use other methods of measurement using more complicated waveforms provided that the total range of the voltages are substan­ tially the same as those covered by the two test signals described above. 2 0 8 TELEVISION TRANSMISSIONS

ANNEX 4

Method of measuring non-linear distortion in a television circuit and tolerances allowed by the Swiss Administration The following measurements are made to determine the extent of non-linear distortion presented by a television transmission system; the results obtained should be within the limits given.

Waveform No. I

F ig u r e 57 Definition (see figure 57). On the classical saw-toothed signal a continuous sine wave of variable frequency is superimposed (2.... 5.5 mc/s) and of constant amplitude (100 mV peak-to-peak). Measurement The sinusoidal part of the signal is separated at the output by means of a high-pass filter (/-i = 1 . 8 mc/s); following which it is passed through an envelope detector and applied to an oscilloscope.

F ig u r e 58 Tolerance (see figure 58). m should be — M. for all frequencies situated in the band considered.

Waveform No. II Definition (see figure 59) A variable continuous sinusoidal frequency (2.... 5.5 mc/s) and constant amplitude ( 1 0 mV peak to peak) is superimposed on the grey level.

o./V i t t

“ i f i f

F ig u r e 59 TELEVISION TRANSMISSIONS 2 0 9

Measurements (a) The signal at the output is connected directly to an oscilloscope. The signal is assumed as adjusted to normal (in other words for an input signal corresponding to nor­ mal level of peak white, the output signal is 1 volt peak-to-peak). Then the ratio of the height h at the input to the height h at the output, with or without*the sinusoidal signal should lie between 0.9 and 1.1 for all values of h between 0.1 and 0.7 V. (b) As for I the output signal is extracted by a high-pass filter and one may detect or not detect its envelope. Then for all values of h between 0.1 and 0.7 V the peak-to-peak amplitude of the sinusoidal signal (or its envelope) should not vary by more than 10%.

Waveform III Definition A square wave of 50 c/s superimposed on the line synchronizing signal, the suppres­ sion signal being accurate. Measurements. Directly to an oscilloscope before the re-establishment of the d.c. component (if the re-establishment exists). Tolerance (see figure 60)

S — 5 ■ 3 T ^ 100

F ig u r e 60

Note. — All the signals are also used for measurements of other characteristics of the circuit.

14 210 TELEVISION TRANSMISSIONS

II. Characteristics recommended by the C.C.I.F. for the transmission of television signals on lines *

Characteristics of the television signal transmitted to line

Carrier frequency and sidebands of modulation **. — It is unanimously recognised that the use of a method of transmission with a residual sideband is necessary for the type of television transmission considered. It is admitted that the video signal to be transmitted corresponds to an image consisting of 405 lines and that at the studio output the spectrum of the video signal has a relatively sharp cut-off at the two extrem­ ities, these being respectively 30 c/s and 3 Mc/s. It is also assumed that the origin­ ating Broadcast Authority has corrected as far as possible for aperture distortion and other distortions in the camera chain. With a coaxial pair of the type standardized by the C.C.I.F. (see the specification in section 3.4.4 of the first part above) and with a repeater spacing of the order of 9 kilometers (enabling the use of telephone carrier systems on coaxial pairs of this type), it is possible to transmit an upper sideband of width around 3 Mc/s and a residual lower sideband of width 500 kc/s (value considered satisfactory even though there is not yet sufficient experience on this subject). If the construction of this pair has been of high quality as regards the regularity of impedance and if the equali­ zation and phase compensation has been good it may be assumed that the signal applied at the input will be faithfully reproduced at the output. For television transmissions of the type considered it is recommended to employ in Europe a carrier frequency of nominal value 1056 kc/s, it being understood that in the course of transmission the carrier frequency should not vary more than a few cycles per second. In the present state of the technique it is not yet possible to establish independ­ ently the types of transmitting and receiving terminal equipment.

Polarity of modulation since the advantage and disadvantage of positive polarity (where the signal is increased with the brilliance) or negative polarity have not yet been established for television transmissions on metallic lines, but, on the other hand, it is desirable not to have to use inversion apparatus at the interconnection of two circuits. Hence it is recommended that the polarity of modulation adopted at the origin of a chain for international television transmission should be conserved throughout the length of this chain.

* The recommendations under this heading only apply to the 405-line standard; there is occasion to study the extension to other standards. ** The establishment of a corresponding recommendation for the 625 and 819-line standard is the object of a new question. TELEVISION TRANSMISSIONS 211

Ratio of amplitude between vision and synchronizing signal. — It is recommended that the ratio: amplitude of vision signal amplitude of synchronizing signal 7 in the modulated wave should equal - .

Depth o f modulation. — The limit allowed for the “reference coefficient of modula­ tion ” defined below is provisionally fixed at 50%. Note. — The coefficient of modulation, for a given signal, is defined as follows: _ Vs is the voltage (peak-to-peak) of the video signal considered. This signal modulates in amplitude a carrier, the amplitude of which varies between the two limites VM and Vm with VM — Vm = Vs when the two sidebands are retained.

V m >.3 Vc

V >.f vs

V,m

t = time V = voltage F ig u r e 61

By definition the coefficient of modulation t is Vs VM - v n Vm + Vm VM + Vm It will be seen that this definition coincides with the usual definition when the signal s is sinusoidal. After partial suppression of the lower sideband the amplitude ratios considered above are approximately retained and the coefficient of modulation 'M v„ VM + Vm remains for all intents and purposes the same. The coefficient of modulation defined above is essentially a function of the type of signal transmitted and differs according to whether the d.c. component of a video signal is or not retained. 2 1 2 TELEVISION TRANSMISSIONS

Be that as it may, the highest coefficient of modulation that it is possible to meet amongst all the possible types of waveform should have an upper limit in order to limit the distortion which appears in the detection on account of the partial suppression of the lower sideband. The choice of this highest modulation coefficient determines the coefficient of modulation of all other types of signal. On the other hand the maximum amplitude of the modulated carrier current that it is possible to meet should have an upper limit in order to limit the non-linear distortion. The ratio between the vision signal and the basic noise is then so much smaller that the modulation coefficient is itself small. It would appear from this .that there should be a lower limit to the modulation coefficient. The choice is there­ fore a compromise between the two requirements. When the complete video signal is as defined above (with for example a negative polarity of modulation) and the d.c. component has been suppressed, it is easy to determine the type of video signal corresponding to the higher coefficient of modu­ lation. It is that which corresponds to the transmission of white spots on a dark background (figure 61) (It might be considered that the average value of the synchro­ nizing signal is negligible compared with V J . The corresponding coefficient of modulation t r is called “ reference coefficient of modulation ”.

d.c. component. — It is recommended that the d.c. component of the complete video signal should be suppressed for transmission to line.

Characteristics of the coaxial pair and of the repeaters

Regularity of impedance. — If coaxial pairs are used for television it is important to have a very regular impedance characteristic. In the present state of the technique of manufacture and of laying coaxial cables, taking all precautions, it would appear possible to obtain for a repeater section a return loss of 5 nepers or 43.5 decibels throughout the frequency band used. The C.C.I.F. considers it is very desirable to make transient measurements of the impedance irregularities on coaxial pairs intended for television transmission.

Reflection coefficients between the input impedance of a coaxial pair and the input and output impedance of repeaters used on this coaxial pair:— Let ZL be the measured impedance (for a frequency /) of the cable as seen from the repeater station (see figure 62); Let ZE be the output impedance (measured at the frequency/ ) of the equipment in the repeater station as seen from the line; Let ZR be the input impedance (measured at the frequency /) of the equipment in the repeater station as seen from the line; A — al the total attenuation (at the frequency b) of the cable between two adjacent repeaters, a being the attenuation coefficient of the coaxial pair and 1 the distance between the two adjacent repeater stations considered. TELEVISION TRANSMISSIONS 213

One considers the number N (in decibels or nepers) defined by the formula

Ze + ZL Zl + ZR N — 2A + 20 log1 0 + 2 0 log1 0 (decibels) Ze — ZL Zl — ZR

Ze + ZL Zl — ZR N = 2A + loge + loge (nepers) Ze — Zl Zl — ZR Provisionally the condition indicated below should be fulfilled. In the case of a television transmission system N should be of the order of 8 nepers or 70 decibels at the frequencies adjacent to the virtual carrier frequencies used for transmission on the line. At frequencies away from the carrier frequency, one might probably accept lower values of N. Note. — As regards the matching of the impedance of the section of amplification to the impedances of the two adjacent repeaters, in the general case of a section of coaxial cable between two adjacent repeaters the C.C.I.F. has only fixed a limit for the sum N of the three terms defined above. It is recommended that Administrations and Private Operating Companies interested in a section of coaxial pair which crosses a frontier should agree, amongst themselves on the admissable values for each of the three terms to give the above limit i.e. agree on the use of as good a match as possible or of one systematic mis-match at the end of the repeater section. It is nevertheless very desirable that throughout the length of the coaxial system Administrations or Private Operating Companies, should always agree to choose the same methods, particularly as regards the matching of impedances in order to facilitate the maintenance of the system. PAGE INTENTIONALLY LEFT BLANK

PAGE LAISSEE EN BLANC INTENTIONNELLEMENT 5TH PART

COORDINATION BETWEEN RADIOTELEPHONY AND LINE TELEPHONY

SECTION 1

Intercontinental radiotelephony connections and the use of radio relay links for international telephone circuits *

T h e international T e l e ph o n e C onsultative C om m ittee, considering (a) that radiotelephony systems now connecting various continents normally use carrier frequencies of about 30 Mc/s **; (b) that the insertion of a radio link in a long-distance telephone circuit, entails certain special conditions creating particular difficulties that are not encountered when line circuits are employed exclusively; (c) that a radiotelephony link differs from a line circuit by the following: c. 1. A radiotelephone connection of this nature is subject to variations in attenuation and to special difficulties resulting from the fading of the signal; c. 2. A radiotelephone connection of this nature is affected by noise arising from atmospherics the intensity of which might attain or even exceed the value of the received signal. c. 3. Special precautions are necessary in the establishment and maintenance of such a connection in order to keep out of the receiver disturbances caused by all transmitters and especially by those which are part of the radio link in question: c. 4. In order to maintain the radio link in the best conditions from the point of view of the quality of transmission, it is necessary to take special measures to ensure that the radio transmitter operates as much as possible always on an even load

* See above in section 2 of the first part of this work for the provisional general characterist­ ics of an intercontinental circuit only comprising land sections. ** Each time that the limit of 30 Mc/s is mentioned in the following text it should be taken to mean “ about 30 Mc/s ”. 2 1 6 RADIOTELEPHONY CONNECTIONS whatever the type and attenuation of the telephone system connected to the radio link;

c. 5. It is necessary to take steps to avoid ,or correct for, abnormal conditions of oscillation and crosstalk;

c. 6 . Whilst the effective transmitted frequency band recommended for inter­ national land circuits has been determined after studyng the needs of the human ear, this band (in the case of a radio link operating on frequencies below 30 mc/s) may be limited by the necessity to place the maximum number of telephone channels in this part of the radio spectrum and not to occupy a larger band than is necessary for the telephone channel.

c. 7. Such a radiotelephony link is, in general, a long-distance intercontinental connection providing a telephone service between two extensive networks and is of great importance from two points of view: c. 7.1. The intercontinental conversations have in general, much impor­ tance to the users and they are exchanged in languages which are not always the mother tongue of the speakers and therefore good quality is particularly desirable. c. 7.2. It is not desirable to deprive the public of a very useful service under the pretext that it will not always satisfy the degree of excellence desirable for long-distance communications as regards the quality of transmission;

Makes the following unanimous recommendation:

1. Connections using frequencies above 30 Mc/s

When it is possible, in order to render less difficult the problems associated with radio frequencies, the telephone communications between fixed points should be jealized by metallic lines or by radio relay systems employing frequencies greater than 30 Mc/s and that when this can be done, the objective should be the obtain the quality of transmission recommended by the C.C.I.F. for international metallic circuits.

2. Connections using frequencies below 30 Mc/s

2 . 1 that, since it is necessary to economise in the frequency spectrum on inter­ continental connections consisting principally of a single radio link over a long distance using frequencies below 30 Mc/s, it is desirable to employ as far as possible single sideband systems transmitting a smaller frequency band than the 300 to 3400 c/s recommended by the C.C.I.F. for land circuits and preferably to reduce the upper frequency to 3000 c/s or less, but not below 2600 c/s except in special cases.

2 . 2 that in spite of the necessity to tolerate large variations in the noise level on such a radio connection, all possible efforts should be made so that the connections have a minimum of noise and fading by employing technical methods such as 1 0 0 % modulation of the transmitter, directional aerials, single sideband transmission. RADIOTELEPHONY CONNECTIONS 2 1 7

In the present state of the technique, it is not yet possible to recommend a minimum signal-to-noise ratio nor a method of measuring the disturbing noise. *

2 .3 that during the times when such a radiotelephone circuit is extended by a circuit equipped with echo suppressors care should be taken that the noise is not of a value sufficient to frequently operate the echo suppressors;

2 .4 that such a radiotelephone connection should be equipped with a reaction suppressor (switching arrangement controlled by the voice) in order to avoid oscilla­ tions or disturbing echos on the complete channel;

2 .5 that such a radiotelephone connection should be equipped with automatic gain equipment to compensate automatically, as far as possible, the phenomenon of fading;

2 . 6 that the terminal apparatus of such a radiotelephone connection should allow the connection of this to any other type of circuit;

2 .7 that in the case where a secrecy device is employed it should not appre­ ciably affect the quality of transmission;

2 . 8 that when no suitable automatic arrangement exists a technical operator should be empowered to ensure the best regulation of the loading of the transmitter, of the output from the receiver and the performance of the reaction suppressors (see section 2 . 1 of the present part entitled “ measurement and regulation of vocal sounds ”).

Note. — Even though the directives set out in the second part of the above recommendation are much less severe than , those imposed on international land circuits, the ideal objective is to reach the same standards of transmission in all cases. This being so, it is desirable that telephone systems connected to a radio link should conform to the recommendations of the C.C.I.F. for the general conditions which should be satisfied by international circuits, notably as regards the equivalent, distortion, noise, echo and transitory phenomenon (see section I of the first part of this work). Taking account of the first and second part of the recommendation above, it is desirable that in each particular case interested Administrations and Private Operating Companies should agree together to what extent the general standards employed for international land circuits may be attained. If the technique recommended in the first part of the recommendation is applied, the objective should be to obtain as far as possible the characteristics recommended by the C.C.I.F. for land international circuits. When this is not possible, interested Administrations and Private Operating Companies should consider the best solution to bring it up to date from the technical and economic viewpoint.

* The C.C.I.R. indicates in its recommendation No. 44, that the ratio between the signal and noise at audio frequency (for an audio bandwidth of 3 kc/s, the audio signal being measured with a v.u. meter) should be at least equal to 33 decibels in the following cases: —■ a radiotelephone circuit with double sideband modulation providing a commercial service, — radio link with single sideband modulation with 1, 2, 3 or 4 telephone channels providing a commercial service. SECTION 2

Arrangements employed to improve transmission

2.1. Measurement and adjustment of the volume

Instrument enabling a special operator at the junction between the radio link and the metallic circuit to measure the volume

The International Telephone Consultative Committee

makes the following unanimous recommendation:

1 ° that the instruments which it is desirable to install at the monitoring point of the special operator depend upon the observations required of the instantaneous volume transmitted on the circuit at the point where it is connected;

2 ° that the present practice is normally to use two different types of instruments. The first indicates the instantaneous power (maximum impulse indicator, peak indicator, maximum amplitude indicator) the second follows the average volume variations (volume indicator or average impulse indicator). In this manner protec­ tion is given on the one hand to the radio transmitter against too frequent overload (sometimes by an automatic limiter) and on the other hand the regulation of the modulation of the radio transmitter to its optimum value. It may be desirable, in certain cases to employ the two types of apparatus simultaneously; 3° that it is possible to use a single instrument which integrates the power on the circuit during an interval of time equal to the maximum interval during which overmodulation is most harmful. The constants of such an instrument may be chosen so that it can function either as a peak indicator or a volume indicator (or indicator of mean impulses) as required. It is then possible to adjust the modulation so that the pointer only exceptionally exceeds a fixed limit; 4° that even though the use of long-distance radio communications does not require the unification of the characteristics of these various instruments, it is never­ theless very desirable to adopt, in the future, uniform characteristics for the instru­ ments used by the different Administrations and Private Operating Companies so as to make possible the comparisons of the readings made at the extremities of the same connection. Information on this subject is given in the Green Book, Volume IV, 3rd part, Section 3.2.2, under the title “ Volume Indicators ”. RADIOTELEPHONE — ADJUSTMENT OF THE VOLUME 2 1 9

Volume regulators

The International Telephone Consulative Committee

Considering:

that it is desirable to ensure approximately constant loading of the radio trans­ mitter during variations of volume arising from the talking subscriber, makes the unanimous recommendation: that it is desirable to insert in international radiotelephone circuits (except short- distance circuits) at the junction point of the land telephone network and the radio link, a manual or automatic volume regulator. Regulators of this type are at present used by various Administrations and Private Operating Companies and give satisfaction in present conditions of service. The note below defines the essential functions which should be fulfilled by a relatively simple automatic volume regulator. The limits, given for indication, correspond to regulators in service. It may be desired to have a more perfect regulator which, in consequence, will be more complex. Annex 27 of the Book of Annexes to Volume III of the Yellow Book describes in a concise manner a regulator constructed and in use by the British Administration.

NOTE Conditions which should be satisfied by an automatic volume regulator placed at the junction of the landtelephonenetwork anda radiotelephony link An automatic volume regulator inserted at the junction of the land network and a radio link should, as far as possible satisfy the following conditions (the numerical values are given as an indication). 1. General conditions. — The speed of adjustment of the gain and the other charac­ teristics of the operation of the regulator should be such that it produces no appreciable reduction, either in the transmission quality, or the naturalness of the conversation, through­ out the frequency band in which the regulator should function. (250 to 2750 c/s). 2. Static characteristics. — An automatic volume regulator may be defined sufficiently by its steady-state characteristics (or static characteristics) and by the duration of the initial and final transit time. The static characteristic (figure 63) should be such that the regulator ensures a mean value of regulated volume at the output; for a variation of the order of 5 nepers or 43.4 decibels of the input volume, the output volume should vary less than ± 0.3 nepers or ± 2.6 decibels. Below this range of input signal the gain of the regulator should remain constant. 3. Transient time. — The initial and final transit times of the regulator may be either different (case a) or equal (case b): Case a. — The initial transit time of the regulator should be of the order of a fraction of the duration of a logatom in order that the apparatus adjusts itself rapidly to the following speech immediately after a period of silence. The final transit time of the regulator should be of the order of several logatoms in order that the variations of volume, during the transmission of several consecutive syllables should be constant and that the naturalness of the voice is thus respected. Furthermore, in order to avoid the false operation of reaction suppressors, it is necessary that the gain of the regulator be not raised, during silent periods, above a pre-determined value. 220 RADIOTELEPHONE — FADING CORRECTOR

Input volume

F ig u r e 63 — Static characteristic o f an automatic volume regulator

Case b. — When the initial and final transit times of the regulator are equal, they should be of the order of the duration of several logatoms in order to respect the naturalness of the voice; but in this case, in order to avoid an overload of the radio transmitter at the start of a new issue of sound, it is necessary that the regulator should hold, during silent . intervals; the same gain as that which it had when the subscriber ceased to speak. 4. Sensitivity to noise. — It is' desirable that the regulator should be insensitive to extraneous noise. Thus for example the regulator might be rendered insensitive to fre­ quencies outside the band from 600 to 2 0 0 0 c/s. It should be understood that the action of the regulating apparatus may be ineffective when the conditions of noise of fading of the signal are particularly unfavourable. In this case, it may be necessary to revert to manual operation.

2.2. Fading corrector

T h e I nternational T e l e p h o n e C onsultative C o m m it t e e considering:

1 ° that it is desirable to ensure approximately constant volume from the radio receiver connected to the land telephone network against variations in the volume of the received signal;

2 ° that fading phenomena of the output signal from the radio link depends considerably on the wave length of carrier employed; 3° that the consideration of fading correctors for international radiotelephone circuits is limited for the moment to the case of short waves to which in consequence this recommendation refers: 4° that the problem of maintaining constancy of the low-frequency output differs for the type of modulation used and that the present recommendation applies to the case of amplitude modulation, which is usually used in commercial radio­ telephony; 5° that in the case of pronounced selective fading, the fading correctors con­ sidered in the present recommendation may not function satisfactorily and that there is occasion in this case to recourse to other technical methods, notably the use of several aerials separated in space and the use of special radio receivers; RADIOTELEPHONE — REACTION SUPPRESSORS 2 2 1

makes the unanimous recommendation: that the fading correctors should have characteristics approaching the following: 1. Regulation during the steady state. — For variations of the carrier at the input of about 6 nepers or 52 decibels the variations of the volume at low frequency (i.e. audio) at the output of the receiver should not exceed ± 0.3 nepers or 2.6 decibels throughout the transmitted frequency band. This is generally obtained by variations of the potential of the valve grid of one or more controlled stages either by the rectified voltage of a single stage by the rectified voltage of several separate stages. 2. Time Constant. — The time constant of these instruments, for the initial transit period as well as for the final transit period should be capable of having several values between 0 . 1 seconds and 1 or 2 seconds or more. It is desirable that the technical operator should have a switch enabling him to choose the most favourable value of the time constant at a given moment. In practice, a small number of points (2 or 3) have been found sufficient. 3. Attenuation distortion. — The fading corrector should only introduce a negligible attenuation distortion at the output of the radio receiver.

2.3. Reaction suppressors and echo suppressors

Classification of various types of reaction suppressors

•The classification of various types of reaction suppressors is based on the state in which these instruments place the connection when idle, i.e. during silent intervals between the two subscribers. Consider one end A of a circuit AB and its two channels. Transmission (E) and Reception (R): firstly it may be said that it is not necessary that the suppressors at each end be identical and connections function perfectly well with apparatus to different principles at A and B. Now at one end, during an idle period there may be the following possibilities: 1° Transmitting and Receiving channels both open (ED & RD); 2° Transmitting channel open, Receiving channel blocked (ED & RBJ; 3° Transmitting channel blocked, Receiving channel open (EB and RD ); 4° Transmitting and Receiving channels both blocked (EB & RB); Now the idle condition of the connection will depend on its last working condi­ tion i.e. of the direction over which the last speech passed over the circuit; thus during the first part of a silent interval there is the following alternative: the subscriber A ceases to talk or ceases to listen (i.e. the subscriber B ceases to listen or ceases to talk). In either of these two cases, the end A may, theoretically at least, be idle in any one of the four states 1 to 4 given above. Thus there are 16 possible systems which may be represented by the following 16 combinations, in which the first figure *) represents the idle condition at the end A

* Use is made of figures in place of letters to avoid confusion that may occur depending upon the language used. 222 RADIOTELEPHONE — REACTION SUPPRESSORS when A ceases to speak, and the second figure the idle condition at the end A when A ceases to listen: 1-1, 1-2, 1-3, 1-4, 2-1, 2-2, 2-3, 2-4, 3-1, 3-2, 3-3, 3-4, 4-1, 4-2, 4-3, 4-4. The four combinations 1-1, 2-2, 3-3 and 4-4 correspond to the systems in which the idle condition of the connection is independant of the last working condition: these are those which have been most used up to the present. The combination 2-3 corresponds to the case of a system in which the circuit is, during the idle condition in the state in which it was at the end of the last speech. One might conceive a method of working where the arrangement corresponds to the reverse combination 3-2 equally interesting: in this case, the fact that one of the two talkers ceases to talk prepares the connection for the speech of the other. No system on this principle is known. The ten other combinations do not present, at first sight, very much interest and would seem to be illogical: they have not been used. This is not so for the five combinations 1-1, 2-2, 3-3,4-4,2-3 which all correspond to existing systems. Certain radiotelephone circuits of small importance and with low traffic (inter­ colonial connections for example) have been used according to the combination 1 - 1 (the two channels always open when idle). The combination 2-2 (transmit channel open and receive channel blocked) might include in particular, the principle of the echo suppressor called “ suppressor- suppressor ” in which, when idle, the transmit channel is open and the receive channel attenuated. The third combination 3-3 (transmit channel blocked receive channel open when idle) corresponds to one of the systems most used — in principle to the reaction suppressors such as the “ vodas (voice-operated device antisinging) ”. The fourth combination 4-4 (the two channels blocked when idle) has been used for a long time on the long French radiotelephone connections. Also on the same principle are the American systems called “ carrier-operated device antinoise ”. Finally the fifth combination 2-3 is the object of an interesting use in Germany (differential reaction suppressor). The design of different types of reaction suppressors is very varied. The classification given above does not make precise the method of realization nor the secondary characteristics. The C.C.I.F. considers it is not necessary to give detailed descriptions and recommends the designation of the various types of reaction suppressors by a combination of 2 figures. In order to avoid confusion one adds the letter U (unilateral to these two figures when the reaction suppressor unilaterally blocks the transmitter from the receiver.

Essential characteristics oj reaction suppressors of the type 3-3 (VODAS). — The essential characteristics of reaction suppressors of the type 3-3 (Vodas) which should be standardized for telephony use between fixed subscribers’ stations are the following:— 1. When there is no conversation the receive channel is open, the transmit channel is blocked. Classification of different types of reaction suppressors

State of the connection Combination when idle Operations performed during working indicating Examples of arrangements the type Principles of operation using these principles of reaction suppressor Transmit Receive Unblocking 1 st blocking 2nd blocking side side functions function function AITLPOE RATO SUPPRESSORS REACTION —RADIOTELEPHONE

1-1 (U) D D RBE Transmission and reception open Connections of little importance (when idle) and transmission and low traffic. blocked (unilaterally) by the re­ ceive side.

1-1 D D RBE EBR Transmission and reception open Connections of little importance (when idle) and blocked bilaterally and low traffic

2-2 D B RDR RBE EBR Reception blocked (when idle) and (Suppressor-Suppressor) blocked bilaterally

3-3 D B EDE RBE EBR Transmission blocked (when idle) VODAS and bilateral blocking

4-4 (U) B B EDERDR RBE Transmission and reception block­ System considered in France ed (when idle) and blocking (uni­ lateral) of the transmission by the reception

4-4 B B EDERDR RBE EBR Transmission and reception block­ System used in France; American ed (when idle) and bilateral blocking system called “ positive control ”

2-3 D or B B or D RDR or EDE RBE EBR “ Flip-flop ” blocking Differential control system used in Germany

to OJK) 2 2 4 RADIOTELEPHONE — REACTION SUPPRESSORS

2. When the subscriber (in one country) starts to talk the receive channel in the same country is first blocked, the transmit channel in the same country opens and the speech currents pass to the radio transmitter. When the speech stops the transmit channel is blocked after an interval of time around 1 2 0 milliseconds, whilst the receive channel is opened only after a short interval of time sufficient to ensure that the sound waves radiated by the transmitter do not enter the receive channel by an echo path at the near end. 3. At the start of the reception of the speech currents (after a sufficiently long silent period) the normal blocking of the transmit channel is maintained and the normal conditions of unblocking of the receive channel allows these currents to pass to the subscriber. When the reception of the speech currents ceases the control of the blocking of the transmit channel is suppressed but after only a short interval of time, sufficient to guard against the risk of an echo of the speech currents originat­ ing from the land circuit on the transmit channel. 4. The detectors which control the operation of the relays of the reaction suppressors “ Vodas ” are designed to differentiate between the speech currents and the noise so as to reduce possible false operation.

Essential characteristics oj reaction suppressors oj the type “ 2-3 ”. — The sup­ pressor of the type 2-3 has the following characteristics: (a) in the idle condition the transmit channel is open for the subscriber who lasts poke. When for example the subscriber in the other country has spoken last the receive channel is open and the transmit channel is blocked. (b) When the subscriber, who last spoke before the idle condition, recommences to speak, his transmit channel remains open and the opposite channel is blocked for the speech voltages which are below (or not much more) than those of the subscriber who is talking. (c) When the subscriber who was not the last speaker before the idle period starts to talk, the speech voltages unblock the new transmit channel with a sensitivity if large and a time of operation if short so that the initial syllables are not appreciably cut. The opposite channel is blocked for the lower speech voltages (or not appre­ ciably greater). (d) When the subscriber ceases to talk, the transmit channel remains open so that the final syllables of the voice are not suppressed. If the receive channel has been open, the transmit channel continues blocked for approximately 125 ms. in order that a possible echo of the last syllable shall not change the position of the suppressor in the extention circuit. (e) The suppressor only in the case where the direction of conversation changes. In this manner a useless operation is avoided at the start and finish of the intervals of silence. (j) A subscriber might intercept by raising his voice if, at the other end of the ratio link a “ flip flop ” suppressor is used. (g) If this is the case a subscriber may surmont the noise on the opposite channel (e.g. noise at the receiver) and raise his voice and thus open his transmit channel. RADIOTELEPHONE — REACTION SUPPRESSORS 2 2 5

(h) The control circuit may be arranged so that they would be influenced less by noise than by the voice. This selective operation may also be effected by automatic volume regulators adapted for this use.

Note. — The above classification may if required be added to if it appears that new types of reaction suppressors have been introduced into service. If so their essential characteristics will be given at the end of this recommendation.

Protection of reaction suppressors on a radiotelephone circuit

T h e I nternational T e l e ph o n e C onsultative C om m ittee

considering:

that as regards the protection of reaction suppressors adjacent to the technical operator against false operation by parasitic noise a measuring instrument is not always necessary, and it is probably sufficient to have a simple peak indicator operat­ ing each time the reaction suppressor has been in action, following this the technical operator adjusts the gain;

considering, nevertheless:

that it may occasionally be found desirable in the case of reaction suppressors, and it will always be the case with echo suppressors on land lines, to use measuring instruments,

makes the unanimous recommendation:

that to protect reaction suppressors on a radiotelephone circuit against false operation by noise, it is recommended to use a volume meter (volume indicator or impulse indicator) whose characteristics are given in the Green Book, Volume IV, 3rd Part, Section 3.2.2 under the title “ Volume Meters ” to ensure that the charac­ teristic, as a function of frequency, of this apparatus corresponds to that of the suppressor that it is intended to protect.

False Operation (caused by disturbing noise) of reaction suppressors or echo suppressors connected in an international telephone connection routed on radiotelephone and line circuits

In consequence of the improvement in radiotelephony circuits effected recently (in general and also as regards noise) the false operation of echo suppressors or reaction suppressors connected in an international telephone circuit routed over radio and line sections is rare. Nevertheless, arrangements are now available to avoid false operation [see annex 28 of the Book oj Annexes of Volume III (of the Green Book)] and on the other hand reaction suppressors are no longer usual in practice on line circuits.

15 2 2 6 RADIOTELEPHONE — REACTION SUPPRESSORS

2.4. Interconnection of two radiotelephony circuits by means of a 4-wire metallic circuit

The International Telephone Consultative Committee considering: that, in a large number of cases it has been possible to join two radiotelephone circuits by means of a four-wire metallic circuit by connecting in tandem the three circuits without introducing any modification of the normal arrangements; that, nevertheless, in other cases, particularly because of high radio noise or of noise produced on one of the extension metallic circuits, this method has not given satisfactory results particularly due to false operation of echo suppressors or of reaction suppressors, makes the unanimous recommendation: that, when the normal method of direct connection of two radiotelephone circuits and an intermediate four-wire metallic circuit does not give satisfactory results, there is need to employ one of the following arrangements which may give considerable improvement: (a) Take out of circuit the reaction suppressors at the junction points of the intermediate metallic circuit and the radio links; (b) Take out of circuit the echo suppressor on the metallic trunk circuit. (This procedure will be facilitated when the two halves of the echo suppressor are at the ends of the circuit.) (c) Readjust the sensitivity of the blocking arrangements to a value so that they discriminate between noise and speech currents assumed to be at a sufficiently different level. Note. — New technical methods are being studied to improve the operation of circuits in tandem. Many Administrations have studied the use of methods of distant control of the blocking arrangements, either by auxiliary signals at a specific frequency or by the carrier wave itself. SECTION 3

Principles of the arrangements employed to ensure secrecy

The International Telephone Consultative Committee

Considering,:

(a) that the arrangements in question are intended to ensure commercial secrecy rather than absolute secrecy of a radiotelephone conversation; (b) that to ensure the maximum secrecy of conversations, the detailed arrange­ ments employed and their operation should be the object of an agreement between interested Administrations and Private Operating Companies; makes the unanimous recommendation:

1 ° that the exposition below completes the study of this subject in so far as it concernes radio telephony circuits using frequencies below about 30 M/s *

1.1. Principles of the arrangements

Two principle types of systems are used to obtain commercial secrecy on radio telephony circuits working on frequencies below about 30 Mc/s.

1.1.1. Double sideband systems These system use the principle of inversion, with or without variation of the carrier frequency of, it is understood, some hundreds of c/s. The audio-frequency band being inverted relative to a fixed frequency.

1.1.2. Single sideband systems With these systems the audio-frequency band is subdivised into a certain number of sub-bands of equal width that are interchanged with or without frequency inversion according to a rhythmic cycle determined in advance to form an unintelligible signal. At the receiver the audio signal is reconstituted by an inverse procedure to that used at the transmitter: it is evidently necessary to ensure strict synchronism between the permutations at the two terminal stations.

* The recommendations concerning the question will be found in documents Nos. 47 and 48 of the Vlth Plenary Assembly of the C.C.I.R. (Geneva, 1951). 2 2 8

1.2. Characteristics of the apparatus 1.2.1 The system of chopping up the bands ensures a greater secrecy than the system of inversion but to be satisfactory the radio frequency distortion such as that caused by selective fading should be less than that for the first type of system. 1.1.2. The apparatus is designed so as to reduce to a minimum the attenuation distortion and also the level of the unwanted modulation products and of the carrier. The allowable degree of distortion due to the presence of commercial secrecy appara­ tus depends on the type of arrangement used and is fixed by common agreement between the interested Administrations and Private Operating Companies.

1.3.- Position of Equipment In order to facilitate the use and maintenance of the apparatus and for economic reasons, the secrecy equipment is generally installed at the point where the transmit and receive channels are brought together.

2° that for the frequencies above about 30 Mc/s the type of systems to be used and their method of operation should be agreed between the Administrations or Private Operating Companies concerned. SECTION 4

Requisite conditions for connections between mobile radiotelephone Stations (for example, cars, aeroplanes and ships) and international telephone lines

THE INTERNATIONAL TELEPHONE CONSULTATIVE COMMITTEE Considering: (a) that the conditions which should be defined by international agreement would appear to be small in number; ' (b) that when realized these conditions will ensure satisfactory connections between mobile radiotelephone stations and international telephone lines, (c) that telephone connections between mobile radio telephone stations [for example vehicles (motor vehicles or railway coaches) aeroplanes and ships] and metallic telephone lines (international or trunk) are still being developed and that for the moment it is not desirable to fix too rigid conditions, makes the unanimous recommendations:

1 ° that radiotelephone connections from mobile stations intended for extension into the international telephone networks should terminate (either 2-wire or 4-wire) at the trunk exchange in such a manner that they may be extended to international lines in the same way as any line circuit. Note. — For information if the termination is 2-wire it is desirable to refer to the C.C.I.F. Recommendation concerning the impedance of trunk and international ^ circuits (section 1 .2.7 of the first part of the present work). This recommendation states:

a) that it is desirable to adopt for the impedance of 2 -wire circuits the same nominal value as for the impedance of 4-wire circuits. P) that the nominal value of the impedance of 4-wire trunk circuits (seen from the switchboard jack or from the selector banks) should be standardized at either 800 ohms or 600 ohms in any one trunk exchange. In the case of a telephone connection a mobile radiotelephone station termin­ ating in 4-wires at the trunk exchange, there is occasion to refer to the conditions fixed by the C.C.I.F. for the reflection coefficient at the'junction between the 4-wire repeaters and the 4-wire line (section 5.2.1 of the first part of this work) to ascertain that the values of the impedance of the repeater, not including the line transformers, 2 3 0 RADIOTELEPHONY — MOBILE STATIONS measured at the input and output terminals of the repeater at 800 c/s should conform to the value allowed by the C.C.I.F. for the impedance of international circuits. The impedance of the repeater, excluding line transformers, will be approxima­ tely equal to that of the circuit on which the repeater is to be connected in service such that the reflection coefficient: Z — W z + w should be not greater than 0.4 for the repeater input impedance and 0.6 for the repeat­ er output impedance, Z being the impedance of the line (including the line trans­ former) and W the impedance of the repeater: these two values should not be exceeded for any frequency effectively transmitted.

2 ° that the speech volume at the input and output of the radio link towards the mobile station should as near as possible conform to the conditions recommended by the C.C.I.F. for long-distance international metallic circuits. 3° as regards the band of frequencies effectively transmitted by a radio link with one mobile station, the ideal would be to have the same conditions as for inter­ national metallic circuits. These conditions are represented by graph No. 1 of figure 64 opposite entitled: Allowable limits for the variation, as a function of frequency, of the equivalent in terminal service relative to its value at 800 c/s (band of frequencies effectively transmitted extending from 300 to 3400 c/s). Even though the limitations on this point do not arise from terminal equipments it should be considered that in the case of telephone connections with mobile radio telephone stations using carrier frequencies below about 30 Mc/s there are limitations due, for example, to the necessity to obtain the maximum telephone channels in this part of the radio spectrum and this necessitates a restriction in the frequency band of the telephone channel: also there is the inevitable noise due to atmospherics and valves. The most important case in practice is that of radiotelephone stations on board ships: it is desirable that for these the frequency band transmitted should not be less than 300-2600 c/s. The cases of mobile radiotelephone stations on motor vehicles, trains and aero­ planes should be reserved for the time being as there is little experience on the subject. 4° that noise arising from the telephone connection with a mobile radio station should not be sufficient to cause false operation of echo suppressors or other apparatus on the long-distance circuit, either national or international forming part of the connection. As regards the false operation of echo suppressors there is occasion to recall (section 1 . 2 . 2 of the 1 st part of the present work) that the minimum value recommended by the C.C.I.F. for the level of operation (referred to zero relative level) of a terminal echo suppressor is equal to —3.5 nepers or —30 decibels. RADIOTELEPHONY — MOBILE STATIONS 2 3 1

On the other hand there is occasion also to consider the operation of other apparatus particularly the calling or signalling equipment on long-distance circuits (national or international) forming part of a connection to a mobile radiotelephone station. 5° that when the mobile radio stations require to communicate with fixed stations in more than one country it is necessary to agree on the method of signalling to be used between the two stations.

Remark. — The curve of variations of equivalent as a function of frequency should lie betwen the hatched lines.

F ig u r e 64, Graph No. 1. — Variation as a function o f frequency, of the equivalent in terminal service, relative to its value at 800 cjs (International circuit transmitting effectively the frequency band between 300 and 3400 c[s)

Essential characteristics of the arrangements controlled by speech in stations on ships and by the carrier frequency in shore stations

T h e I nternational T e l e ph o n e C onsultative C om m ittee considering: a) that the essential characteristics of the arrangements controlling the carrier by the speech in ships and the arrangements controlling the carrier in the receiving shore stations are their operation times and their times to return to the idle condition; b) that the operation delays should be short in order to reduce to a minimum the clipping of syllables and the time of return to the idle condition so long that they remain in the working state during the intervals between words of a conversation. Makes the unanimous recommendation:

1 ° that on ships, the time of operation and return to normal of the voice-operated apparatus controlling the carrier should be the following: 2 3 2 RADIOTELEPHONY — MOBILE STATIONS

input level net operation time time to return to idle condition (note a) (note b) (note c) —30 db less than 25 ms between 75 and 170 ms —20 db less than 15 ms between 75 and 170 ms

2 ° that the operating time (note d) of the apparatus controlled by the carrier in the shore receiving station should also be as short as possible in order to allow an incresase in the operating time of the apparatus on board the ship and should not exceed 5 milliseconds when the level of the carrier at the input to the receiver at the shore station is 1 decibel above the value which is just sufficient to operate the apparatus. The required values for the time to return to idle condition (note c) depend amongst other factors on the time constant of the automatic gain control of the shore receiver. It may generally be considered satisfactory to have a value between 10 and 50 milliseconds. Remark (a) Input Level. — Level of a sinusoidal test signal of frequency corres­ ponding to the middle of the audio-frequency band, relative to that which produces 1 0 0 % modulation. Remark (b) Net operation time. — Period between the instant when the test signal is applied to the input of the modulator of the transmitter and the instant when the carrier reaches 50% of its maximum amplitude. Remark (c) Time to return to idle condition. — Period which separates the instant when the test signal is ceased to that when the attenuation of the carrier is within 5 decibels of its maximum attenuation or the moment when it is theoret­ ically suppressed. Remark (d) Operation time oj the apparatus controlled by the carrier. — Period between the short application of a test signal simulating the carrier transmitted from the ship and the unblocking of the receiving channel (instant when the attenuation of the receiving channel is within 5 decibels of its final value when in the receiving position). Remark (e) Time to return to the idle condition oj the apparatus controlled by the carrier. — Period which elapses between the stoppage of the test signal simu­ lating the carrier transmitted by the ship and blocking of the receiving channel (instant when the attenuation of the receiving channel is within 5 decibels of its final value when it is unblocked.) 6 T H PART

MAINTENANCE

General recommendation relative to the maintenance of international circuits

T h e I nternational T e l e ph o n e C onsultative C o m m ittee,

Considering, that, to ensure satisfactory cooperation between Administrations and Private Companies concerned with the maintenance of international telephone circuits, and to ensure the maintenance of good transmission in the international telephone service, and in particular in the European telephone network, it is necessary to unify the essential arrangements to be taken for the establishment and maintenance of international telephone circuits, including requirements for the periodicity of maintenance measurements and for the localisation of faults;

makes the unanimous recommendation: (1) that all the Administrations and Operating Companies in Europe apply strictly the “ Maintenance Instructions ” which follow and which have been established by the C.C.I.F. taking account of the long experience obtained in particular by Administrations and Operating Companies having well-developed networks, (2) that these Administrations and Operating Companies in Europe should bring to the notice of their technical services the literature assembled by the C.C.I.F. on methods and measuring apparatus for maintenance already in use in certain countries, which is given in the Book of Annexes to Volume III of the Green Book, (3) that pending the establishment of detailed instructions for the maintenance of semi-automatic circuits, Administrations and Operating Companies in Europe should work on the basis of the “ Guiding Principles for the maintenance of semi­ automatic circuits ”, which appear at the end of the “ Maintenance Instructions ”. PAGE INTENTIONALLY LEFT BLANK

PAGE LAISSEE EN BLANC INTENTIONNELLEMENT MAINTENANCE INSTRUCTIONS

SECTION 1

General

S u m m a r y o f S e c t io n 1

Introduction. — Role of the 9th Study Group responsible for maintenance questions Chapter I: Control and subcontrol stations 1. Control station 2. Subcontrol stations 3. Role of control and subcontrol stations in localizing and clearing of faults Chapter II: Designation of international circuits or of assemblies of international circuits 1. Telephone circuits 2. Circuits used for voice-frequency telegraphy 3. Circuits designated specially for phototelegraphy or facsimile 4. Circuits intended for programme or television transmissions 5. Groups of telephony circuits; 12-circuit group supergroup Chapter III: Periodic maintenance programme 1. Preparation 2. Modifications to the programme

Chapter IV: General precautions to be taken to improve the transmission stability of lnter- nation circuits in Europe Note. — Method of making vibration tests 2 3 6 MAINTENANCE — GENERAL RECOMMENDATIONS

INTRODUCTION

Role of the 9th Study Group, responsible for maintenance questions

To coordinate the arrangements required for the maintenance of international circuits a Study Group has been made responsible for all questions concerning the application of the recommandations relative to setting-up and maintaining international circuits.

This Study Group has made the following recommendations: (1) The strict application of the following recommendations for setting-up and maintenance of international circuits; these recommendations are based on long experience obtained in particular by Administrations and Operating Companies having well-developed national telecommunication networks; (2) Use by the personnel concerned of the phrases contained in the booklet published under the title “ List of phrases to be used by maintenance and fault services and in repeater stations for the maintenance of international telephone communications (3) Division of responsibilities for the maintenance of good transmission on an international link in accordance with the principles defined in Chapter 1.

CHAPTER I

Control and sub-control stations

1. Control Station For each international circuit or group of circuit (in particular a 12-circuit group and a supergroup) subject to common maintenance, one station is responsible for the setting up and maintenance of the circuit or group of circuits under con­ sideration. This station which is called the “ control station ” is designated by agreement between the technical services of the Administrations and Operating Companies concerned. The control station is one of the terminal stations of the circuit or group of circuits being considered. The control station is responsible in particular for: (a) Seeing that the maintenance tests are carried out in accordance with the methods and on the dates agreed between the various technical services and in such a way that the interruptions to traffic are kept to a minimum; MAINTENANCE — CONTROL STATIONS 2 3 7

(b) Calling in the stations concerned in the event of a fault; carrying out the different tests and investigations to ensure as soon as possible either the temporary replacement of the faulty line or equipment or the clearance of the fault;

(c) Recording, on forms provided for the purpose, of all incidents which arise; time of fault report, exact location, action taken, time' of restoration to service.

A control station is designated:

— for each international circuit,

— for each group or supergroup link: “ group control station ” or “ supergroup control station ”,

— for each line used for a carrier system: “ control station for the line” (in particular a coaxial line, a symmetrical pair line or a radio link).

A control station for a group, supergroup or line is normally one of the two terminal stations of the group, supergroup or carrier system line.

2. Subcontrol stations

For setting up and maintaining circuits (or groups of circuits) the control station calls in “ sub-control stations ”. In principle a subcontrol station is designated for each country other than the control station country. Each time cooperation is required in one of these countries, for the clearance of a fault for example, the control station is responsible for calling in the subcontrol station of the country concerned. The subcontrol station is responsible for seeing that the transmission on the national section with which it is concerned is within the prescribed limits. A group subcontrol station, in the case of a transit country, is one of the group transfer stations, and in the case of the terminal country, is the distant terminal station (from the control station) where the group is demodulated to audio frequencies. A subcontrol station for a supergroup is, in the case of a transit country, one of the supergroup transfer stations and in the case of the terminal country the distant terminal station (from the control station) where the supergroup is reduced to groups. Each group or supergroup subcontrol station is responsible for the setting-up and the maintenance of that part of the link between the transfer stations nearest to the two frontiers. A subcontrol station for a line is, in the case of a transit country, one of the intermediate repeater stations and in the case of a terminal country, the line terminal station remote from the control station. 2 3 8 MAINTENANCE — CONTROL STATIONS

3. Role o f control and subcontrol stations in localising and clearing o f faults The occurrence of a fault is reported to one of the terminal stations, either the control or subcontrol station, by the operating service; it is desirable that when making the report the traffic staff give the reason for the report; bad transmission in one or both directions, noise, loss of communication, etc. The terminal station which first receives the report should promptly advise the other terminal station. Tests for the location of faults always take priority over periodical maintenance tests. The operations are carried out in the following order at the request of the control station: (a) The control station and the terminal subcontrol station determine if the fault is on the line or in the equipment at the terminal station or exchange. (b) If the fault is not in the terminal equipment it must be localised either to a national section, or to a section crossing a frontier, by means of tests made at the transfer stations (or repeater stations) nearest the frontier, by calling in the control and subcontrol stations in the different countries concerned *. (c) Each subcontrol station must proceed quickly with the clearance of a fault located to its national section. (d) When a subcontrol station has cleared a fault on a national section or on a section crossing a frontier it should immediately advise the control station to ensure restoration to service without delay; this advice should be given by any available means of telecommunication. e) When the fault has been cleared the control station should restore the link (or circuit) and at its request overall measurements will be made: — with the group or supergroup reference pilot in the case of a group or super­ group link; — with an 800 c/s test tone in the case of a circuit.

(f) If these tests show that the levels at the end of the link (or circuit) have not their nominal values as shown on the line-up record of the link (or on the circuit hypsogram) the control station proceeds with the necessary adjustments to restore the levels to their nominal value. (g) The link (or circuit) is then restored to the operating service. In the case of unusual faults or those very difficult to locate or in the case where the same kind of fault occurs very frequently on a particular section, the control and subcontrol station should advise their respective technical services without delay, who in co-operation will take any useful action to prevent such faults in the future. The control and subcontrol stations should provide their technical services, at fixed periods, with a summary of faults which have affected international circuits.

* If it appears that the fault will cause a serious interruption to traffic, the faulty section should be made good with a reserve section provided for this purpose. If no reserve section is available the control station should be informed; the control station will advise the exchange which will advise any other exchanges concerned. MAINTENANCE — DESIGNATION OF CIRCUITS 2 3 9

CHAPTER II

Designation of international circuits or assemblies of international circuits The following rules should be applied for the designation of international circuits or groups of international circuits. The place names should always be written in Roman characters taking the official name of a town as used in the country to which it belongs. 1. Telephone circuits Telephone circuits used in manual operation: the circuit number follows immed­ iately the names of the two international terminal exchanges placed as a general rule in alphabetical order: Example: London-Paris 1. Circuits used for international semi-automatic or automatic operation: these circuits are designated by the names of the two international terminal exchanges arranged in the order corresponding to that in which the circuit is operated and the number of the circuit is preceded by the characteristic letter Z. The numbering of semi-automatic or automatic circuits operated in one or the other direction must therefore be distinct. Circuits operated in the direction corresponding to the alphabetical order of the international terminal exchanges should have odd numbers. Circuits operated in the direction corresponding to an inverse alphabetical order of the international terminal exchanges have even numbers. For example: For a circuit operated in the London-Paris direction (alphabetical order): London-Paris Z21. Fo a circuit operated in the Paris-London direction (inverse alphabetical order): Paris-London Z 18. Telephone circuits for private services or special purposes (military, diplomatic, meteorological, civil aviation, electric power distribution, banks, permanent speaker circuits between repeater stations, permanently used control circuits for sound or television broadcasting, etc.) the characteristic letter P is used. Example: London-Paris PL 2. Circuits used for voice-frequency telegraphy Circuits for voice-frequency telegraphy for normal civil operation: the letter T is used, Example: London-Paris Tl. Circuits for voice-frequency telegraphy for private or special services: the letters TP are used. Example : London-Paris TP1. In the case of telephone circuits used as reserve circuits for voice-frequency telegraphy, the designation of such a circuit as a telephone circuit (in accordance 2 4 0 MAINTENANCE — DESIGNATION OF CIRCUITS with the above) is followed by a supplementary indication, in brackets, comprising the letters ST followed by the number of the voice-frequency telegraph circuit for which the circuit under consideration is normally used as a reserve. ' Example: London-Paris 65 (ST 1) describes the circuit designated as a reserve for the London-Paris T1 voice-frequency telegraph circuit. For telephone circuits used as reserve circuits for voice-frequency telegraph circuits used for private or special purposes, the normal designation of the circuit is used followed in brackets by the letters STP and the number of the voice-frequency telegraph circuit for which the circuit under consideration is normally used as a reserve.

3. Circuits specially designated for phototelegraphy or facsimile In the case of a circuit specially designated for phototelegraphy or facsimile the designation of the circuit as a telephone circuit (in accordance with the above) is followed by a supplementary indication, in brackets, comprising the letter F followed by the number of the circuit, when it is used for photo telegraph transmission. Example: London-Paris 23 (FI).

4. Circuits for programme or television transmissions ■ The letter R is used in the case of a unidirectional sound programme circuit and the letters RR in the case of a reversible sound programme circuit. In the same way the letters V or VV are used for television circuits. The names of the terminals in the designation for an unidirectional circuit (for sound or television) are placed in the order corresponding with the direction of transmission (instead of alphabetical order). Examples: — circuit transmitting only in the direction London-Paris, London-Paris R 1 or London-Paris V 1 — circuit transmitting only in the direction Paris-London, Paris-London R 1 or Paris-London V 1 — reversible circuit London-Paris RR1 or London-Paris VV1.

5. Groups of telephone circuits: 12-circuit group, supergroup 12-circuit groups are designated by the numbers 901, 902 . . . 999 (for example London-Lugano 901), the 900 series being reserved exclusively for numbering these groups. Supergroups are designated by the numbers 6 001 . . . (for example Amsterdam- London 6 001), the series 6 001 to 6 999 being reserved exclusively for the numbering of supergroups. The above numbers are used for the group or supergroup from the point where it is assembled to the point where it is broken down, independently of the position it occupies in the band of frequencies transmitted on the line. PERIODICAL MAINTENANCE PROGRAMME 2 4 1

CHAPTER III

Periodical maintenance programme 1. Preparation In order to reduce to a minimum the correspondence and discussions required for the organization of periodical maintenance measurements on the, international telecommunication network, a Commission, on which all the interested countries should be represented, meets each year to prepare a “ Programme of periodical maintenance ” and to discuss questions arising from the execution of this programme. This “ Programme of periodical maintenance ” shows simply the days (and not the times) when the periodical maintenance tests should be carried out; it gives the days for testing international circuits (telephone, telegraph, sound and television programme circuits), as well as the days for testing international group and super­ group links. The dates for test fixed in the Programme of periodic maintenance are determined in accordance with the rules relative to the periodicity of tests to be made on inter­ national circuits or on carrier systems. These rules are given in sections 2 and 3 of the Maintenance Recommendations. The Programme of periodic maintenance is sent to the technical services of the different Administrations concerned and by them to the control and subcontrol stations. This document is also sent to the Operating Services in place of the “ List of international circuits ”. 2. Modifications to the programme The days for testing new international links or circuits as well as modifications to the days for testing existing international links or circuits are determined by the technical service to which the control station is responsible, in agreement with other interested technical services. If the technical service responsible for a subcontrol station considers it necessary to alter the testing days for an international link or circuit, it should contact the technical service of the control station, who will make the necessary arrangements. It is the responsibility of the President of the above Commission to call a special meeting of this Commission if particularly important questions arise relative to the execution of the Programme of periodic maintenance or if particular difficulties in maintenance frequently recur. 2 4 2 TRANSMISSION STABILITY

CHAPTER IV

General precautions to be taken to improve the stability of transmission on international circuits in Europe

There are grounds for the application of the following arrangements:

1. After each periodical maintenance measurement, and each time a fault is cleared on an audio circuit, the equivalent of the circuit should be brought back to as near its nominal value as possible, even though the measurement shows that the equivalent is within the permissible limits. In the case of a circuit routed on one or more carrier sections, the channel terminal equipment should not be readjusted until the levels of the reference pilots of the groups and supergroups on which the circuit is routed have been checked and adjusted to their nominal values if they are out of limits. This having been done the equivalent of the circuit should be adjusted to as near as possible its nominal value by adjusting the gain control potentiometer individual to the circuit, if one exists; so far as possible small adjustments should be avoided when these involve changes to soldered connections. After each periodical maintenance measurement and each time a fault is cleared on a group or supergroup the line-up should be systematically readjusted even if within the permissible limits, on condition that the restoration to service of the group or supergroup is not unduly delayed.

2. If any item of equipment at an intermediate station is changed the sub­ control station for the section concerned should be informed.

3. Naturally the stability of transmission on carrier circuits can be improved by the use of automatic regulation or by more careful manual regulation.

4. Vibration tests in accordance with the note which follows entitled “ Method of making vibration tests ” should be made: (a) at the time new equipment is put into service. (b) as a periodical preventive maintenance measure. The periodicity of such vibration tests (e.g. once a year or once every two years) will be determined by the Administration concerned and other tests made in between if there are special reasons for doing so. Concerning point (a), the urgent requirements of operating services have sometimes resulted in equipment being put into service (for audio and carrier circuits) which have not been sufficiently tested beforehand (in particular for soldered joints, valve contacts etc.), in these cases the equipment must be temporarily taken out of service and a thorough inspection made as soon as possible to remove all causes of incipient faults. VIBRATION TESTS 2 4 3

On the other hand, in the future, equipment must not be put into service until after the most thorough inspection by means of vibration tests, and it is necessary to ensure that the pressing needs of the operating services do not result in the suppression or reduction in time of these tests. The vibration testing foreseen under (a) and (b) above naturally necessitates sufficient technical staff being available, but it is only at this cost that an international service with a satisfactory quality of transmission can be guaranteed.

5. Every precaution should be taken in repeated stations to avoid interruptions, even very short, on working circuits. This is particularly important for telephone circuits for automatic or semi-automatic operation and circuits used for voice- frequency telegraphy or for phototelegraphy.

NOTE

Method of making vibration tests used by the British Telephone Administration

I ntroduction

Contact defects are, undoubtedly, one of the major causes of instability and short interruptions on circuits. These defective electrical connections are liable to be introduced at all stages from the design and manufacture of a component to the time the circuit is put into service. Although the clearance of a contact defect is straightforward its location may present many difficulties: the type of defect which causes intermittent short duration faults is particularly difficult to locate. The seriousness of the difficulty, with the mainten­ ance methods available, can be appreciated when it is realised that a London-Glasgow circuit routed over a 12-circuit carrier group has approximately 7 000 soldered connections and 6 000 pressure contacts (U-links, valveholder connections, etc.), distributed over 24 repeater stations, many of which are unattended. The investigation carried out in 1945-1950 by the British Administration has shown that preventive maintenance methods for locating a defect before a service failure occurs need revision, the most important of which is the introduction of continuous monitoring over a period at several points on a circuit in service. This enables a defect or recurring fault to be located to a repeater station with certainty and in the shortest possible time, without withdrawing the circuit or system from service. Contact Defects. — The family of contact defects so far encountered includes many which are unwittingly cleared, temporarily, by the application of normal maintenance methods. The degradation of service due to them has not been fully revealed in normal fault data. The types of contact defects found include: — Unsoldered joints; defectively soldered joints (“ Dry joints ” or “ H.R.s ”)• — Variable wire-wound potentiometers. — Defective spot welding of resistance wires and valve electrodes. — Valveholder contacts and valve pins. — U-link springs and sockets. 2 4 4 VIBRATION TESTS

— Dry riveted and screwed connections. — Plug and jack sleeve or springs. — Spring contacts in jacks and keys. — Unwetted relay contacts. — Broken wires in loose mechanical contact. — Spurious contacts between wires, or between wires and earth. — Loose connections on copper oxide rectifiers. — Bad contacts on pressure-mounted crystals in crystal filters. — Poor connection of screened conductors. — Poor connections on heat coils and mountings. — Poor connection between line fuses and mountings.

The performance of a transmission system is generally assessed in terms of the failure of the service to the user or operating service. This may give rise to a fault report and the cause will not be found unless the fault persists long enough to be located by transmission measurements. Because of the time factor many contact defects, which degrade the service, may cause a series of fault reports over a period of months before location is possible by the maintenance engineer. During this period needless out-of-service time and expenditure of engineering effort is incurred. A clean contact connection such as that between U-link and socket or a clean wire wrapped tightly round a clean tag, will cause no degradation of circuit performance until atmospheric corrosion, dirt, damp or mechanical fatigue produces contact resistance. The contact defect then becomes unstable and the slightest displacement of the contact may either partially restore the electrical connection or cause a complete disconnection. A greater displacement will sometimes temporarily clear the defect so that it will lie hidden until sufficient time has elapsed to allow the contact resistance to develop again. A very troublesome contact defect in line plant is the type which may be temporarily cleared when a normal testing power of 1 mW is applied to the circuit. A few defects have been found, particularly in crystal filters and rectifiers, which could be temporarily cleared by a testing power as low as 10 db below the normal level. These faults invariably recur.

Vibration testing technique

Principle of test. — Without special testing equipment the location of contact defects is often a matter of chance. Unless inside a component, most unsoldered connections can be located by very careful visual examination; this is the only way of locating a clean, tight, unsoldered connection. On the other hand, a dry soldered connection often looks perfect yet gives rise to a series of short duration faults the clearance reports for which are “ Right when Tested ” or “ Found O.K.”. To achieve the design performance of a circuit it is essential that every contact defect be eliminated. This can be done by first applying a vibration test to every point in the circuit, followed by a very careful visual inspection, and then by making continuous observ­ ation of a signal on the circuit for at least 24 consecutive hours during weekdays by means of a recording decibelmeter. If the overall loss of the circuit is not stable further tests are made to locate the cause of the variations. The principle of vibration testing is to pass a signal through the equipment to be exam­ ined and apply, in stages, gradually increasing intensity of vibration to each part of the equipment. The sidebands, resulting from the disturbance of a contact defect, are then detected by a suitable device connected to the output of the equipment under test. VIBRATION TESTS 2 4 5

For audio-frequency equipment the sidebands are passed to a high-gain loudspeaker amplifier and there produce audible clicks. For carrier and coaxial equipment it is necessary to detect the sidebands and reduce one of them to the audio-frequency band before connec­ tion to the loudspeaker amplifier. In the particular case of amplifiers, except for the input and output circuits, a contact defect is associated with D.C. from the normal power supplies. Vibration of a contact defect in a D.C. circuit generates a square-top waveform and a wide band of frequencies results. These may be detected as clicks in a loudspeaker. To detect contact defects in the input and output circuits of amplifiers and other equipment where no D.C. flows it is essential that the contact passes a “ wetting ” A.C. signal.

Test Signals. — Some dry-soldered connections and contact defects due to crystal mountings in filters, that have completely failed a transmission path, have been cleared by the application of the normal testing power of 1 mW, but no defect has yet been encountered which could be cleared with a power lower than 10 db below 1 mW. If this type of contact defect is to be located it is advisable in the first stage of the vibration test to ensure that at no point in the transmission path under test is the level of the “ wetting ” signal higher than — 10 db (ref. 1 mW). In the final stage the power may be raised to the maximum that the equipment under test will transmit without overloading. It is essential that the equipment under test should effectively transmit the test frequency. The sideband energy developed by disturbing a contact defect is a function of the A.C. voltage across the defect and the ability to detect it depends on the effective gain between the defect and the loudspeaker. For shunt resonant circuits, such as those in equalisers which affect the transmission most at a particular part of the transmitted frequency spectrum, some increase in sensitivity can be obtained by choosing a test frequency in this part of the band.

Fault Detection. — The resistance of a good soldered connection measured with low level A.C. is about 0.001 ohms, whilst that of a dry soldered connection may be anything between 0 . 1 ohms and infinity, depending upon its condition and the period that has elapsed since it was last disturbed. In the early stages of development the contact defect may have a relatively low resistance and if it exists in a high impedance path the sideband energy developed when the defect is disturbed may be below the noise level of the equipment. It is essential therefore that the detecting device shall be as sensitive as possible. A suitable apparatus developed for this purpose is the Loudspeaker-Amplifier, 6 B, described below.

Loudspeaker-Amplifier 6 B. This portable, mains-operated loudspeaker-amplifier (fig. 1) employs a conventional three-stage R-C coupled negative feedback circuit with wafer-type control switch allowing the gain to be reduced 40 db in steps of 8 db; a toggle switch allows a further reduction of 30 db for monitoring speech or music circuits. At maximum gain, with an input voltage of 17.4 mV at 1 000 c/s, the output voltage across 3 ohms is 1.74 volts with a harmonic content less than 2 per cent. The frequency response of the amplifier has a spread of 3.5 db from 300 c/s to 10 000 c/s. With the input closed with 2 4 6 VIBRATION TESTS

600 ohms and at maximum gain the noise level across 3 ohms is less than 6 mV unweighted, and 1 mV when measured with a broadcast weighting network. Under normal room noise conditions an input signal 96 db below 1 mV at 1,000 c/s can be heard 2 yards from the loudspeaker. In the design the size (131/2 in. X 93 / 8 in. x 5 % in.) and weight (7 lb) have been reduced to a minimum by the use of a wafer-type loudspeaker and electrolytic decoupling capacitors. Although the loudspeaker is very close to the valves the choice of a non-microphonic valve for the first stage eliminates the risk, with such high gain, of an acoustic howl between the loudspeaker and the first stage valve. To reduce A.C. mains pick-up from the mains transformer and external sources the input transformer has a mumetal can.

Method of Applying Vibration. — In applying the vibration test the nature of contact defects and the ease with which many of them can be temporarily cleared must always be uppermost in mind. Defects have been found which are to unstable that by gently blowing on them their presence is revealed by clicks in the loudspeaker. At the same time this extremely light disturbance is sufficient to break them down completely. To avoid breaking down contact defects it is therefore most essential to avoid disturbing as a whole the equip­ ment under examination. The first stage of the vibration test, before any covers or U-links or similar connections are disturbed, is the connection of the test signal and detecting device to the equipment. External U-links are then moved imperceptibly whilst listening for clicks in the loudspeaker. The equipment cover is removed by easing-off as carefully as possible. Clicks from the loudspeaker indicate that a contact defect has been disturbed. Experience has shown that components and wiring on panels are best tested in a systematic sequence, e.g., variable gain-controls are rotated slowly, valves displaced care­ fully and slowly with a very slight rotary action in the valveholders, then with the aid of a small insulated tool, such as the handle of a screwdriver, cable form, tags, soldered con­ nections and components are gently touched (not tapped) whilst listening for clicks from the loudspeaker. A click may be heard when a connection is lightly touched but may not recur when touched a second time because the defect has been broken down. However, such a defect might be revealed at a later stage in the test. The procedure is then repeated, very lightly tapping all connections, tags, components (including valves) and the cause of any clicks is investigated. When located the defect is cleared before continuing the test. Valves that are abnormally microphonic or have loose electrodes are replaced. The procedure is then repeated a third time, tapping harder so that contact defects mechanically held by resin or rivets are disturbed and so that sufficient vibration is trans­ mitted from cans to components and wiring inside the can. Finally, all wires and tags are pulled gently both ways along the axis of the wire and at right angles to it. This detects rigid mechanical joints which are unsound electrically. The pull is adapted to the type of wire and component involved so that no damage is caused. Wire “ nicked ” in the process of removing insulation or wire brittle with age may easily be fractured by this process, but it is preferable that this fracture occurs whilst the equipment is under observation rather than during cleaning operations by non-technical staff. A VIBRATION TESTS 2 4 7 fractured wire still in electrical contact would in due course corrode and develop into an unstable defect. Carbon resistors and small capacitors suspended in the wiring, that are liable to touch tags, earth points or covers when lightly disturbed are repositioned. Completely sealed crystal filters using pressure-contact crystal mountings are gently struck with the closed hand along the length of the cover. Excessive vibration, however, may either temporarily clear an existing defect or completely displace a crystal in its mounting. When testing equipment which uses valves, the power supply connections also are tested. Defects in bus-bar connections, fuses and voltage regulators give rise to clicks in the loudspeaker when disturbed. When the equipment has been freed from all defects the cover is replaced and struck quite hard with a closed hand and no clicks should be heard from the loudspeaker. A small wooden-handled screwdriver weighing about 2 ounces is suitable for applying vibration to the equipment but, unless the handle is covered with rubber, it may not be possible to distinguish between the direct noise due to tapping and simultaneous faint clicks from the loudspeaker. Generally, however, the ear can discriminate between a click expected at the moment of tapping and random clicks, even in the presence of steady noise. A special pair of pliers with long flexible insulated jaws has been designed. This tool produces a tweezer action for gripping a wire from the normal grip action applied to the handle.

Acoustic Coupling between Loudspeaker and Equipment under Test. — Some difficulty is experienced when vibration testing is applied to equipment having microphonic valves. Although clicks can be heard above the microphonic noise the high gain in the loudspeaker- amplifier necessary to detect small contact defects is liable to set up an acoustic howl between the loudspeaker and the valves. This may be overcome by the use of an extension valve adaptor, consisting of a valve base wired with 9 in. flexible leads to a valveholder. The valve and valveholder are first vibration tested and the valve is then carefully removed from the panel. The valve base of the adaptor is inserted in the equipment valveholder and the microphonic valve in the extension valveholder. This eliminates microphonic noise which would otherwise occur when the equipment is tapped. The valve is enveloped in cottonwool to reduce the acoustic coupling.

Application to Transmission Testing

Audio-Frequency Equipment. — The use of a very low-level signal, just above the equipment noise level, and a simple, high gain loudspeaker-amplifier has the disadvantage of a continuous audible tone from the loudspeaker and a very low-level “ wetting ” signal passing through the contact defect. By introducing a low-pass filter before the loudspeaker-amplifier to cut off the testing frequency and its harmonics, the level of the test signal through the contact defect can be increased in proportion to the suppression of the test signal by the filter, with a corresponding increase in sideband level. However, the maximum value at any point in the equipment should not be greater than — 10 db (reference 1 mW) or exceed the overload point of a component. Figure 65 shows the arrangement of the testing equipment. 2 4 8 VIBRATION TESTS

SB

F ig u r e 65. — Block schematic diagram o f fault detector for audio-frequency equipment E = Equipment under test 48 A = Transformer No. 48 A 6 B = Loudspeaker-amplifier 6 B

Pending development of standard equipment a low-pass filter giving an attenuation of 65 db at 2 800 c/s and 5 600 c/s (second harmonic) has been constructed from standard equaliser components. This filter is used with a 2 800 c/s testing signal from a standard audio-frequency oscillator through a variable attenuator. After an initial vibration test at a level not exceeding — 1 0 db at any point in the equipment under test, a test is made at the maximum level the equipment can handle, with a corresponding increase in sideband level. For testing 2-wire repeaters, a test signal of 1 900 c/s and a low-pass filter with a cut-off frequency of 1 900 c/s are necessary.

Multi-Channel Voice-Frequency Telegraph Equipment. — The low-level “ making ” tone is used as the test frequency appropriate to the looped telegraph channel. Due to the unsmoothed H.T. supply to the detector-amplifier and the narrow bandwidth of the filters the detection of the sidebands generated when a minor defect is disturbed is difficult. Experiments have shown that defects of sufficient importance to affect the performance of the telegraph system can be detected by taking off the sidebands developed across the anode decoupling condenser, through a 0.1 jj.F capacitor and 100 c/s low-pass filter to the loudspeaker-amplifier.

Carrier and Coaxial Line Equipment. — The experimental equipment in figure 6 6 has proved effective in detecting defects in carrier line equipment. The 40 kc/s test signal is derived from the test equipment normally provided at unattended carrier stations. This is fed through an attenuator to the equipment under test and the output connected to the detector.

9 A ; 0J

F ig u r e 66. — Circuit diagram o f fault detector for carrier-frequency equipment (1) = In (2) = Out 9 A = Equaliser 9 A PH = High-Pass Filter 48 A = Transformer No. 48 A VIBRATION TESTS 2 4 9

The experimental detector is made up from standard components. It incorporates a high-pass filter for suppressing microphonic noise below 16 kc/s. a bridge rectifier for detecting the sidebands generated when a contact defect is disturbed, and a loudspeaker- amplifier. The meter is included so that the test signal level from the equipment under test can be adjusted to ensure that the rectifiers are not overloaded, and that they operate on the most sensitive part of their characteristic. Tests with half-wave rectifiers of the Westector type showed that frequently under field conditions they were permanently damaged by high level signals. With an output level of — 10 db (reference 1 mW) from the equipment under test, the gain control is adjusted to give 0.1 mA steady rectified current. Under these con­ ditions 0.1 db change in level can easily be detected with the Loudspeaker-Amplifier No. 6 B. Coaxial line equipment and group and supergroup equipment have been vibration tested using a coaxial connection direct to the bridge rectifier. If a defect was located to a panel whose components and wiring were inaccessible with the panel on the rack, the final location had to be made on a test bench.

Carrier and Coaxial Terminal Equipment. — For vibration testing both 12-circuit carrier and coaxial terminals the equipment must be made spare by withdrawal from service. The equipment is looped at the H.F.R.D.F. or G.D.F., and audio-frequency vibration testing technique applied at the channel ends. All equipment in the transmission path is tested with the oscillator and detector connected to each channel in turn. For modems in which both directions of transmissions are on one panel, it is advantageous to test both at the same time. Equipment common to all channels is tested only once by observing on one channel. The carrier-frequency generating equipment may be tested when observing on the appropriate channel. Alternatively, where no terminal equipment is available, the detecting equipment shown in figure 6 6 can be used straight across the output of the carrier frequency generating equipment. The setting of the gain control is at minimum initially, and is then increased to give the appropriate rectified current.

Typical Test Results. — With the aid of experimental vibration testing equipment, coupled with visual inspection, the defects on line equipment eliminated by the Area Trans­ mission Efficiency Officers and their staffs between March 1948 and March 1949 were 219 239 on 24 218 bay sides. This represents an average of 9.0 defects per bay side. Based on the percentage of the work completed it may be anticipated that the total defects eliminated in Great Britain and Northern Ireland will be about 350 000. A broad classification of the defects is given in Tables 1 and 2. 2 5 0 - VIBRATION TESTS

T a ble 1 Average number of defects per bay side (or vertical)

Coaxial 12-Circuit carrier Audio equipment M.C.V.F. Telegraph Distribution frames

10.2 8.9 12.1 5.4 4.6

T a ble 2 Average number of defects per bay side (including frames)

Components Wiring Valves and miscellaneous

Carrier equipment ...... 5.6 0.7 3.1

Audio equipment ...... 9.3 1.7 2.3

Proposed Application to New Equipment. — Application of vibration testing to new equipment has shown that some contact defects have escaped detection at all stages in production from component tests to final acceptance tests, and have become a maintenance liability until finally cleared as a result of fault reports. It has been shown that by eliminating defects prior to functional tests the time for acceptance tests can be reduced and programmes of acceptance testing arranged with reasonable accuracy. Factory experiments are being carried out with a view to eliminating faulty components after the panel wiring stage. The time between the manufacture of components and com­ pletion of a wired panel may be several months and dry soldered connections in components may have had time to develop, and can be located by a vibration test. With a view to new equipment going into service with a reduced fault liability it is proposed to include vibration testing in the specification for installation of transmission equipment. A trial of this nature has been carried out on all the equipment provided on a London-Manchester route which was converted from 12-circuit to 24-circuit working. Vibration tests followed by observations on recording decibelmeters have shown that a very high degree of stability has been achieved.

Continuous Monitoring

General Arrangements. — Vibration testing is a highly skilled operation, but even with qualified staff the elimination at the first attempt of all contact defects on the equip­ ment used in a circuit is not certain. Continuous observation on a circuit is therefore most desirable to confirm that the circuit is free from fault. Such observations are made on circuits by means of a test tone and one or more recording decibelmeters (Decibelmeter No. 14). VIBRATION TESTS 2 5 1

The decibelmeter is a moving-coil recording meter which has a range of — 10 to + 5 db, relative to 1 mW in 600 ohms, over the frequency band 50 c/s to 12 kc/s. The chart, calibrated in decibels, is driven by a synchronous motor at a speed of 1 in. or 6 in. per hour, and the pen system allows interruptions of 10 mS to be recorded as approximately 1 / 8 in. movement of the syphon pen. High and low level adjustable alarms are incorporated, operated by the movement. If the circuit to be monitored is spare, a stabilised test tone at a level of 20 db below 1 mW is applied to the circuit and the decibelmeter, preceded by a suitable amplifier, is connected to the circuit at the distant end. Provided a transmission path is free from contact defects and unstable components and provided the equipment is operated from stabilised power supplies, then the variations in level on the path are due to fundamental changes such as the variation in attenuation with temperature of coaxial and carrier cable. For audio-frequency circuits the temperature effect is very small and therefore it would be expected that day-to-day changes in level of a test signal transmitted over the transmission path would also be small. The use of the recording decibelmeter has shown that if a circuit is free from defects the changes in level over long periods may vary between 0 . 2 db and 1 db, depending on the length of the circuit and type of amplifying equipment. A 400-mile circuit using Ampli­ fiers No. 32 should not vary by more than ± 0.2 db. A circuit with contact defects, faulty components or valves shows considerable variation in level over short periods, and if the defects are disturbed, transient changes occur which may disconnect a circuit for a few milliseconds. A contact defect in the feed-back path of an amplifier may cause momentary rises in recorded level. In general, a defect likely to affect the overall loss of a circuit behaves in a characteristic manner and exhibits a characteristic trace. It is sometimes possible, therefore, to diagnose the type of defect from a record taken over a period of time on a decibelmeter.

Continous Monitoring on Transmission Paths in Service. — One contact defect of a recurring transient nature can be, and often is, the cause of the bad fault record of a circuit. From the point of view of the maintenance and operating staffs, however, it is unsatisfactory to withdraw a circuit from service for vibration testing and continuous monitoring for long periods, to find perhaps only one defect; this aspect is even more serious when the H.F. path of a carrier or coaxial system is concerned. Continuous monitoring on transmission paths in service is, therefore, attractive from both the service and the maintenance points of view. It allows lost circuit time'due to faults to be reduced and improvement in circuit performance to be made between reported faults. It permits the maintenance engineer to carry out his work in clearing faults without being pressed to restore a circuit or system to service before he is satisfied it is fault free. It also allows him periodically to check the performance of circuits in service, and to detect and clear a defect before service is seriously affected or a fault reported.

M.C. V.F. Telegraph Circuits. — Figure 67 shows a method of monitoring both direc­ tions of a V.F. telegraph circuit in service. It utilises a band-pass filter 2 500 ± 50 c/s (Filters, Frequency 38 A), and a monitoring test signal of 2 580 c/s. This signal is capable F ig u r e 67. — Block schematic diagram of equipment for monitoring M.C.V.F. telegraph circuitsin service, indicatinglevels of test tone

High impedance _ 2 / attenuator (25 db) ZftO -Bf - u - 8/ ZiRO db meter / 1 y |m | No. 14 ® ~ K i <3D— T — < <3> db meter [HI No. 14 -2D — | | ....

f - C f b - ^ “ 1 — j— i i 1 l > TESTS VIBRATION 1 HI t l i n o ( d t ) Terminal repeater t station t M.C.V.F. M.C.V.F. Telegraph Telegraph equipment equipment 8/ i

Terminal Intermediate V.F. change-over repeater repeater u links I station station ■ lL.jJ < «

Line Line

Stabilised OSC VIBRATION TESTS 2 5 3 of being transmitted by Tariff E circuits and will not interfere with the V.F. telegraph channels. The bridging loss of the monitoring equipment to V.F. telegraph channels is negligible. A similar arrangement to that shown at the receiving end may be connected at various points in the circuit. All recording decibelmeters on the circuit up to the defect will show a straight line record and those beyond should show the same irregular trace.

Speech Circuits. — One monitoring arrangement, shown in figure 6 8 , is similar to that used on V.F. telegraph circuits except that additional filters are required to prevent the monitoring signal reaching the subscriber and prevent speech frequencies entering the monitoring circuit. The additional filters are band-stop, 2 580 ± 100 c/s, and one is required at each end of each direction of transmission. Alternatively, “ Filters, Composite, Speech + Duplex ” may be used. These filters incorporate a band-stop filter (1 470 c/s to 2 080 c/s) and a band-pass filter (1 560 c/s to 2 000 c/s), the frequency of the monitoring signal being 1 740 c/s. For monitoring one direction of transmission at a time the composite filters can be connected in the 2 -wire path but if echo-suppressors are used in the circuit they have to be rendered inoperative and the circuit degraded by 3 dfi. From a maintenance aspect it is preferable to monitor each direction of transmission separately from the 2 -wire side and to place the responsibility for checking circuit performance at the receiving end where the recording decibelmeter provides the performance data. This can be achieved by introducing 2 800 c/s L.P. filters in both the 2-wire ends, and connecting on the line side of these filters a band-pass filter 3 kc/s ± 50 c/s at one end and 3.2 kc/s ± 50 c/s at the other end (fig. 69).

F ig u r e 69. — Block schematic diagram o f equipment for simultaneous monitoring in both directions on 2-wire ends of speech circuits A = Station A B = Station B A -*■ B = Control direction A-B db = db meter No. 14

Speech Circuits Routed over Carrier Channels. — Equipment similar to that shown in figure 6 8 or figure 69 is connected in the channel ends. This arrangement will check the stability of the channel, but if there is a defect on the H.F. path it can only be located by pilot monitoring equipment at all amplifying points. High impedance ZERO - 8 / ~ ig - / o -S S ’ / 8SS ZERO ® 4

I i j »—‘jf.*

F igure 68. — Block schematic diagram of equipment for monitoring on speech circuits, indicating levels of tests tone VIBRATION TESTS 2 5 5

Experimental equipment which monitors a 60 kc/s pilot injected at the sending end of the H.F. path has proved a reliable method of locating contact defects.

Music Circuits. — As these circuits are unidirectional only one set of composite filters is necessary. Experiments have been made using a 10 kc/s low-pass filter in the music path and a 1 2 kc/s high-pass filter for taking off the monitoring signal of 1 2 kc/s at the receiving end (fig. 70). For music circuits which cut off at 8 kc/s or 15 kc/s other filter combinations will be required.

m >

F ig u r e 70. — Block schematic diagram o f equipment for monitoring music circuits

Interpretation of Chart Records. — Typical specimens of the traces on circuits in service are shown in figure 71. Figure 71 a) shows the trace of a 400-mile looped audio Y.F. telegraph circuit 2 !/ 2 years after overhaul using vibration testing. Figure 71 b) shows the trace of a 450-mile carrier circuit routed over 250 miles of 12-channel carrier and 200 miles of coaxial path. Using 60 kc/s pilot monitors defects were observed on the 12-circuit carrier path and finally located by vibration testing. The coaxial path was overhauled at all stations, involving the withdrawal of the system from service week-end for 6 months.

F ig u r e 71 Typical traces recorded on circuits in service

(> 5% i c 1(0 11 48 /3 IH h5 hS 4 9

F i g u r e 71 (a). — Taken on 400-mile looped audio V.F. telegraph circuit 21/ 2 years after vibration testing 2 5 6 VIBRATION TESTS

Temporary gain Carrier system adjustment in service

Temporary increase in gain 6 db beyond fault

Faulting

4 ends of circuit open-circuit

V.F/system failure; changed to reserve circuit Taken Taken on 450-mile carrier circuit — —

Carrier cable fault (b). developing 71 igure F

Maintenance working party

+ + + + + VIBRATION TESTS 2 5 7

Figure 71 c) shows the trace of successive failures of a V.F. system on a carrier channel due to interelectrode contact in the valve in the carrier channel panel. The clear of the fault in both cases was recorded as “ measured and found O.K.”

Gain correction

F ig u r e 71 (c). — Traces produced by inter-electrode contact in valve in a carrier channel panel

F ig u r e 71 (continued) Typical traces recorded on circuits in service

3 S/ 3 2 § 3 OU 3o o °.S T3 S

Interelectrode Interelectrode contact channel-amp. contact

F ig u r e 71 (d). — Traces produced by a contact defect

17 2 5 8 VIBRATION TESTS

Figure 71 e) shows.the effect of a typical carrier cable fault.

F ig u r e 71 (continued) Typical traces recorded on circuits in service

First appearance , Second appearance of a contact defect of contact defect

Conclusions.

Work is proceeding on equipment designed to give improved facilities and greater sensitivity in fault detection, and a new mains-operated H.F. fault detector developed by the Research Branch will enable a defect causing a level change of 0.01 db to be located. Standard oscillators of the neon-stabilised milliwatt type are being designed for use on continuous monitoring; these will replace the modified Oscillator No. 13 employed at present. Meanwhile, there seems no doubt that vibration testing will help to reduce the faults due to line plant and improve the performance of line plant in the period between successive faults. Elimination of contact defects in new equipment will enable a true measure of the design performance to be assessed. Continuous monitoring will provide an invaluable aid to the maintenance engineer in solving recurring fault problems. It will also check the stability of new equipment or new circuits. As a means of providing basic data for study of weaknesses in maintenance methods and fault-reporting procedure, it should be of great assistance to the Engineering Department. SECTION 2

Establishment and maintenance of international carrier systems providing at least one primary (12-circuit) group

S u m m a r y o f S e c t io n 2

Preliminary note. — Designation of groups or supergroups and numbering of telephone channels of a carrier system.

Chapter I: General. 1. Definitions concerning international carrier systems 2 . Types of measurements to be made.

Chapter II: Bringing itito service of an international carrier system. 1. Preliminary exchange of information.

2. Establishment of high-frequency line for the bringing into service of a carrier system. 3. Establishment of international group and supergroup links. 4. Establishment of telephone channels.

Chapter III: Periodic maintenance of an international carrier system. 1. Coaxial line regulating section. 2. Regulating section on a symmetrical pair. 3. Supergroup link. 4. Group link. 5. Apparatus use for maintenance measurements. 6. Measurements of non-linear distorsion and tests on valves. 7. Check of master oscillators. 260

Basic Group A Basic Group B

Channel No...... A 8 A 3 A U A 5 A 6 A * A 8 A 9 A f°A H A tS VK 18 K U K W K 8 K 6' K 5 K U K 3 K 8 K 1

Frequency kc/s .....->■ 18 60 108 CHANNELS AND GROUPS OF DESIGNATION

F ig u r e 72. — Designation of channels in groups

(basic supergroup)

1 2 Group No. -v. ^ ^ 5 ^ k ^ 3 . 8 .7 L s . 3 , r r . 5 J . 3 . S J Supergroup No.

Channel No. •Frequency kc/s 6o 108 186 80k- 888 300 318 ~ 360 U08 bS6 50k ’558 58k 618 660 fo8 $56 80k

F ig u r e 73. — Designation o f groups in supergroups DESIGNATION OF GROUPS AND CHANNELS 26 1

PRELIMINARY NOTE

Designation of groups or supergroups and numbering of telephone channels in a carrier system

1. Designation of telephone channels within a group

The position occupied by a telephone channel within a group is defined by a number (in Arabic figures) from 1 to 12. The numbers of the different channels are taken in frequency order: — in ascending order when the group is “ erect ”, in which the audio frequencies are in ascending order in the different channels (as in basic group A), — in descending order when the group is “ inverted ”, in which the audio frequencies are inverted in the different channels (as in basic group B, and in basic groups C, D and E considered later). This designation is shown in figure 72.

2. Designation o f groups within a supergroup

The position of a group within a supergroup is defined by a number (Arabic figure) from 1 to 5. These numbers are taken in frequency order: —- in ascending order when the groups are “ erect ” groups (as in the groups of the basic supergroup 312-552 kc/s), — in descending order when the groups are “ inverted ” groups (as for groups within the other supergroups). This designation is shown in figure 73.

3. Coaxial systems

(a) Numbering of a supergroup. — The different supergroups used in a coaxial system are defined by numbers (Arabic figures) corresponding to their respective position in the frequency spectrum transmitted on the line. This numbering is shown in figure 74. (b) Numbering of, a group. — The position of a group is designated by the number of the supergroup in which it is placed, followed by the number of the group within the supergroup. (c) Numbering of a channel. — The position occupied by a telephone channel in a coaxial system is designated by three figures (e.g. 3 — 4 — 11); the first indicates the number of the supergroup, the second the number of the group and the third the number of the channel. oto to

10 11 is 13 ik 15 46 Supergroup No. CHANNELS AND GROUPS OF DESIGNATION

kc/s

Basic supergroup transmitted directly to line

Figure 74. — Numbering of supergroups, groups and channels in a coaxial system DESIGNATION OF GROUPS AND CHANNELS 2 6 3

4. Systems on symmetrical pair cables *) (This text applies only to systems using a pair for each direction of transmission. In systems of the n + n type, 12 go and 12 return channels constitute the same group.)

There are two cases to distinguish: 1. The case where the groups transmitted on the line are all in the same sense. 2. The case where one of the groups is inverted relative to the others. The first case is the normal case for systems with 5 groups; the second case is normal for systems with 1, 2, 3 or 4 groups. 4.1. Systems where all the groups are in the same sense For systems with 5 groups on symmetrical pair cable, this is the normal case which corresponds with diagram 2 of figure 75.

Supergroup No.-

Group No. Channel No. kHz

Diagram No. 2 (recommended by the C.C.I.F. for systems with five primary groups)

Supergroup No.-

Group No. Channel No. 60 408 156 kHz Diagram No. 1 bis (may be used by agreement between Administrations for systems with 4 primary groups, as a variation of diagram No. 1 of figure 76)

F ig u r e 75. — Numbering of primary groups and channels in carrier systems on symmetrical pairs in cable, where all groups are assembled in the same sense

(a) Numbering o f the groups. — The 5 groups, all in the same sense, are num­ bered (in the direction of ascending frequency) 5, 4, 3, 2, 1 and the assembly const­ itutes a supergroup corresponding to a displacement by 48 kc/s towards the lower frequencies of supergroup 1 of a coaxial system as defined by the C.C.I.F. For this reason the assembly of groups in the figure is designated by the number 1 * in order to integrate this supergroup with the general numbering for supergroups. (b) Numbering of channels. — The place occupied by a telephone channel in such a carrier system is also designated by three numbers e.g. 1 * — 4 — 11. (c) The case of system with 4 groups. — By agreement between the Administ­ rations concerned one group of supergroup 1 * may be omitted, but the above numbering of the groups and channels in the groups should be retained as if no group had been omitted. (See diagram No. 1 bis of fig. 75.) 2 6 4 > DESIGNATION OF GROUPS AND CHANNELS

4.2. Systems where one group is inverted relative to the others For systems with 1, 2, 3 or 4 groups, this is the normal case which corresponds to diagram No. 1 of figure 76.

Primary Group Channel No. kHz

Diagram No. 1 (recommended by the C.C.I.F. for systems providing 1, 2, 3 or 4 groups)

Supergroup No. Primary Group Channel No. kHz

Diagram No. 2 bis (may be used by agreement between Administrations for systems with 5 groups as a variation of diagram No. 2 of figure 75)

F ig u r e 76 Numbering of groups and channels in carrier systems on symmetrical pair cable where one of the groups is inverted relative to the others

(a) Designation of groups. — In these systems with 1, 2, 3 or 4 groups the following indications are used to define the position of the group on the line: group 12-60 kc/s group 60-108 kc/s group 108-156 kc/s group 156-204 kc/s Groups A and B are the basic groups A and B defined by the C.C.I.F. (b) Numbering o f channels. — The designation of the position occupied by a telephone channel of a carrier system is by means of a letter giving the position of the group (transmitted on the line) containing the channel and by means of the number of the channel within this group. The designation of a channel on such carrier system is therefore of the form A — 7, C — 9, D — 4, etc. (c) The case of systems with 5 groups. — By agreement between the Administ­ rations concerned, in the case of an international carrier system providing 5 groups on symmetrical pairs, instead of the arrangement shown in diagram 2 of figure 75, that shown in diagram No. 2 bis of figure 76 may be used which follows from dia­ DESIGNATION OF GROUPS AND CHANNELS 2 6 5 gram No. 1 by adding a 5th group. In this case the following designations are used giving the position of the group transmitted on the line: A: group 12- 60 kc/s 1 (identical with groups A and B B: group 60-108 kc/s J defined by the C.C.I.F.) C: group 108-156 kc/s D: group 156-204 kc/s E: group 204-252 kc/s. The supergroup formed by these 5 groups is designated by the number 1 The designation of a channel in such a carrier system is therefore of the form: 1 *' — A — 1, 1 *' — C — 9, etc . . .

5. The case where a supergroup, conforming to diagram No. 2 bis for symmetrical pair systems, is transmitted on a coaxial system By agreement between the Administrations concerned the assembly of 5 groups from a symmetrical pair system conforming to diagram No. 2 bis of figure 76 may be transferred en bloc to a coaxial system. This assembly of 5 groups in fact constitutes a supergroup which is designated in the coaxial system according to its position as a supergroup on the fine, followed by a mark' to distinguish it from the normal supergroups. For example in the case of figure 11 a) this supergroup is transmitted on the line (between 564 and 804 Mc/s) as supergroup 3 of a coaxial system. For this reason the supergroup is designated 3'. Figure 11 b) shows several possible positions for such a supergroup in the band of frequencies transmitted on a coaxial line. The position occupied by a telephone channel is designated by three numbers or letters e.g. 2' — B — 11.

6 . Open-wire lines and radio links When a radio link using frequency division multiplex has to be connected to the international carrier cable network, the same system of designation and numbering should be used as for the corresponding cable. The same arrangements should be used for carrier systems on open-wire lines providing at least 1 2 telephone channels. Supergroup No. 3'

Primary Group

Channel No. EINTO O GOP AD CHANNELS AND GROUPS OF DESIGNATION

M(f 612 660 ft8 f56 80k kc/s (a) Numbering of groups and channels (as an example the supergroup is shown in position 3)

Supergroup No. 3 '

Primary Group kc/s 60 300 318 552 56k Soil 818 m e

Supergroup No. 2'

Primary uroupGrouo ...... -> [S.5 J sjN// 3 y f^ 2 lX. ^ ______D G 5 /i 8 . / .5 At 3 8 # efc... kc/s 60 300 3/2 552 56k 8ok 8i8 d058

(b) Example of the possible positions in the coaxial frequency band, of a supergroup conforming to diagram No. 2 bis

F ig u r e 77 Arrangement o f groups in a supergroup which may be used on coaxial systems interconnected with symmetrical pair systems MAINTENANCE OF CARRIER SYSTEMS 267

CHAPTER I

General

1. Definitions concerning international carrier systems

Figure 78 which follows represents a long group provided on several carrier systems on symmetrical pairs or coaxial cable in tandem. It gives an example of the way in which the terms defined below are applied to the component parts of a circuit of the group. The definitions used for maintenance of international carrier systems are:

Group section. — Part of a group link between two consecutive group distri­ bution frames or equivalent points.

Group link. — A transmission path of defined bandwidth (48 kc/s) connec­ ting together two group distribution frames or equivalentpoints. It extends from the point where the group is assembled to the point where it is dispersed. The term ordinarily covers both directions of transmission.

Group transfer point. — A group fink normally consists of several group sections connected together by “ group transfer equipment ” at points called “ group transfer points ”.

Supergroup section. — Part of a supergroup link between two consecutive supergroup distribution frames or equivalent points.

Supergroup link. — A transmission path of defined bandwidth (240 kc/s) connecting together two supergroup distribution frames or equivalent points. It extends from the point where the supergroup is assembled to the point where it is dispersed. The term ordinarily covers both directions of transmission.

Supergroup transfer point. — A supergroup link normally consists of several supergroup sections connected together by “ supergroup transfer equipment ” at points called “ supergroup transfer points ”. Transfer point Transfer point Transfer point to ooOs RBF RP RS RS RS RS RP RP RP RP RS RS RP RBF EMV EMP EMS EMS ETS EMS EMS EMP ETP EMG EMG ETP EMP EMS EMS EMP EMV

I tl FCH (SoaauaF QxuxuaP (3w r ie r (Sbaaaaf d u e d o e d n e due ANEAC O CRIR SYSTEMS CARRIER OF MAINTENANCE dale dnJe u GnS

Sujier-tjzoHft dectjon ^ j j j

______^ujter-jtonjt Sid______^ fSufuer-^jzoup GvJe

^couft /oectuM

(Q cmier fele{tJbm& cdvcumeF

EMV — Channel translating equipment (translation of the audio frequency band to the primary group and vice versa) (CTE) EMP — Group translating equipment (translation from basic group to basic supergroup and vice versa) (GTE) EMS — Supergroup translating equipment (translation from basic supergroup to coaxial line frequency range and vice versa) (STE) EMG — Group modulating equipment (GME) ETS — Through supergroup filter — supergroup transfer equipment (SGF) ETP — Through group filter — group transfer equipment (IGF) RBF — Repeater distribution frame (RDF) RP — Group distribution frame (GDF) RS — Supergroup distribution frame (SDF)- (This figure represents only one direction)

F ig u r e 78 MAINTENANCE OF CARRIER SYSTEMS 2 6 9

Carrier line link. — A transmission path for a carrier system on symmetrica! pair cable for carrying one or more primary groups. It may be divided into two or more carrier line sections connected in tandem at intermediate points. The term ordinarily covers both directions of transmission.

Coaxial line link. — A transmission path in a coaxial cable system. It extends from the point where the supergroups are assembled into the line frequency range to the point where this frequency range is dispersed.

Regulated line section. — (symmetrical pair or coaxial). — A line section of a carrier system over which the line regulating pilot or pilots are transmitted from end to end without regulation, particular to these pilots, at intermediate points.

2. Types of measurement to be made

Taking account of the definitions given in paragraph 1 above, the present section describes measurements to be made on carrier systems before putting in service and for periodic maintenance, considering successively the following parts of the systems: — Coaxial line regulated section — Carrier line regulated section — Supergroup links — Group links — Telephone channels. For each of these parts, Chapter II which follows describes the setting up and reference measurements to be made before putting the system into service and in Chapter III the periodic maintenance measurements. Reference measurements are chosen from the “ setting-up measurements ” which are made in a detailed manner (at a fairly large number of test frequencies) at all useful points at the time the line or link is established. These reference measurements will be used later for localising a fault or for checking the line-up conditions of the line or link. Maintenance measurements required for periodic preventive maintenance proper can be made at fewer points and with fewer test frequencies. The present section does not contain a chapter covering the localisation of faults on carrier systems; for this localisation it is sufficient to apply the general principles given in Section 1, Chapter I, paragraph 3 using the reference measurements. 270 MAINTENANCE OF CARRIER SYSTEMS

CHAPTER II

Bringing into service of an international carrier system

1. Preliminary exchange o f information

As soon as the Administrations or Operating Companies concerned have decided to put a nev£ international carrier system into service, they should advise their res­ pective technical service without delay and by agreement they will designate the control and subcontrol stations on the line for the new carrier system. The technical services responsible for the various subcontrol stations will send all the technical details required for setting up the system to the technical service responsible for the control station. The technical service responsible for the control station then prepares' a general diagram of the carrier system which is sent to the technical services responsible for the subcontrol stations.

2. Establishment o f high-frequency line for the bringing into service of a carrier system

Each technical service is responsible for setting up the line section in its territory. To set up a line section which crosses a frontier the following method will be applied.

(a) The case o f a coaxial line section The Administrations concerned with setting up the line section which crosses a frontier will have to make bilateral agreements which should be inspired by the C.C.I.F. recommendations given in the Green Book Volume III section 3.4.6. In particular consideration should be taken of the following points: — line regulating pilots in the framework of the C.C.I.F. recommendations so far as frequency and level is concerned each country sending the pilots required by the equipment of the other country; — speaker circuits and alarm or remote control circuits; — arrangements for power feeding in the case where a power feeding section crosses the frontier; —' regulating systems used; — nominal value of the level at various frequencies at the output of the frontier repeater. On this last point each Administration should accept, on the receive side, the conditions which normally apply for the system used in the other country. MAINTENANCE OF CARRIER SYSTEMS 271

As an indication it could be arranged that the level on the frontier side, at any frequency, at the output of the repeater equipment at the frontier stations does not differ by more than ± 0 . 2 nepers or ± 2 decibels from the nominal value, as recom­ mended in the above-mentioned recommendation of the C.C.I.F. The frequencies used for the level measurements should be agreed between the Administrations concerned. Experience shows that to avoid too many test frequencies, it is useful to make measurements at closer intervals at the extremities of the working frequency band and around points where there are irregularities to correct, and at wider frequency intervals in the remainder of the band. For information Annex 3 of the Book of annexes of Volume III of the Green Book describes a method of equalisation studied by the British Administration.

(b) The case of a carrier line section

For the initial setting up of a carrier line section crossing a frontier measurements should be made at well defined frequencies in order to obtain the “ loss frequency ” characteristic of the line. As an indication frequencies spaced as follows could be used: 4 kc/s between 12 and 60 kc/s 8 kc/s between 60 and 108 kc/s 12 kc/s between 108 and 252 kc/s. The conditions for making measurements at the line pilot frequencies should be defined by agreement between the Administrations concerned. The measurements of level at the frequencies chosen will be made in particular at the frontier stations at the output of each line amplifier. The value measured at each of the above frequencies should normally be + 0 .5 nepers or + 4.5 decibels for systems providing 1, 2 or 3 groups and + 0.2 nepers (+ 1.75 decibels) for systems providing 4 or 5 groups (excluding the case where special cables are concerned, such as submarine cables, or where a special method of equalisation is used, e.g. by the use of systematic pre-equalisation). No value of measured relative power level should differ from the above nominal value by more than ± 0 . 2 nepers or ± 2 decibels.

(c) Reference measurements on the complete line

When the sections crossing fronters and the national sections have been set up they are connected together and reference measurements made between the two ends of the high-frequency line for the carrier system, the terminal equipments being excluded.

(c.l) The case of a coaxial line. — The reference measurements are made at the line regulating pilot frequencies and at additional available measuring frequencies. As many as possible chosen from the following frequencies should be used: 60, 308, 556, 808, 1 056, 1 304, 1 552, 1 800, 2 048, 2 296, 2 604 kc/s for systems providing 1 0 supergroups. 2 7 2 MAINTENANCE OF CARRIER SYSTEMS

60, 308, 556, 808, 1 056, 1 304, 1 552, 1 800, 2 048, 2 296, 2 544, 2 792, 3 040, 3 288, 3 536, 3 784, 4 092 kc/s for 16 supergroup systems. Level measurements at these frequencies will be made at all stations (attended and unattended) at the output of each line amplifier. In the case of automatic regulation systems with regulation in the amplifiers it is useful to arrange also for reference measurements to be made at the input to these amplifiers. In practice, in the event of a line fault, a measurement at the output of such an amplifier will give no information because the regulation will mask the fault. In the case of manually regulated sections, it is desirable to note the settings of equalisers at the time the reference measurements are made; it is also desirable to note the temperature of the cable or the resistance of a conductor as a reference to the temperature. (c.2) The case of a carrier line. — Reference measurements are made at well defined frequencies in order to obtain the “ loss-frequency ” characteristic of the line. As an indication frequencies spaced as follows could be used: 4 kc/s between 12 and 60 kc/s 8 kc/s between 60 and 108 kc/s 12 kc/s between 108 and 252 kc/s. In addition measurements will be made at the frequencies of the line pilots following methods agreed between the Administration concerned. Level measurements at the frequencies chosen will be made at the output of each line amplifier at all attended stations and at unattended frontier stations. In the case of automatic regulation systems with regulation in the amplifiers it is useful to arrange also for reference measurements to be made at the input to these amplifiers. In practice, in the event of a line fault, a measurement at the output of such an amplifier will give no information since the regulation will mask the fault. In the case of manually regulated sections, it is desirable to note the settings of equalisers at the time the reference measurements are made; it is also desirable to note the temperature of the cable or the resistance of a conductor as a reference to the temperature. Reference measurements at unattended stations other than the frontier station are left to the discretion of each Administration. (c.3) Record of the reference measurement results. — For both carrier and coaxial lines the results of these level measurements made at the ends of the high frequency line and at the output of frontier repeaters should be recorded on a “ line-up record form for the line

3. Establishment o f international supergroup and group links

(a) Preliminary exchange o f information Before the supergroup or group link is established details of the routing should be exchanged and agreed between the Administrations concerned. Agreement must

* See, as examples, the model forms in Appendix I (Line-up record for a coaxial line) and in Appendix II (Line-up record for a carrier line). MAINTENANCE OF CARRIER SYSTEMS 2 7 3 also be reached on the control and subcontrol stations for the group or supergroup links. In principle only one subcontrol station will be designated for each transit country. In addition agreement must be made on the group or supergroup reference pilot or pilots and the points noted where regulators will be inserted if required together with an indication of the types of regulators, manual or automatic, which it is proposed to use. This information will be shown on a “ routing form ” * giving the following details: routing of the group or supergroup, designation of the transfer points (and in particular the transfer points nearest the frontiers), nominal levels at the transfer points. This form will be prepared by the Control station for the whole link, using the information provided by each subcontrol station for the national part for which it is responsible. The technical service responsible for the Control station will send, with the utmost possible despatch, two copies of this routing form to all the technical services responsible for the subcontrol stations (one for the technical service and one for the subcontrol station).

(b) Preliminary measurements

After agreement on the routing the technical service of the country in which the supergroup or group control station is situated should ask all the other Administrations to proceed with the setting-up of the link. All the repeater stations concerned, i.e. the stations at the end of each super­ group or group section forming part of the link, should set up and check the equip­ ment to be used to provide the link, e.g. through supergroup filters, through group filters, etc.

(c) Overhauls

All new equipment should be generally inspected after installation using vibra­ tion testing methods as recommended by the C.C.I.F. (see Section 1, Chapter IV). These tests should be regarded as part of the normal acceptance testing for new equipment. If the equipment has been installed for some time and not been used, it may be desirable to repeat the above tests before putting the equipment into service.

(d) Establishment o f a supergroup link

(1) On completion of the above preliminary work and overhauls the insertion loss of each supergroup section should be measured at the following frequencies: 313, 317, 333, 381, 411, 429, 477, 525, 545 and 549 kc/s. The results of the measurements on each supergroup section should be sent to the subcontrol station designated for the national part of the supergroup link

. * See, as examples, the models in Appendix III (routing form for a supergroup link) and Appendix IV (routing form for a group link).

18 2 7 4 MAINTENANCE OF CARRIER SYSTEMS under consideration. Each country having set up the national part in its own territory, each international supergroup section is set up by the stations situated at the ends of this section in the two countries concerned (these are the supergroup transfer stations nearest the frontier). The national parts and the international sections are connected together and the subcontrol stations concerned advise the control station of the supergroup.

(2) Before beginning the initial line-up of the overall link, the control station should advise the distant terminal of the link. It will usually save time if arrangements are made for both directions of transmission to be lined up simultaneously.

(3) The supergroup sections are connected together by means of the appropriate through supergroup filters and the supergroup reference pilot applied at the origin of the supergroup link, under the normal operating conditions and in particular at its nominal level (see Table 1 which follows). If this reference pilot has not yet been provided a test signal at 411 kc/s is sent. The level of the pilot (or of the test signal) is measured at the transfer stations adjacent to the frontiers and if necessary at intermediate subcontrol stations, and adjustments made to bring the level as near as possible to the nominal value. Each Administration can make more detailed reference measurements on the link in its own territory for the national sections and the frontier sections; these measurements can be useful for the exact localisation of a fault on the national territory. The measuring points should be the points at which subsequent periodical maintenance or reference measurements (for the initial localisation of a fault) will be made. It is recommended that a single measuring point only should be provided at each station for this purpose. Each Administration can make reference measure­ ments at other points to facilitate the exact localisation of faults in its territory; but to avoid any confusion these should not be shown on the “ line-up record form ” held by the control station.

(4) The following frequencies should be sent from the terminal supergroup distribution frame of the link, at an absolute power level corresponding to the relative level at this point, taking account of the nominal impedance at this point (see Table 2 which follows): 313, 317, 333, 381, 429, 477, 525, 545 and 549 kc/s. Level measurements should be recorded at each intermediate supergroup transfer station and at the terminal supergroup distribution frame at the incoming end of the supergroup link. It is desirable that the total spread of the overall “ loss frequency ” characteristic of the link should not exceed 4 decibels or 0.45 nepers.

(5) Reliability test. — When the initial line-up of the supergroup link has been completed and the automatic regulators provided where necessary, the performance of the link should be checked before putting it in service, by tests for a sufficient time, if possible using a recording meter. These tests are made by using the supergroup reference pilot, or if this pilot has not yet been provided, by using a test frequency MAINTENANCE OF CARRIER SYSTEMS 2 7 5 near 411 kc/s. The transmitted level and the received level at the distant end of the supergroup link are recorded continuously. All variations in level should be investigated and satisfactory explanations obtained for these variations.

Table 1 Frequencies and levels of group and supergroup reference'pilots

Group and supergroup reference pilots Frequency Absolute power level at a point corresponding to in kc/s ■ of zero relative level

Basic group A ...... 35.860 — 25 db or — 2.9 N 35.920 — 20 db or — 2.3 N

Basic group B ...... 84.080 — 20 db or — 2.3 N 84.140 — 25 db or — 2.9aN

Basic supergrou p ...... 411.860 — 25 db or — 2.9 N 411.920 — 20 db or — 2.3 N

N ote. — To avoid confusion in interpreting results, when making measurements, they should be given as a difference (relative to the nominal value) for the absolute level of the pilot at the point under consideration.

(e) Establishment of a group link

(1) On completion of the preliminary work detailed in paragraphs (a) to (c) the insertion loss of each group section should be measured at frequencies spaced at 4 kc/s in the frequency band of the group and determined by agreement between the Administrations concerned. The results of the measurements on each group section should be sent to the subcontrol station designated for the national part of the group link under consideration. Each country having set up the national part in its own territory, each inter­ national group section is set up by the stations at the end of this section in the two countries concerned (these are the group transfer stations nearest the frontier). The national parts and the international sections are connected together and the subcontrol stations concerned advise the control station of the group-.

(2) Before beginning the initial line-up of the overall link, the control station should advise the distant terminal of the link. It will usually save time if arrange­ ments are made for both directions of transmission to be lined-up simultaneously.

(3) The group sections are connected together by means of the appropriate through group filters and the group reference pilot applied at the origin of the group link, under the normal operating conditions and in particular at its nominal level (see Table 1 above). If this reference pilot has not yet been provided a test signal at 84 kc/s (or 36 kc/s) is sent. The level of the pilot (or of the test signal) is measured at the transfer stations adjacent to the frontiers and if necessary at intermediate sub­ control stations, and adjustments made to bring the level as near as possible to the 276 T able 2 Relative power levels at group and supergroup distribution frames in carrier systems of various countries

Relative power level at supergroup Impedance Relative power level at group Basic Impedance distribution frame distribution frame at group at group Supergroup Country distribution Transmit Receive at Transmit Receive distribution frame frame frame N | db N | db N | db N | db

Germany (Federal Republic) — 4.2 — 3.5 B 150 ohms — 4.0 — 3.5 75 ohms balanced unbalanced Belgium, Cuba (Cuban Tele­ — 37 — 8 B 75 ohms — 35 — 30 75 ohms SYSTEMS CARRIER OF MAINTENANCE phone company), Denmark unbalanced unbalanced United States — 42 — 5 B (American Telephone and Telegraph Company) At group or super­ — 6 — 0.2 A or B 150 ohms — 5.2 — 4.1 75 ohms group distribution balanced unbalanced frame France ■ At measuring point of supergroup dis­ tribution frame System 1 — 37 — 8 B 75 ohms — 35 — 30 75 ohms unbalanced unbalanced System 2 — 4.2 — 3.5 B 150 ohms — 4.0 — 3.5 75 ohms Italy balanced unbalanced System 3 — 5.4 — 47 — 1.1 — 10 B 150 ohms — 5.4 — 47 — 2.8 — 24 75 ohms balanced unbalanced Mexico (Telefonos de Mexico) — 5.4 — 47 — 1.1 — 10 B 150 ohms — 5.4 — 47 — 2.8 — 24 75 ohms balanced unbalanced Netherlands and United King­ — 37 — 8 B 75 ohms — 35 — 30 75 ohms dom unbalanced unbalanced Sweden — 5.4 — 47 — 1.1 — 10 B 150 ohms — 5.4 — 47 — 2.8 — 24 75 ohms balanced unbalanced Switzerland — 4.7 — 0.9 A or B 75 ohms — 4.0 — 3.0 75 ohms unbalanced unbalanced MAINTENANCE OF CARRIER SYSTEMS 2 7 7 nominal value. Each Administration can make more detailed reference measure­ ments on the link in its own territory for the national sections and the frontier sections; these measurements can be useful for the exact localisation of a fault on the national territory. The measuring points should be the points at which subsequent periodical maintenance or reference measurements (for the initial localisation of a fault) will be made. It is recommended that a single measuring point only should be provided at each station for this purpose. Each Administration can make reference measure­ ments at other points to facilitate the exact localisation of faults in its territory; but to avoid any confusion these should not be shown on the “ line-up record form ” held by the control station. (4) The frequencies indicated in paragraph (1) above are sent from the terminal group distribution frame. Level measurements should be recorded at each intermediate group transfer station and at the terminal group distribution frame for the group link. It is desirable that the total spread of the overall “ loss frequency ” characteristic of the link should not exceed 3 decibels or 0.35 nepers. (5) Reliability test. — When the group link has been established and the automatic regulators provided where necessary, the performance of the link should be checked before putting it in service, by tests for a sufficient time, if possible using a recording meter. These tests are made by using the group reference pilot, or if this pilot has not yet been provided, by using a test frequency near 84 kc/s (or 36 kc/s). The transmitted level and the received level at the distant end of the group link are recorded continuously. All variations in level should be investigated and satisfactory explanations obtained for these variations.

(f) Precautions to be taken in connection with the supergroup reference pilot

To avoid interference between pilots of different links it is desirable to avoid as far as possible routing the same group links as group No. 3 over two different supergroup links. When this cannot be avoided the supergroup reference pilot should be stopped at the transfer point of the group.

(g) Reference measurements

The measurements described above in paragraph d-(4) (for setting up a super­ group link and in paragraph e-(4) (for setting up a group link) also constitute the reference measurements. At each subcontrol station of the supergroup link (or group link) and at the transfer stations nearest the frontiers, the following information should be recorded: measured levels, points where the measurements are made, impedance at each of these points, measuring equipment used. Each station concerned will communicate these details to the control station which will prepare a “ line-up record form ” * on which all these details are recorded and will

* See as examples the forms shown in Appendix V (line-up record form for a supergroup link) and in Appendix VI (line-up record form for a group link). 2 7 8 MAINTENANCE OF CARRIER SYSTEMS send it to the technical services of the other countries. The latter will compare the information given on the “ line-up record ” with that obtained by the stations of their countries and in case of differences advise the technical service responsible for the control station.

4. Establishment o f the telephone channels

(a) Preliminary checks Before connecting the terminal equipments to the ends of a group link, it should be ensured that these terminal equipments meet the recommendations of the C.C.I.F. {Green Book, Volume III, section 3.1.2). In particular to avoid interference between reference pilots of different links, it should be ensured that in the channel modulating equipment, the attenuation at 3 860 and 3 920 c/s through the modulator and demodulator of each channel is at least 4.6 nepers or 40 decibels.

(b) Establishment of telephone channels. Measurement of levels To establish a section of a circuit consisting of a channel of a carrier system group link, proceed as follows: When the group link has been established connect the channel modulating and demodulating terminal equipment at each end and line up the telephone channels. To do this an 800 c/s * tone with a power of 1 milliwatt at a point of zero relative level is sent on each channel in turn and the gain of the channel terminal equipment adjusted so that the received level is as near as possible to its nominal value. If the Administrations consider it useful, the gain frequency characteristic of each channel should then be measured at the following frequencies: 300, 500, 800, 1 400, 2 000, 2 400, 3 000, and 3 400 c/s. The checks mentioned in paragraph 4 (a) above having been made there is no need to make reference measurements on individual channels of the carrier systems.

(c) Measurements o f non-linear distorsion (cl) Carrier systems on symmetrical pairs. — By way of information annexes 41 and 42 of the Book of Annexes of Volume III of the Green Book describes methods used in various countries for checking linearity on a carrier system before putting it in service.

* This frequency of 800 c/s was chosen at a time when it was a typical speech frequency for the instruments and lines in use at that time. Since then telephone instruments and circuits provided by carrier systems have improved in respect of the uniform transmission of the components (at various frequencies) of the human voice. At the present time, there is no frequency particularly typical for telephone speech. Since, on the other hand, numerous documents have been established referring to 800 c/s there is no real reason to change the measuring frequency. MAINTENANCE OF CARRIER SYSTEMS 2 7 9

(c2) Coaxial carrier systems. — The object of measuring non-linear distorsion on a coaxial system is to ensure that, before putting the system in service, the con­ struction and setting-up have been carried out in a satisfactory manner. Each country concerned in an international route can verify this, based on the results of earlier tests. There is no need to standardize internationally a particular method for measuring the overall non-linear distortion on a coaxial system. By way of inform­ ation, annex 43 of the Book of Annexes to Volume III of the Green Book, describes the method used in Great Britain for these tests.

CHAPTER III

Periodic maintenance on an international carrier system

1. Coaxial regulated line section (a) Maintenance measurements stations at the ends o f the regulated line section Daily record of the level of the two line pilots. (b) At attended stations (including the stations at the ends of the regulated line section) (b.l) For sections with manual regulation. Daily readings of the level of the two line pilots. (b.2) Monthly, measurement of additional measuring frequencies (inter­ supergroup) and of the two line pilots (for manual or automatic regulation sections). The measurement of these additional measuring frequencies in unattended stations and checking the operation of the regulating system are left to the discretion of the Administrations concerned.

2. Carrier regulated line section

Maintenance measurements (a) In the stations at the ends of the regulated line section Daily reading of the level of the line pilot or pilots. Where necessary level measurements at other frequencies by agreement between the Administrations concerned. (b) At attended stations (including the end stations) Measurement of level at the line pilot frequencies. Measurements of these frequencies will be made at periods (weekly, fortnightly, monthly or longer intervals) to be fixed by agreement between the Administrations 2 8 0 MAINTENANCE OF CARRIER SYSTEMS concerned according to the length of the regulated line section and the number of groups that can be routed on the section.

(c) At unattended stations Measurements left to the discretion of each Administration.

3. Supergroup link

{a) Maintenance measurements The use of the supergroup reference pilot enables the supergroup link to be kept under continuous observation, without interfering with the working traffic circuits. This reduces the need for periodic maintenance tests. Where individual meters which continuously indicate the level of the reference pilot at the receiving end are not provided, measurements of the level of the reference pilot should be made at least once daily, preferably at the same time each day. In addition, each month, the level of the supergroup reference pilot should be measured at each subcontrol station and at each transfer station at the end of a frontier section of a supergroup, these measurements being made under the direction of the control station. If these measurements show the levels to be different from the nominal values shown on the line-up record the minimum adjustments should be made to restore the levels.

(b) Adjustments to be made after maintenance measurements The level at the receiving end of the supergroup should be adjusted after each periodic maintenance test to as near as possible its nominal value. This may be done by an adjustment at the terminal station in all cases where the adjustment required does not exceed ± 0.4 nepers or ±3.5 decibels relative to the original setting of the gain control at the time the group was first established. In all other cases measurements should be made at subcontrol and other intermediate stations where the supergroup sections are interconnected to determine if a fault exists. If a fault exists it should be located and cleared. If no fault exists, but the change in equivalent is due to normal causes e.g. temperature variations, the cumulative effect of adjustments made on coaxial systems or carrier systems on symmetrical pair cables on which the supergroup is routed, etc., adjustments should be made at each supergroup link transfer point to bring the level to as near as possible its nominal value before making a final adjustment at the terminal station.

4. Group link

(a) Maintenance measurements The use of group reference pilots enables the group link to be kept under con- tinous observation without interfering with the working traffic circuits. This reduces the need for periodic maintenance tests. MAINTENANCE OF CARRIER SYSTEMS 2 8 1

Where individual meters which continuously indicate the level of the reference pilot at the receiving end are not provided, measurements of the level of the reference pilot should be made at least once daily, preferably at the same time each day. In addition each month the level of the group reference pilot should be measured at each subcontrol station and at each transfer station at the end of a frontier section of the group, these measurements being made under the direction of the control station. If these measurements show the levels to be different from the nominal values shown on the line-up record, the minimum adjustments should be made to restore the levels.

(b) Adjustments to be made after maintenance measurements The level at the receiving end of the group should be adjusted after each periodic maintenance test to as near as possible its nominal value. This may be done by an adjustment at the terminal station in all cases where the adjustment required does not exceed ± 0.3 nepers or ± 3 decibels relative to the original setting of the gain control at the time the group was first established. In all other cases measurements should be made at subcontrol and other intermediate stations where the group sections are interconnected to determine if a fault exists. If a fault exists it should be located and cleared. If no fault exists but the change in equivalent is due to normal causes e.g. temperature changes, the cumulative effect of adjustments made on supergroup links, coaxial systems or carrier systems on symmetrical pairs on which the group is routed, etc., adjustments should be made at each group link transfer point, to bring the level to as near as possible its nominal value before making a final adjustment at the terminal station.

5. Apparatus used for maintenance measurements

Annexes 31 to 36 of the Book of Annexes of Volume III of the Green Book describe, by way of information, apparatus used by various Administrations or Operating Companies enabling the maintenance measurements described in para­ graphs 1 to 4 above to be made.

6 . Measurement of non-linear distortion and tests on valves

If changing a valve in the carrier system interrupts a large number of circuits, it is desirable to avoid the systematic replacement of valves and it is recommended that measurements of non-linear distortion should be made on the line sections, using a method which does not interrupt service, in order to localise the section where the faulty valve can be found. If the system is arranged so that removing a valve does not interrupt service, these distortion measurements are not necessary. It is then desirable to check the valves periodically using methods appropriate to the type of valve used. 2 8 2 MAINTENANCE OF CARRIER SYSTEMS

7. Checker of master oscillators If a country has a national frequency standard, it is desirable to use it for checking the frequency of the master oscillators used for carrier systems. This frequency standard is guaranteed to about 1 0 ' 8 by reason of the triangulated frequency comparisons organised by the C.C.I.R. If a country has no national frequency standard, there are two possibilities: (1) To receive by radio the standard signals transmitted in accordance with C.C.I.R. recommendations. (2) To receive from a neighbouring country, over a metallic circuit, a frequency stabilised by comparison with the national standard of that country. For the latter purpose, the use of a frequency of 1800 kc/s has been provided for. It may happen that the transmission system will not pass this frequency of 1 800 kc/s. In all such cases the methods described in annexes 45 and 46 of the Book of Annexes to Volume III of the Green Book, may be used provided a telephone circuit is available. Annex 44 describes a method requiring the transmission of a frequency of 60 kc/s. If does not seem necessary in any case to compare directly the frequencies of the master oscillators of the carrier systems of different countries. SECTION 3

Establishment and maintenance of international circuits used for telephony and for voice-frequency telegraphy

Su m m a r y o f Se c t io n 3

Chapter I: Putting an international circuit into service. 1. Preliminary exchange of information. 2. Measurements before bringing the circuit into service.

3. Conditions for the establishment and changeover arrangements for a voice-frequency telegraph circuit.

Chapter II: Periodic maintenance of an international circuit. 1. Organization of periodic maintenance measurements. 2. Periodicity of maintenance measurements. 3. Method of making periodic measurements. a) measurements of equivalent. b) tests of signalling. c) determination of stability of circuits (only for circuits or sections of circuits con­ taining 2-wire repeaters).

Chapter III: Maintenance of circuits used for voice-frequency telegraphy.

1. Recommendations for the organisation of periodic maintenance measurements on circuits used for voice-frequency telegraphy. 2. Measurements to be made. 2 8 4 CIRCUIT MAINTENANCE

CHAPTER I

Bringing an international circuit into service

1. Preliminary exchange of information

As soon as the Administrations or Operating Companies concerned have decided to put a new circuit into service, they should advise their technical services without delay, so that these.technical services can reach immediate agreement on the technical details necessary to establish the circuit; the group to be used, or the circuit if entirely audio, the proposed signalling system, and the provision of echo suppressors if necessary. The technical services of the terminal countries should also designate by agreement the control station, and the technical service of each transit country should advise the other technical services concerned of the name of the subcontrol station chosen for its territory. The technical services of all the countries concerned should immediately send the details relative to the section of the circuit in their respective territories, to the technical service responsible for the control station. The rapid provision of this information will enable the technical service res­ ponsible for the control station to prepare quickly the “ routing form ” * in the case of a circuit entirely on carrier or “ the hypsogram ” * in the case of an entirely audio circuit. In the case of a mixed circuit a form is used which serves as a routing form for the circuit and as a hypsogram for the audio sections.* The technical service responsible for the control station should send the routing form or the hypsogram to all the technical services responsible for subcontrol stations on the international circuit in question, with the minimum delay. Two copies should be sent—one for the technical service and one for the subcontrol station.

2. Measurements before putting the circuit into service

After lining up the various national sections and' sections crossing frontiers, the control station in co-operation with the various stations concerned (by arrange­ ment with the subcontrol stations) proceeds with the overall line-up of the circuit.

* See, as an example, appendix VII below, which can serve as a routing form or a hypsogram as required. CIRCUIT MAINTENANCE 2 8 5

Measurements are then made at all frequencies at least from 300 to 3 400 c/s using automatic level measuring recording sets at the ends of the circuit. If no automatic level measuring set is available the equivalent of the circuit should be measured at least at frequencies 300; 500, 800, 1 400, 2 000, 2 400, 3 000, and 3 400 c/s.* It is desirable to measure also at 400, 600, and 3 200 c/s. Technical services can agree, if necessary, to make measurements at other frequencies than those shown above. During these measurements the signalling connections to automatic equipment should be disconnected if the signalling units are incorporated in the carrier terminal equipment. Measurements of noise on the circuit, and of “ go to return ” crosstalk, should then be made. The tests should be continued until the equivalent, relative levels, stability, noise and crosstalk are in accordance with the C.C.I.F. recommendations. When the circuit is put into service, a check should be made to ensure that the absolute power level of the signalling current transmitted at the origin of the circuit, is at the specified nominal value. These levels and the permissible tolerances are given in the table below:

Signalling frequency Absolute power level at a point of zero relative level

Nominal Tolerance Nominal Tolerance value value Nominal Tolerance value N N db db

Manual signalling 500 c/s ± 2% uninter­ uninter­ interrupted rupted rupted at 20 c/s (500) (500) 0 ± o.l 0 ± 1

± 2% interrupted interrupted (500/20) (500/20) — 0.35 ± 0.1 — 3 ' ± 1 ■

1-frequency signal­ ling 2 280 c/s ± 6 c/s — 0.7 ± 0.1 — 6 ' ± 1

2-frequency signal­ 2 040 c/s ± 6 c/s — 1 ± 0.1 — 9 ± 1 ling 2 400 c/s ± 6 c/s — 1 ± o.l — 9 ± 1

3. Conditions for the establishment and changeover arrangements for a voice-frequency telegraph circuit (a) A four-wire circuit for voice-frequency telegraphy differs from a telephone circuit by the absence of terminating units, signalling units and echo suppressors. When it is necessary to provide a circuit for 24 voice-frequency telegraph channels on a carrier channel, a telephone channel routed on a single group link should be used for preference.

* In the case of circuits which do not transmit frequencies above 2 400 c/s, measurement should be made at 300, 500, 800, 1 400, 2 000, and 2 400 c/s. 2 8 6 CIRCUIT MAINTENANCE

(b) Points A and B (fig. 79), where the changeover between the voice-frequency telegraph circuit and its reserve circuit takes place (and which are considered con­ ventionally as the origin and extremity of the four-wire circuit used for voice-fre­ quency telegraphy), should, be at the same relative levels for the two circuits, the levels being determined from the level diagram of the telephone circuit. The relative level at point A should not exceed — 0.4 nepers. The relative level at point B should be at least + 0.4 nepers. (c) The relative power level at the point on the receive side where the change­ over between the voice-frequency telegraph circuit and its reserve circuit takes place, should be as stable as possible with time. Furthermore any interruption of the circuit, even of very short duration, spoils the quality of the telegraph transmission.

A B V1X1* "| n,^-o,U N [ n ' ^ + o,#N J QiwcwJt (1) D

© '/ccait(8) M ------

< 1 F ig u r e 79 A and B = Points where: (1) Connection is made between the circuit (1) used for voice-frequency telegraph and the telegraph equipment. (2) Changeover takes place between the circuit (1) used for voice-frequency telegraphy and the telephone circuit (2) used as a reserve for circuit (1). T = “ Transmit ” voice-frequency telegraph equipment. R = “ Receive ” voice-frequency telegraph equipment.

CHAPTER II

Periodic Maintenance of an International Circuit

1 i Organization of periodic maintenance measurements Dates for measurement. — The dates for periodic maintenance are fixed by the “ Programme of periodic maintenance ”. When a circuit terminates at the same points as the group link on which it is routed, the measurements on the circuit should be made, whenever possible, immediately following the adjustment of the group link. CIRCUIT MAINTENANCE 2 8 7

Times of measurement. — The times for measurement are fixed by agreement between the control and subcontrol stations. It is essential to choose the times for periodic measurements so as not to interfere with the telephone traffic. In particular, it is necessary in the case of heavily loaded circuits that Administrations or Operating Companies study the possibility of carrying out the measurements during times of light traffic.

2. Periodicity o f maintenance measurements

Periodic maintenance measurements made on a complete circuit comprise measurements of: (a) Equivalent and levels at one frequency. (b) Equivalent and levels at several frequencies. (c) Stability (for 2-wire audio circuits or sections of circuit only). (d) Signalling current and operation of signalling units. The periodicity at which the measurements are made is given in Tables 1 and 2 which follow.

Table 1 shows the periodicity for making measurements on the types of circuit normally used in the international telephone network of Europe (except for frontier circuits). These circuits are: . — 4-wire-audio frequency circuits. Included also in this category are circuits on carrier systems providing a small number of telephone channels. No distinction is made between circuits in underground cables and circuits on open-wire lines unless the open-wire section is equipped with a repeater. — or 4-wire carrier circuits on telephone channels of systems providing at least one primary group. — or 4-wire circuits of mixed constitution i.e. consisting of a mixture of audio and carrier sections. To determine the periodicity of maintenance measure­ ments a distinction is made between circuits routed mainly on carrier systems and circuits routed mainly on audio-frequency sections. In the case of circuits of mixed constitution, it is desirable that the dates and times of the measurements made on the overall circuit and on the audio frequency sections should be the same as on the carrier section, in order to cause the least possible interference to the traffic services.

Table 2 shows the periodicity of measurements to be made on short-distance international circuits, which are generally used for terminal traffic, but which can when necessary be used to extend more important international circuits. It is desirable that the same recommendations are applied to national circuits frequently used for international communications. T able 1 Periodicity of measurements to be made on international telephone circuits (circuits normally used for the international network)

C o l 1 Col. Col. 3 Col. 4 Co . 5

Measurements Signalli ng tests of equivalent Type of circuit Number o f repeaters or carrier sections Measurements of equivalent and levels at and levels at one frequency t several frequencies Manual circuits Automatic circuits

Audio frequency 4-wire circuits with 1 to 14 repeaters Monthly Half yearly 4-wire circuits * MAINTENANCE CIRCUIT 4-wire circuits with 15 or more repeaters Weekly Half yearly

4-wire circuits including an open wire sec­ at least monthly as agreed Half yearly tion with at least one repeater between Administrations and Operating Companies At the same See the concerned time as the “ Guiding measurement principles Circuits wholly Circuits routed on a single group link and Every two months Yearly of equivalent for the carrier terminating at the same points as the link and levels maintenance at several of semi­ Circuits routed over several group links in Monthly Half yearly frequencies automatic tandem (see column 4) circuits ’’ 4-wire circuits of Circuits routes mainly on carrier systems Monthly Half yearly mixed constitution Circuits routed mainly on audio sections Weekly or monthly as Halt yearly agreed between Admin­ istrations and Operating Companies

N ote — i Measurements of equivalent and levels at one frequency shown in column 3 are included in the measurements made at several frequencies shown in column 4. 2 Included also in the category “ audio-frequency circuits ” are circuits on carrier systems which provide a few telephone channels. Table 2 Periodicity of measurements to be made on international telephone circuits (Types of circuit not normally used in the international network)

Col. 1 Col. 2 Col. 3 Col. 4 Col. 5 Col . 6 Measurements Measurements Signalli ng tests of equivalent of equivalent Measurements MAINTENANCE CIRCUIT Category of circuit ; Type of circuit and levels and levels of at one at several stability frequency i) frequencies Manual circuits Automatic circuits

2 -wire circuits with one repeater Yearly Yearly Yearly at the 2-wire circuits with 2 or 3 repeaters Half yearly Yearly Half yearly As agreed same time as between measurements 2-wire circuits with at least 4 repeaters Quarterly Half yearly Quarterly Administ­ of equivalent rations and and levels Audio-frequency : 2 -wire circuits including an open wire Monthly Half yearly Monthly Private at several circuits section with at least one repeater Companies frequencies concerned (see column 4) 4-wire circuits with a 2 wire section As agreed between Adminis trations and having at least one repeater Operati ng Companies concerned

N ote. — 1) Measurements of equivalent and levels at one frequency shown in column 3 are included in the measurements at several frequencies shown in column 4. 289 290 CIRCUIT MAINTENANCE

3. Method of making periodical measurements

(a) Measurements of equivalent (and in the case of audio circuits, of relative levels). Measure the equivalent (and when necessary the relative levels at frontiers) for each circuit: — at a frequency of 800 c/s * in the case of measurements at one frequency, — at frequencies 300, 500, 800, 1 400, 2 000, 2 400, 3 000, and 3 400 c/s (if necessary 400, 600, and 3 200 c/s) in the case of measurements at several frequencies.*) Whenever automatic level measuring recording sets are available at the ends of the circuit the measurements should be made using this equipment at all frequencies over the range 300 to 3 400 c/s at least. The subcontrol stations responsible for the circuits in the different national sections should be called in by the control station to take part in all the measurements. All the results of the measurements should be recorded by the control station and by the subcontrol station concerned. When possible the tests on international circuits terminating at the same points should be made at one time. The equivalent of the circuit should be adjusted after each periodic maintenance test as near as possible to its nominal value, after checking as far as possible the levels of the group reference pilots. This may be done by an adjustment at the terminal station in all cases where the adjustment required does not exceed ± 0 . 2 nepers or ± 2 decibels relative to the original setting of the gain control at the time the circuit was first established. In all other cases, measurements should be made at sub­ control and other intermediate stations as necessary to determine if a fault exists. If a fault exists it should be found and cleared. If no fault exists but the change in equivalent is due to normal causes e.g. temperature variations, adjustments made on group or supergroup links on which the circuit is routed, etc. adjustments should be made, under the direction of the control station, at each circuit transfer point to bring the level as near as possible to its nominal value before making a final adjust­ ment at the terminal station. When testing groups of circuits as mentioned above, all the circuits should be tested before returning to the faulty circuit or circuits.

* This frequency of 800 c/s was chosen at a time when it was a typical speech frequency for the instruments and lines in use at that time. Since then, telephone instruments and circuits provided on carrier systems have improved in respect of the uniform transmission of the components (at various frequencies) of the human voice. At the present time there is no frequency particularly typical for telephone speech. Since on the other hand numerous documents have been established referring to 800 c/s there is no real reason to change this measuring frequency. **) In the Pase of circuits which do not transmit frequencies above 2 400 c/s, measurements should be made at 300, 500, 800, 1 400, 2 000, and 2 400 c/s. MAINTENANCE — VOICE-FREQUENCY TELEGRAPH CIRCUITS 2 9 1

(b) Signalling tests (b.l) Manually operated circuits The power of the voice-frequency signalling current, in its normal operating condition, should be measured at the same time as the equivalent at several frequencies is measured.*) Level measuring sets can be used for these tests. The operation of the voice-frequency signalling units is tested as an in-station test.**) . (b.2) Semi-automatic or automatic circuits (See the “ Guiding principles for the maintenance of semi-automatic circuits ”). (c) Determination o f the stability o f circuits (only for international circuits (or sections of circuits) including 2 wire repeaters). The gain of one of the repeaters in the circuit is increased in one or both direc­ tions until singing occurs; this test gives the singing margin of the circuit for the direction of transmission and . from the values of the singing margin for the two directions of transmission the stability of the circuit is deduced. For more details see the Book of Annexes to Volume III of the Green Book, 2 nd part, section 1 . 6 .

CHAPTER III

Maintenance of circuits used for voice-frequency telegraphy

1. Recommendations for the organization o f periodic maintenance measurements on circuits used for voice-frequency telegraphy Any interruption of such a circuit, even of very short duration, spoils the quality of the telegraph transmission. It is therefore desirable to take great care when making measurements on circuits used for voice-frequency telegraphy. To draw the attention of staff to this matter, channel equipment for circuits used for voice-frequency telegraphy should be specially marked in the terminal exchanges and where necessary in repeater stations where the circuits are accessible.

*) If “ n ” is the relative power level at the point of measurement, the measured absolute power level of the sent 500/20 c/s signalling current (interrupted signalling current) should be between the following limits

(n — 0.35) q-2 nepers or (n — 3) ^ decibels assuming the use of signalling units conforming to the new specification (Green Book of the C.C.I.F. Volume V, page 4). **) For information, the operating limits of the signal receiver are as follows: If “ n ” is the relative power level at the point in the circuit where the receiver is connected, it will operate reliably when the absolute power level N of the signalling current at the input to the receiver is between the following limits: — 0.95 + n < N < + 0.25 + n nepers or — 8.5 + n < N < + 2.5 + n decibels. 2 9 2 MAINTENANCE — VOICE-FREQUENCY TELEGRAPH CIRCUITS

It is desirable that the maintenance measurements on the “ reserve circuit ” should be made just before the maintenance measurements are made on the “ normal circuit ”, so that the reserve circuit can replace the normal circuit whilst the latter is tested. For the changeover it is recommended that as far as possible a procedure is used which avoids any mutilation of the telegraph signals. When several voice-frequency telegraph systems are in use between two repeater stations, if the maintenance measurements on the telephone circuits between these stations are spread over several days, the tests on the circuits carrying the telegraph systems should also be spread over these days; this makes the execution of the tele­ graph tests easier. The maintenance measurements to be made on international voice-frequency telegraph channels concern only the telegraph Services. •* Further, in connection with measurements on voice-frequency telegraph channels, it is for the telegraph Services to determine what documents should be exchanged. Measurements on telegraph circuits used in switched networks (telex circuits, amongst others) concern only the telegraph Services.

2. Measurements to be made The maintenance recommendations for four-wire telephone circuits are also applicable to circuits used for voice-frequency telegraphy. Periodic measurements of level at one frequency (800 c/s) should be made at the periodicity recommended for international telephone circuits (see Table 1, section 3, Chapter II). Measurements at different frequencies should be made once every three months. All these measurements should be made by sending at the origin of the circuit used for voice-frequency telegraphy, a power corresponding to 1 milliwatt at a point of zero relative level on the telephone circuit. The measuring frequencies are as follows: Circuits providing an 18-channel telegraph system: 300, 400, 600, 800, 1 400, 2 000, 2 400, 3 000 c/s. Circuits providing a 24-channel telegraph system: 300, 400, 600, 800, 1 400, 2 000, 2 400, 3 000, 3 200, 3 400 c/s. SECTION 4

Periodic measurements to be made on the line for circuits or sections of circuit entirely at audio frequency or for low- frequency carrier systems *

Su m m ary of Section 4

1. Periodicity.

2. Method of carrying out: (a) measurements of repeater gain. (b) tests for rejection of valves. (c) determination of singing point or of the stability margin of 2 -wire repeaters.

1. Periodicity

(a) At all repeater stations: Measurements and tests made on repeaters:

Measurement of the gain of repeaters Except where there is no feedback or very little feedback there is no need to make gain measurements, but simply tests for the rejection of valves, as shown below.

* Covers audio-frequency circuits or channels of carrier systems providing 1, 2, 3, or 4 carrier channels. 2 9 4 MAINTENANCE — AUDIO CIRCUITS

Tests for rejection of valves The periodicity should be fixed by Administration and Operating companies so that it is reasonably certain that from the time of a test for rejection of valves the variation of gain will not exceed the permissible limits (see Volume III of the Green Book, Section 5, Repeaters) until the following test.

(b) At intermediate stations:

Determination o f relative levels — relative levels at frontier repeater stations ...... during corresponding measurements on the circuit — relative levels at other repeater stations ..... during corresponding measurements on the circuit (but only in the case of a request from the control or subcontrol).

Determination o f singing points or stability margin

At the same time as determining the stability of the circuit (see Table 2, Section 3, Chapter II).

2. Method of carrying out

(a) Measurements o f repeater gain The insertion gain of the repeaters (under their normal working conditions) between 600 ohm pure resistances (for each direction of transmission) (line trans­ formers excluded) should be measured at 800 c/s and at sufficient other frequencies over the band that the circuit should effectively transmit. Having noted the result of these measurements the maintenance officer should advise the responsible control or subcontrol station of the actual gain measured (in nepers or decibels) and not in terms of the setting of the gain control of the repeater. In the case of amplifiers with sufficient feedback, where the gain is practically independent of the state of the valves, the measurement of gain is of no interest and should be replaced by a test for the rejection of valves carried out as indicated (b) below. MAINTENANCE — AUDIO CIRCUITS 2 9 5

(b) Tests for the rejection o f valves These tests are carried out at a periodicity determined for the type of valve used. There is a choice between the two following methods: — If it is decided periodically to carry out measurements of the mutual con­ ductance of the valves it will be sufficient to measure the anode current for normal heater current from time to time between two successive measure­ ments of mutual conductance and check that the anode current is within the specified limits. — If no periodic measurements of mutual conductance are made, the following method should be used. Measure the anode current for two values of heater current; the normal working value and a value 5 % lower for example. Check that the difference between the two measured values is within the specified limits. . If there is no feedback or very little feedback the gain of the amplifier should also be measured. (c) Determination o f singing points or of the stability margin o f two wire repeaters These tests should be carried out in accordance with the methods given in the Book of Annexes to Volume III of the Green Book. PAGE INTENTIONALLY LEFT BLANK

PAGE LAISSEE EN BLANC INTENTIONNELLEMENT SECTION 5

Line-up and maintenance of international programme transmissions

Su m m a r y of Section 5

Chapter I: General organization of international programme transmissions

1. Technical responsibilities during a!n international programme transmission. Definition of the constituent parts of an international programme link. 2. Various types of circuits used for programme transmissions. 3. Various classes of programme transmissions. Use of control circuits. 4. Definition and duration of line-up period and preparatory period.

Chapter II: Establishment and maintenance of permanent circuits for programme transmissions. 1. Control and subcontrol stations. 2. Establishment of the circuit. 3. Reference measurements. 4. Periodic maintenance measurements.

Chapter III: Constitution, line-up, supervision and clearing down the international programme link. 1. Measurements to be made before the line-up period which precedes a programme trans­ mission.

2. Measurements to be made during the line-up period which precedes a programme transmission. 3. Measurements made by the Broadcast Authority during the preparatory period. 4. Maximum power to be transmitted during the programme transmission. 5. Identification signal.

6 . Supervision of the transmission. 7. Procedure at the end of the programme transmission. Broadcasting Broad­ authority Terminal Outgoing Transit Incoming Terminal casting receiving the repeater country country country repeater Local authority Local programme station station line transmit­ line ting the programme EDINBURGH LONDON PARIS LYON TORINO MILANO MESTRE B KC | k D E I

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0 Volume meter or peak meter Equalizer.

F ig u r e 80. Diagram of an international programme line. MAINTENANCE — CIRCUITS FOR PROGRAMME RELAYS 299

CHAPTER I

General Organization of international programme transmissions

1. Technical responsibilities during an international programme transmission. Definition o f the constituent parts o f an international programme link

In order to apportion the responsibilities during the transmission of a broadcast programme there is need to distinguish (see fig. 80 below):

(a) The Broadcasting Authority which is the source of the programme (studio, outside broadcast point or programme switching centre) and which in the figure is at some distance from Edinburgh repeater station;

(b) the outgoing local line, which connects the broadcast authority to the first repeater station;

(c) “ the (long-distance) international programme line ” consisting in principle of a chain of national and international programme circuits, the national circuits being of the same type as international circuits. In the figure this long-distance line is “ Edinburgh-Mestre ”, and consists of the national circuit Edinburgh- London, the international circuit London-Paris, the national circuit Paris-Lyon, the international circuit Lyon-Torino and the national circuits Torino-Milano and Milano-Mestre;

(d) the incoming local line, which connects the last repeater station to the receiving broadcast authority;

(e) the receiving Broadcasting Authority for which the programme is intended and which in the figure is at Venezia some distance from Mestre.

The assembly of the long-distance international programme fine and the local lines constitutes the “ international programme link ”. The international programme line is, in all cases, the sole responsibility of the Telephone Administrations. The local lines may be the responsibility of either the Telephone Administration, the Broadcast Authority or the two together according to arrangements in each country. 3 0 0 MAINTENANCE — CIRCUITS FOR PROGRAMME RELAYS

The country at the incoming end designates a single control station for the long­ distance international line and for the international link (in principle the last repeater station at the receiving end). The country at the outgoing end designates a subcontrol station for both the international long-distance line and the international link. Each transit country designates a subcontrol station for the international long­ distance line. Each circuit comprising part of the international long-distance line retains its normal control station.

2. Different types of circuit used for programme transmissions

For broadcast programme transmissions it is recommended to use either: (a) Normal programme transmission circuits, or ■ (b) Old-type programme transmission circuits. Exceptionally if neither of the two types of circuit is available use can be made of ordinary telephone circuits. These circuits can be distinguished in particular by the band of frequencies they effectively transmit. ■

(a) Normal circuits. — When a normal circuit for programme transmission is used, the band of frequencies effectively transmitted by the complete link should extend from 50 to 10 000 c/s. For a frequency to be effectively transmitted the equivalent at the frequency must not exceed the equivalent at a frequency of 800 c/s by more than 0.5 neper (4.3 decibels).

(b) Old type circuits. — When an old-type circuit for programme transmissions is used, the band of frequencies 'effectively transmitted by the complete link should extend from 50 to at least 6 400 c/s (the upper limit of course being less than 1 0 0 0 0 c/s).

(c) Telephone circuits. — These circuits should transmit effectively a band of frequencies from 300 to 3 400 c/s. In telephony a frequency is said to be effectively transmitted if the equivalent at this frequency does not differ by more than 1.0 nepers (8.7 decibels) from the equivalent at 800 c/s.

3. Different types o f programme transmission. Use of control circuits (a) Different types of programme transmission

For the establishment of “ control circuits ” {circuits de conversation), defined later, it is convenient to distinguish between: — “ Regular transmissions ”, transmissions ordered once for all, because they are to take place at regular intervals at fixed times on established links and always between the same points; MAINTENANCE — CIRCUITS FOR PROGRAMME RELAYS 3 0 1

— “ Occasional transmissions ”—transmissions not covered by the above definition. These transmissions may each be direct programme transmissions or multiple programme transmissions.

(b) Definition and use o f a control circuit A control circuit, a telephone circuit distinct from the special circuit for the programme transmission, provides a direct link between the programme source and the point where it is used (recording equipment, switching centre or broadcast transmitter). This link is used to control the transmission of the broadcast programme and enables the necessary action to be taken rapidly to remedy difficulties or interrup­ tions observed during the transmission: it also enables the programme circuit to be released at the right moment and provides appropriate means for accurately determining the chargeable time for the programme transmission. Broadcast Authorities are required to use a control circuit in certain cases as specified below.

(c) Direct programme transmissions In the case of regular transmissions, especially if the programme is such that the Broadcast Authority is prepared to tolerate any difficulties which arise due to the lack of a control circuit during the transmission of the programme, the use of a control circuit should only be compulsory during the “ preparatory period ” (see paragraph 4 below). For certain well-established regular transmissions, the use of a control circuit, even during the preparatory period, may be dispensed with at the request of the Broadcast Authorities concerned. In the case of an occasional programme transmission, the use of a control circuit should, in principle, be required during the preparatory period and strongly recommended throughout the duration of the programme transmission: in fact, Broadcast Authorities are interested in reducing as much as possible the duration of incidents during the broadcast programme transmission, and on the other hand telephone Administrations must see that the programme does not rise to too high a level with the risk of giving trouble on telephone circuits on the same route.

(d) Multiple programme transmissions (or multiple relays) (1) Multiple programme transmissions with a single programme source. —■ If the first branching point of the programme circuits serves a broadcast transmitter (or a switching centre or recording centre) in the same town and taking part in the multiple transmission, it is strongly recommended that control circuits be provided at least: (a) between the programme source and the first branching point of the pro­ gramme circuits, (b) between the first branching point and the various broadcast transmitters (or switching centres or recording centres). 3 0 2 MAINTENANCE — CIRCUITS FOR PROGRAMME RELAYS

In other cases it is recommended that whenever possible, control circuits be provided between the programme source and the various broadcast transmitters (or switching centres or recording centres). In both the above cases the control circuits should always be provided during the preparatory period and their use throughout the programme transmission is strongly recommended. (2) Multiple programme transmissions with several programme sources. — A preliminary study should be made by the Broadcast Authorities and the Telephone Administrations concerned, to determine what control circuits will be required during the preparatory period and recommended during the programme transmission. Experience has shown that for multiple programme transmissions with bothway transmission and several programme sources, to ensure that the programme runs smoothly it is desirable to have control circuits between the studio directing the broadcast and the various programme sources.

(e) Programme transmissions accompanying an international television trans­ mission The requirements for control circuits for sound programme transmissions associated with an international television transmission should be fixed by agreement between the Administrations or Operating Companies and Television Broadcast Authorities concerned, on similar principles to those given above in paragraphs (c) and (d) for programme transmissions.

4. Definition and duration of line-up period and preparatory period

For each international programme transmission a distinction is made between: (a) the line-up period during which the Administrations and Operating Com­ panies line up the international programme link before handing it over to the Broad­ cast Authorities; (b) the preparatory period during which the Broadcast Authorities carry out their own adjustments, tests and other work before the programme transmission itself commences.

(a) Line-up period Duration. — The duration of the line-up period should be fixed in principle at 15 minutes. However, in the case of programme transmissions involving more than two countries, the duration may be increased. On the other hand, in certain cases, by agreement between the Administrations concerned, the duration may be less than 15 minutes if this does not harm the quality of the line-up. This may be done, for example, when there are two successive international programme trans­ missions on the same route and the second involves extending the link, already lined up for the first. Note. — In the case of programme transmissions required in association with a multiple television programme, broadcast by several transmitters, the line-up period can have a longer duration, fixed by agreement between the Administrations con­ cerned, e.g. of about 25 to 30 . minutes. MAINTENANCE — CIRCUITS FOR PROGRAMME RELAYS 3 0 3

At the end of the line-up period the international programme link and the control circuits are handed over at the same time to the Broadcast Authorities. (b) Preparatory period Beginning and duration. — When the tests during the lining-up period are com­ pleted, the international programme link is not made available to the Broadcast Authorities at the two ends, until the time fixed for the beginning of the preparatory period. The chargeable time for the programme transmission commences at the beginning of the preparatory period. As a general rule, in Europe, the duration of the preparatory period e.g. the time between handing over the international programme link to the Broadcast Authorities and the moment when the programme proper begins, should be about a quarter of an hour, to allow the Broadcast Authorities to carry out all the tests and adjustments necessary before proceeding with the programme transmission. However, the duration of the preparatory period may be extended to half an hour at the request of the Broadcast Authority using the link. Note. — In the case of very Complicated multiple transmissions, or programme transmissions accompanying a multiple television programme, broadcast by several radio transmitters, the preparatory period for the sound programme transmission may have a longer duration fixed by the Administrations concerned and lasting about half an hour.

CHAPTER II

Establishment and maintenance of permanent circuits for programme transmissions

1. Control and Subcontrol Stations For the establishment of a unidirectional international programme circuit, the station at the receiving end is the control station. The other terminal station is a subcontrol station. If the international circuit transits one or more countries, a subcontrol station is also designated for each transit country. The functions of the control and subcontrol stations are the same as in the case of a telephone circuit. Note. — In the case when a reversible programme circuit is established, reference measurements and maintenance measurements are made for each direction “of transmission. The control station remains the same for whichever direction the circuit is used.

2. Establishment of the circuit Each national section of the circuit and each section crossing a frontier having been equalized for attenuation distortion and where necessary compensated for 3 0 4 MAINTENANCE — CIRCUITS FOR PROGRAMME RELAYS phase distortion, to satisfy C.C.I.F. recommendations, the different sections are connected together to provide the overall circuit and the following measurements made: (a) Attenuation measurements The “ attenuation frequency” characteristic of the circuit is obtained using a “ hypsograph If no “ hypsograph ” is available, steady-state measurements should be made at sufficient frequencies to obtain a well-defined characteristic. Equalizers should be adjusted to bring the curve within the limits prescribed by the C.C.I.F. (Green Book, Volume III, part 3, graphs No. 8 , 9 and 10 of figures 44 to 46). (b) Phase distortion measurements If it appears necessary, the phase distortion index for the circuit should be measured using one of the methods recommended by the C.C.I.F. {Book of Annexes to Volume III of the Green Book, 2nd part, Section 1.7). (c) Measurement of non-linear distorsion Until the C.C.I.F. standardizes a method for measuring non-linear distortion for a programme circuit, harmonic distortion measurements should be restricted to the method recommended (Book of Annexes to Volume III of the Green Book, 2nd part, Section 1.8.1). (d) Measurement of circuit noise Measure: — the flat noise at the end of the circuit using a set covering a frequency range of at least 50 to 20 000 c/s and — the psophometric noise, using a psophometer for programme circuits (see Green Book, Volume IV, Section 3.2.3 B). When, after any necessary adjustments, the circuit meets the C.C.I.F. recom­ mendations, reference measurements are made.

3. Reference measurements

The relative voltage.level at the terminal station and at the frontier stations is determined for the following frequencies: — for a normal circuit:

50, 80, 100, 200, 500, 800, 1 000, 2 000, 3 200, 5 000, 6 000, 8 500, 10 000 c/s and if considered useful 30, 40, 11 000, 12 000 and 15 000 c/s; — for an old-type circuit:

50, 80, 100, 200, 500, 1 000, 2 000, 3 200, 5 000 and 6 400 c/s. The results of these measurements are carefully noted on a “ line-up record ” * and also of the flat noise and psophometric voltage measured at the end of the circuit.

* See as example, the model in Appendix VIII. MAINTENANCE — CIRCUITS FOR PROGRAMME RELAYS 3 0 5

4. Periodic maintenance measurements The following periodic maintenance measurements are made every two months: (a) Measurements of relative level The relative voltage level at the end of the programme circuit is measured at the following frequencies: — for a normal circuit:

50, 100, 200, 800, 3 200, 6 000, 8 500 and 10 000 c/s; . — for an old-type circuit:

50, 100, 200, 800, 3 200, 5 000 and 6 400 c/s. If it is found that the relative voltage level at the end of the circuit is incorrect for a particular frequency, the reference measurements should be repeated bringing in frontier stations to determine the faulty sections, and the circuit restored, making further overall measurements to ensure that the normal values are obtained. (b) Measurement of circuit noise As part of the periodic maintenance measurements made every two months, the noise at the end of the circuit should be measured using the psophometer for programme circuits specified by the C.C.I.F. (Green Book, Volume IV, Sec­ tion 3.2.3.B).

CHAPTER III

Constitution, line-up, supervision and clearing down the international programme link

Assuming that the international programme link is as shown in the diagram of figure 80. Assuming also that the various circuits to be interconnected to provide the international link are permanent circuits which are subject to regular periodic maintenance.

1. Measurements to be made before the line-up period which precedes a programme transmission The local lines should be adjusted such that when they are connected to the long-distance international programme fine the hypsogram of the international programme link shall be met.* For example, in figure 80 the station Edinburgh effects the equalization and line­ up for the local line from the British Broadcasting Corporation (B.B.C.).

* According to the definition of the hypsogram of an international circuit it follows that a sine wave of maximum amplitude equal to the peak voltage (at a specified point) transmitted by the studio has a nominal absolute voltage level of + 1.04 nepers ( + 9 db) at a point of zero relative level on the long-distance international programme circuit.

20 3 0 6 MAINTENANCE — CIRCUITS FOR PROGRAMME RELAYS

2. Measurements to be made during the line-up period which precedes a programme transmission The C.C.I.F. recommends the use of the line-up method known as “ constant voltage ** After the connection of the various circuits to form the international programme link (conforming to the hypsogram of these circuits) it is sufficient to verify, by means of a “ hypsograph ” or by measurements at discrete frequencies that the relative voltage level at the distant incoming repeater station has the correct value at the following frequencies: for a normal circuit ...... 50,800 and 10 000 c/s for an old-type circuit ...... '. 50,800 and 6 400 c/s for a telephone circu it ...... 300,800 and 3 400 c/s. Also, and only if the control stations asks, a measurement of the psophometric noise is made at the distant incoming repeater station. These preliminary adjustments having been made, the local lines are connected to the long-distance international programme circuit at the terminal repeater stations. This is the end of the “ line-up period ” and the beginning of the “ preparatory period ” which corresponds to the instant when the complete link is placed at the disposal of the Broadcast Authorities. The latter then proceeds to measure and adjust as necessary.

3. Measurements to be made by the Broadcast Authorities during the “preparatory period ” After the Broadcast Authority has taken possession of the international link, it makes measurements on the complete link in the band of frequencies effectively transmitted, from the point where the programme is picked up to the point where the programme is received. It is desirable to recommend to the Broadcast Authorities when making measurements, to apply at the origin of the international programme link (point A of figure 80 above) a sine wave of which the maximum amplitude should be 9 decibels or 1 neper below that of the peak voltage (i.e. the instantaneous voltage maximum that should never be exceeded at this point in the course of a programme transmission) and if necessary verify that the nominal output voltage level at each repeater is + 6 decibels or + 0.7 nepers, i.e. the absolute level of zero voltage at a point of zero relative level of the international programme link. It is not necessary to re-adjust the output level of intermediate repeaters since these have already been set during the line-up period. Note. — The numerical values given above ensure that during the programme transmission the peak voltage at a point of zero relative level will not exceed the amplitude of a sine wave having a power of 8 milliwatts.

** If certain Administrations or Private Operating Companies have programme amplifiers which are not suitable for use for line-up by the constant-voltage method there is no objection to using the method of equalization of constant electromotive force—even though it may cause incon­ venience from the point of view of maintenance—provided that Administrations or Operating Companies make the necessary arrangements at frontier stations to changeover from the constant electromotive force method to the constant-voltage method recommended by the C.C.I.F. At the same time new amplifiers installed for programme transmissions should be designed to provide for lining-up using the constant-voltage method. MAINTENANCE — CIRCUITS FOR PROGRAMME RELAYS 3 0 7

The reasons for which during this final line up a voltage 9 decibels or 1 neper below the peak voltage is applied at point A are: (a) It is not desirable to overload the terminal equipment of carrier systems by transmitting continuously a test signal corresponding to the peak voltage which is only attained momentarily during the transmission of the programme proper. (b) If Administrations and private Operating Companies make their initial and maintenance measurements with a nominal absolute voltage of + 6 decibels or + 0.7 nepers at the repeater output, it is useful to use the same voltage if it is necessary to check during the preparatory period.

4. Maximum power transmitted during a programme transmission

To check that the maximum power transmitted during a programme transmission never exceeds the limits allowed by Administrations and Private companies, it is recommended that Broadcast Authorities and the terminal repeater stations of the international link, connect volume-meters or peak meters, the same type of meter being used preferably by the telephone Administration and the Broadcast Authority of the same country.

5. Identification signal

(a) For a programme transmission relay not using radiotelephone circuits, to indicate, during the “ preparatory period ”, at times when no test transmission is taking place, that the circuits are through, it is very desirable for Broadcast authorities to arrange that their studios and transmitting stations transmit an “ identification signal ” over the international link whilst it is not in use. This identification signal will not be broadcast, so that it will not be heard by listeners, but will be transmitted from end to end of the link used for the programme transmission, from the studio to the broadcast station. It is recommended that if this identification signal consists of a steady sinusoidal signal, it be transmitted at a level low enough to avoid overloading the programme circuit amplifiers. As an example, a succession of sinusoidal signals at different frequencies in the audible band effectively transmitted could also be used, following each other in accordance with a predetermined law, or a suitable phrase recorded on disc or sound film or a short spoken phrase indicating the name of the broadcast station also recorded on disc or sound film.

(b) In the case of programme transmissions using one or more radio telephone circuits (inter-continental circuits) the Administrations and Operating Companies concerned should agree if the Broadcast Authorities concerned are to be asked to 308 Model of daily record

of international programme transmission Date

Exchange. London... ANEAC — ICIS O PORME RELAYS PROGRAMME FOR CIRCUITS —MAINTENANCE

CIRCUITS NAME or sections of circuits used TIME NUMBER OF of Broadcast for the transmission TIME Authority OBJECT TYPE to be charged not or of the of of counted CHARGE TOTAL programme circuits (faults, per Unit charge Administration transmission used Circuit Circuit interruptions) or Operating handed released etc. Minutes Charge Company To 2) over to From by charged Units which should Broadcast Broadcast collect the Authority Authority charge

Concert London Bruxelles from London broadcast to Bruxelles Berlin, Kobenhavn (See diagram below 0

-

1) In the case of a multiple transmission using simultaneously a number of circuits, it is useful to attach a diagram showing the arrangement, to the daily record. 2) Broadcasting stations at the receiving end are underlined. MAINTENANCE — CIRCUITS FOR PROGRAMME RELAYS 3 0 9 transmit an identification signal and if so the type of signal to be used. The use as an identification signal of a steady-state sinusoidal signal, on radiotelephone circuits may give the following difficulties: (1) On a long-wave radiotelephone circuit (suppressed carrier and single sideband transmission) the transmission of a steady sinusoidal signal will result in the transmitter for the radiotelephone circuit transmitting appreciably more power than that required for a test transmission using speech. (2) On a short-wave radiotelephone circuit, the transmission of a steady sinusoidal signal is less satisfactory than a test transmission using speech or music for judging the effect of fading and in particular of selective fading.

6 . Supervision of the transmission It is desirable to identify circuits and equipment used for programme trans­ missions by special marking in exchanges (and in repeater stations where the circuits' are accessible) and to recommend that the staff monitor these circuits with a high- quality loudspeaker of high impedance so that the control station for the inter­ national programme link can be advised quickly when a fault occurs. If it is necessary to monitor a number of programme transmissions simult­ aneously, silent monitoring equipment giving a visual indication may be used instead of loudspeakers (volume meters, oscilloscopes, etc . . .).

7. Procedure at the end o f the programme transmission If the technical staff are responsible for determining the chargeable time for the programme transmission, the following principles should be applied: The technical staff at the terminal repeater station should cooperate to determine accurately at the end of the programme transmission: (a) The time when the circuits comprising the international programme link were placed at the disposal of the Broadcast Authorities (beginning of the charge­ able time). (b) The time when the circuits were released by the Broadcast Authorities (end of the chargeable time). (c) Where necessary, the times and duration of each interruption or incident (to establish a rebate). Broadcast Authorities cannot establish a claim for a rebate if contrary to the requirements of paragraph 3 of Chapter I of the present section, they have not requested a control circuit. The times of the beginning and end of the chargeable period, and of the time and duration of any interruptions, are entered on a daily record—an example of which is given on p. 308. SECTION 6

Maintenance of circuits used for television

(This question is to be studied by the C.C.I.F. in cooperation with the Television Broadcast Authorities). APPENDIX I Line-up record for a coaxial line Technical Service o f ...... Designation of lin k ...... Date of measurements ...... Control Station .... -...... •■■■...... Subcontrol Stations...... Resistance of conductors used for temperature reference...... Issue ...... ANEAC — ARE SYSTEMS CARRIER —MAINTENANCE

Relative level *) at frequencies (kc/s) Re­ marks Station 60 308 556 808 1 056 1 304 1 552 1 800 2 048 2 296 2 544 2 792 3 040 3 288 3 536 3 784 4 092 *)

DIRECTION OF TRANSMISSION

“A” “ B ” “C” “ D ” “ E ” “F”

DIRECTION OF TRANSMISSION

“ A ” “ B ” “ C ” “ D ” “E” “ F ”

*) The appropriate indication to be given in the “ Remarks ” column for each station, using the following abbreviations: N = nepers t = relative voltage level db = decibels p = relative power level 312 MAINTENANCE — CARRIER SYSTEMS

APPENDIX II Line-up record for carrier line (symmetrical pair) Technical Service of -...... Designation of link -...... Date of measurements ...... Control station -...... Subcontrol stations ...... Resistance of conductors used for temperature reference Issue ......

A/B direction B/A direction Test frequencies / + « •) kc/s Repeater Stations a = *) Relative level ***)

/ = 12 16 20 24 28 32 36 40 44 48 52 56 60 68 76 84 92 100 108 120 132 144 156 168 180 192 204 216 228 240 252 256

Line pilots **)

Additional frequency **)

Equalizers

Remarks ***)

*) The value for a is arranged by agreement between Administrations. **) Indicate frequencies of these pilots. ***) The appropriate indication to be given in the column for each station using the following abbreviations: N — nepers t = relative voltage level db = decibels p = relative power level APPENDIX III Routing form for a supergroup link Technical Service of ...... Supergroup link ...... ,.... Control station for supergroup ...... Subcontrol stations for supergroup ...... Date of issue ......

Sections in cable Nominal Sections on radio links levels Stations Carrier sections Coaxial sections at Remarks **) SYSTEMS CARRIER —MAINTENANCE (Symmetrical pair) transfer ***) *) Designation points of cable Position Number Position Designation Position Pair of of coaxial of of of ***) number supergroup system supergroup radio link supergroup

*) Underline supergroup transfer points. **) Mention any special types of carrier system, e.g. submarine cable system. . In such cases state the frequency bands for the two directions of transmission. Show type of transfer equipment and supplementary information of necessary. ,_i ***) The appropriate indication to be given in the “Remarks” column for each station using the following observations: U) N = nepers . t — relative voltage level db = decibels p = relative power level \ 314 APPENDIX IV Routing form for a group link Technical Service of...... Group link ...... Control station for group ..... -..... Subcontrol stations for group...... Issue dated ...... '......

Primary group section **)

Supergroup sections ***) SYSTEMS CARRIER —MAINTENANCE Nominal levels Stations Remarks Designation at o 1 cables Position of the supergroup, transfer points ***«) Pair Position of primary group Supergroup followed by the position •) t) numbers (A, B, C, D, E) number of the t) group in the supergroup

•

*) Underline the group transfer points. **) Sections in cable, open wire or radio link not providing a supergroup. . ***) Sections in cable or radio links with at least one supergroup. *«**) Mention the type of carrier system: 12,24 . . . 12 + 12 . . . channels and if not on underground cable state— open wire, radio link, submarine cable. In such cases give the frequency bands for the two directions of transmission. Show the type of transfer equipment, f) The appropriate indication to be given in the “ Remarks ” column for each station using the following abbreviations. N = nepers t = relative voltage level db = decibels p = relative power level APPENDIX V Line-up record for supergroup link

Technical Service of ...... Supergroup link ...... D irection...... -...... D ate of measurements ...... Control sta tio n ...... Subcontrol stations ...... -...... Issue dated ...... ANEAC — ARE SYSTEMS CARRIER —MAINTENANCE

Relative levels < « Nominal relative Impedance Remarks Measuring level at Measuring equipment at measuring point ***) Stations Test frequencies kc/s I point measuring 1 **) ****) point ***♦) 313 317 333 381 429 477 525 545 549 *) •)

Frequency (kc/s) of supergroup reference pilot ...... Absolute power level of supergroup reference pilot at a zero relative level point ......

*) Show in these columns the differences relative to the nominal values. **) State if the equipment is selective or not. ***) Indicate the presence of supergroup automatic gain control. **»*) Theappropriateindication tobe givenin the “ Remarks ” column for each station using the following abbreviations: N = nepers t = relative voltage level t/i db = decibels p = relative power level 316 APPENDIX VI Line-up record for a group link

Technical Service of...... Group link ...... Direction ...... Date of measurements ...... Control station...... Subcontrol stations ......

Issue dated -...... SYSTEMS CARRIER —MAINTENANCE

Relative levels ***) Measuring Nominal relative Impedance Remarks Measuring level at Stations < equipment measuring point at *•*) Test frequencies in kc/s 0 point measuring ) «***) point (4 kc/s spacing) ffi Pilot B *)

Frequency of group reference pilot kc/s ...... Absolute power level of group reference pilot at a point of zero relative level

*) Show in these columns the differences relative to the nominal values. **) State if the equipment is selective or not. ***) Indicate the presence of group automatic gain control. ***♦) The appropriate indication to be given in the “ Remarks ” column for each station, using the following abbreviations: N = nepers t = relative voltage level db = decibels p = relative power level MAINTENANCE — CARRIER SYSTEMS 3 1 7

APPENDIX VII Circuit routing form (hypsogram) Technical Service of ...... Routing of circuit ...... Date of measurements (or of putting into service) ...... Control station ...... Subcontrol stations ...... Signalling frequency...... Issue dated ......

Relative level Group delay at repeater output direction * *** time at 800 c/s Remarks Stations Constitution (milliseconds *** ** A — B B — A

An asterisk placed after the relative level indicates that the nominal value of the impedance at the measuring point differs from 600 ohms. This column will only be completed if there are long audio sections. The appropriate indication to be given in the “ Remarks ” column for each station, using the following abb re­ viations: N = nepers t = relative voltage level db = decibels p = relative power level 318 MAINTENANCE------CARRIER SYSTEMS

APPENDIX VIII

Line-up record for a programme circuit

Technical Service of ...... Circuit designation —...... Control station - ...... Subcontrol stations:...... Type of circuit: normal°old type ...... Date of measurements ...... Issue dates — ...... ,.

“ Loss-frequency ” characteristic

A B C D E F | Remarks Station i (Relative voltage level in nepers or decibels)

Frequency (c/s) 30 *** 4 0 *** 50 80 1 0 0 2 0 0 500 800 1 0 0 0 2 0 0 0 3 200 5 000 6 0 0 0 ** 6 400 * 8 500 ** 10 0 0 0 ** 11 0 0 0 *** 12 0 0 0 *** 15 000 ***

Noise: Psophometric noise Flat noise ......

* Old-type circuits only ** Normal type circuits only. *** Measurements at these frequencies will only be made if considered useful. GUIDING PRINCIPLES FOR THE MAINTENANCE OF SEMI-AUTOMATIC CIRCUITS

S u m m a r y

Chapter I: Definitions.

Chapter II: General Rules for the Organization of the Maintenance of Semi-automatic Circuits. 1. Principles. 2. International Maintenance Centre (I.M.C.). 3. Control (repeater) Station.

Chapter III: Preventive Maintenance. 1. Functional Tests. 2. Limit testing. 3. Installation of the testing apparatus foreseen in the specifications.

Chapter IV: Corrective Maintenance. Location and Clearance of Faults. 1. General considerations. 2. Reporting of faults to the I.M.C. 3. Blocking of the circuit. 4. Broad localization of faults.

5. Priority of the localization tests.

6 . Fault clearance. 7. Record of the nature of faults when cleared. OJ OK> Maintenance

Preventive maintenance Corrective maintenance (systematic maintenance) (fault finding) Entretien preventif Entsetien correctif (entretien systematique) Determination of the quality (recherche des derangements) CIRCUITS SEMI-AUTO OF MAINTENANCE of the service Determination de la qualite du service

Functional tests Limit testing Broad localization Detailed fault Essais Essais aux limites Localisation sommaire localization and clearance de fonctionnement Verification des reglages Recherche de Vorgane derange et relive du derangement

Diagram showing various operations implied by “ maintenance ” MAINTENANCE OF SEMI-AUTO CIRCUITS 321

Guiding principles for the maintenance of semi-automatic circuits

CHAPTER I

Definitions

International line: Telephone transmission system contained between the test jack panels of the two terminal repeater stations. International circuit: The whole of the international line and the outgoing and incoming equipment proper to the line. Automatic switching equipment: That part of an international centre concerned with switching operations for directing the call in the desired direction. Maintenance: All the operations concerned with maintaining telephone circuits and automatic switching equipment in a good functional state. (See for this defin­ ition and the following the diagram on the opposite page.) Preventive Maintenance: Tests, measurements and regulations to specified values effected before the appearance of a fault. Corrective Maintenance: Tests, measurements and regulations effected as a result of a fault. Determination o f the quality of the service: Tests carried out under normal functional conditions to ascertain the probability of the appearance of faults. Functional tests: Tests carried out under normal operating conditions to verify that a circuit or a particular part of the equipment functions correctly. Limit testing: Tests carried out under conditions more severe than those corres­ ponding to the specified nominal values, to dermine the margin of security which exists under normal operating conditions.

Localization of faults: The broad localization of a fault consists of determining the division of technical responsibility in which the fault exists. Fault finding consists of determining the faulty part of the equipment.

21 3 2 2 MAINTENANCE — CARRIER SYSTEMS

Organization Chart for maintenance oj automatic circuits

International, maintenance centre I.M.C.

Transmission technical service

Country A

Transmission (x) technical Country B service

Country C

Switching Transmission technical technical service service

International maintenance centre I.M.C.

* The connection between switching technical services of different countries is marked by an asterisk, because such a connection is not considered as indispensable. The staff of international automatic exchanges do not normally need to know foreign languages. However this connection may be realized and may be of good service when it is possible to do so. MAINTENANCE OF SEMI-AUTO CIRCUITS 3 2 3

CHAPTER II

General rules for the organization of the maintenance • of semi-automatic circuits

1. Principles With international semi-automatic operation, each Administration shall assume responsibility for the testing and clearance of faults on its outgoing circuits. The other administrations will co-operate with tests and clearance of faults on these circuits at the request of the responsible Administration.

2. International Maintenance Centre (I.M.C.) 2.1. The body which will exercise this responsibility for the outgoing circuits of an international terminal centre is the “ International Maintenance Centre ” or in short I.M.C. The person in charge of the International Maintenance Centre will be referred to hereafter under the name of “ Officer-in-Charge of I.M.C.” or in short by the expression “ officer-in-charge ”. 2.2. To carry out the maintenance of outgoing circuits the International Maintenance Centre may give directions to the competent services of:— — the international automatic exchange, — the repeater station. 2.3. The operating services should report all faults affecting the international service to the International Maintenance Centre and to this Centre only. 2.4. The responsibilities of the International Maintenance Centre are as follows. 2.4.1. To receive all reports of faults on its outgoing international circuits and to conduct tests with a view to the broad localization of faults limited to ascertaining the division of technical responsibility for their clearance. 2.4.2. To entrust the clearance of faults to the appropriate technical division as determined by the broad localization. 2.4.3. To ensure that the out-of-service times of its outgoing inter­ national circuits (due to faults or other causes) are kept to a minimum compatible with the needs of the service. 2.4.4. To return the circuit to the operating services after having verified its correct functioning. 2.4.5. To keep detailed records of the faults, localizations and clearances- with which it has been concerned. 324 MAINTENANCE OF SEMI-AUTO CIRCUITS

2.4.6. To co-operate with the I.M.C.s of other countries in respect of the broad localization of faults on its incoming international circuits and to accept responsibility for the clearance of faults found to exist in or beyond the centre considered. 2.4.7. To be advised of the need to put any of its incoming international circuits out of use and to inform the I.M.C. of the outgoing centre of the fact. 2.4.8. To ensure that the tests prescribed for its outgoing international circuits are carried out at the specified times and that any faults revealed by such tests are dealt with expeditiously. 2.4.9. To ensure that new outgoing international circuits are satisfactory in operation before being brought into service and co-operate with the I.M.C.s of other countries with any tests which may be necessary on new incoming circuits. 2.5. To ensure that the International Maintenance Centres are operated efficiently,, it is desirable that the following conditions shall, as far as possible, be applied: 2.5.1. The officers-in-charge (and possibly their direct assistants) should possess a thorough knowledge of the switching equipment with which they will be concerned and have an adequate knowledge of transmission. In addition, these officers should be selected with a view to avoiding language difficulties. 2.5.2. The officers-in-charge should possess sufficient authority to direct the clearance, of faults. 2.5.3. The officers-in-charge should be attached to the I.M.C. and should not be distracted from their normal duties by- other occupations which may impede the accomplishment of their principal tak. These officers should be appointed from the beginning of the introduction of semi-automatic circuits into service and should not be subjected to frequent change of duties. They should be authorized to establish personal relations with their opposite numbers in other countries. 2.5.4. To facilitate exchange of views on the clearance of faults, the I.M.C. of the outgoing centre should possess circuit diagrams of the switching equipment installed in the corresponding incoming centres together with any other useful information. It is also desirable that the officers-in-charge of the I.M.C. should be able to visit the switching installations of other corresponding international centres.

3. Control (repeater) Station • The repeater station attached to each international terminal centre should be the control station for the semi-automatic circuits outgoing from this centre. Con­ sequently, in the case of an international route AB comprising semi-automatic circuits operated in the direction A to B and semi-automatic circuits operated in the direction B to A, there will be a control station at each end A and B of the group of circuits:— — at A for the circuits A to B. — at B for the circuits B to A. MAINTENANCE OF SEMI-AUTO CIRCUITS 3 2 5

CHAPTER III

Preventive maintenance

1. Functional tests

1.1. “ Functional tests ” are carried out under conditions similar to normal operating conditions and their purpose is to verify that a circuit or a particular part of the equipment functions correctly. The test conditions are such that a circuit or item of equipment will not be withdrawn from service as faulty if, apart from the test, it would be considered as satisfactory in service.

1.2. Functional tests are carried out locally, or from one end of an inter­ national circuit to the other.

1.3. The tests carried out locally will be left to the discretion of the Administ­ ration responsible for the international centre. The actual tests carried out will depend on the type of equipment concerned and the extent to which alarms and monitoring devices are provided to indicate failures in the establishment of calls. Functional tests of common equipment in the international automatic exchange come into this category.

1.4. Functional tests made from one end of, an international circuit to the other are effected in such a manner that they can be made from the outgoing end of the circuit without co-operation of technical personnel at the incoming end of the circuit. The tests carried out from end to end of a circuit are described in para­ graphs 1.4.1, 1.4.2, and 1.4.3. ‘ 1.4.1. Verification of satisfactory signal transmission; checking that a seizing signal is followed by the return of a proceed-to-send signal and that a forward- clear signal is followed by the return of a release-guard signal. 1.4.2. Rough tests of the transmission conditions, if this is considered useful, by means of the loop test.

The above two tests, being of a simple nature, can be carried out- quickly and consequently as frequently as desired, for example, daily. Signalling tests made by sending seizing and forward-clear signals do not need the provision of any special equipment at the incoming international centre. On the other hand, the international signalling and switching specifications specify the compulsory provision of the loop at the incoming end of an international circuit. 3 2 6 MAINTENANCE OF SEMI-AUTO CIRCUITS

1.4.3. Finally, if any Administration wishes to make functional tests which include the exchange of signals over the international circuit other than those mentioned in 1.4.1 above, use may be made of the test call answering devices (robot subscribers) which exist for the national service of the incoming country. Inform­ ation concerning the calling numbers of these devices will be communicated to other international terminal centres.

2. Limit testing 2.1. The object of these tests is to verify whether the operating margins specified for a particular type of equipment effectively exist or not. If necessary these tests may be followed by the readjustment of the equipment as near to the specified nominal values as is practicable. 2.2. Limit tests of the signalling will, in principle, be carried out locally. The frequency of such tests and the test conditions to be applied will be determined by the Administrations concerned. These tests will be made using, in particular, the calibrated signal generator and the signal measuring apparatus provided for in Chapter VII of the “ Interna­ tional Signalling and Switching Equipment Specifications ”. The verification of the adjustment of signal receivers will be carried out locally but, by special agreement between Administrations, this adjustment can be carried out by tests made from end to end of the circuit when the signal receiver cannot be dissociated from the terminal equipment of the carrier-current system of which it is an integral part. The limit signalling tests will not normally be planned to be made from end to end of the circuit but it may nevertheless be desirable to be able to make such tests, for example, where technical disagreement arises between the two International Maintenance Centres concerned. 2.3. This section 2, Limit testing, does not concern routine maintenance tests made on the line and which are normally followed by a readjustment of the line, for example, to restore it to its planned nominal value of equivalent. Such steps are proper to the repeater stations and are carried out in conformity with the “ Maintenance Instructions ” of Volume III of the Green Book.

3. Installation of the testing apparatus provided for in the specifications It is desirable that Administrations should apply the methods which are indicated in Chapter VII of the International signalling and switching equipment specifications in order that more experience may be gained as to the best methods to be recom­ mended. MAINTENANCE OF SEMI-AUTO CIRCUITS 3 2 7

CHAPTER IV

Corrective maintenance Localization and clearance of faults

1. The localization and clearance of faults on automatic circuits will be carried out in accordance with the general rules described under II for the organization of maintenance. Within the framework of this organization four categories of technical personnel may be called upon for the clearance of faults. (a) The I.M.C. personnel comprising one or more officers-in-charge of main­ tenance. (b) At the repeater station (control), the transmission testing service. (c) At the international automatic exchange, the personnel concerned with the maintenance of the international signalling and switching equipment. (d) In the national automatic exchanges of the incoming country, the personnel concerned with the maintenance of the national switching equipment. The functions of the maintenance personnel at the international and national automatic exchanges do not call for any particular comment except to say that this staff will not need to know foreign languages.

2. Reporting o f faults to the I.M.C. All faults affecting the international service will be reported to the International Maintenance Centre. These faults are reported:— — by operators, — by the maintenance personnel of the international automatic exchange, — by the repeater station staff, — by the officers-in-charge of the I.M.C. of an incoming country. The conditions under which operators will report circuits as faulty will be defined by Administrations. Fault reports can result from functional tests of the equipment and can also arise from faults exposed during tests of the quality of service if this is the practice followed by an Administration for such tests. If an incoming centre is affected by a fault which concerns an important part of the installation at this centre and which is liable to impede the flow of traffic, the I.M.C. of the incoming centre should immediately inform the I.M.C.s of the outgoing centres which are working into the centre considered. 3 2 8 MAINTENANCE OF SEMI-AUTO CIRCUITS

3. Blocking o f the circuit Every circuit reported as faulty to the I.M.C. should be blocked on the initiative of the officer-in-charge if this has not already been done. (For example, in the case where automatic blocking is effected under the conditions described in Chapter VI of the Specifications.) Every intervention of the maintenance personnel which entails the blocking of a circuit should be brought to the notice of the outgoing I.M.C. possibly through the incoming I.M.C. or the control station. The blocking of a circuit by the incoming centre by means of the blocking signal (single-frequency system) or by the continuous emission of one frequency (in the two-frequency system) should not exceed a duration of 5 minutes. If the intervention on the circuit must exceed this duration, the circuit should be withdrawn from service at the outgoing end and the I.M.C. of the incoming centre should make a request to the outgoing centre to this effect.

4. Broad localization of faults The maintenance officer-in-charge of the I.M.C. will first verify whether a fault exists and, if so, will then proceed with the broad localization of the fault. He will determine whether the fault is:— (a) on the international switching equipment at the outgoing centre, (b) on the line, (c) in the incoming country. In carrying out this localization he will avoid, as much as possible, having recourse to the intervention of the I.M.C. of the incoming country and he will use the means put at his disposal which are described in Chapter VII of the Specifications. The experience already acquired from the international point of view confirms the excellent results obtained by the use of loop test in carrying out this broad localization.

5. Priority of the localization tests As a general rule the fault localization tests should have priority over main­ tenance routine tests of individual circuits.

6. Fault Clearance Faults will be passed:— (a) to the maintenance personnel of the international automatic exchange if the fault is localized to the international switching equipment of the outgoing country: (b) to the control station of the international line if the fault is localized to the line. (The control station is situated in the same country as the I.M.C.); (c) to the I.M.C. of the incoming country if the fault is localized to the incoming country. This centre will in turn pass the fault:— — either to the maintenance personnel of the international automatic exchange, MAINTENANCE OF SEMI-AUTO CIRCUITS 3 2 9

— or to any other national transmission or switching service concerned. In so far as it is possible for the I.M.C. personnel to determine that a fault exists in the national network of a foreign country discretion should be used as to whether or not it will be useful to inform the I.M.C. of this country of such a fault. Normally, no attempt will be made to report faults found to exist in the national network of the incoming country except faults of a persistent nature or those affecting local zones which are particularly subject to faults.

7. Records of the nature of faults when cleared The I.M.C. responsible for an outgoing circuit should, after a fault has been cleared, receive particulars of the cause of the fault when this has been determined without ambiguity. These particulars should be limited to a few words, for example, in the case of an international automatic exchange (incoming, transit or outgoing):— — automatic switching equipment, — register, — incoming or outgoing circuit equipment, — signal receiver, or such a report as:— — line fault, — national network. From this, statistics might be derived, to bring to light any weak points in the equipment of an international centre. PAGE INTENTIONALLY LEFT BLANK

PAGE LAISSEE EN BLANC INTENTIONNELLEMENT INDEX C.C.I.F. Green Book, 1954 — Volume III

Alarms: The transmission of — in the case of carrier systems on coaxial pairs .... 99 Amplification: (see: Gain) Amplifiers: — for old-type programme c ir c u its ...... 180 Amplitude o f signal: (in television) ...... 197, 209 Arrangements controlled by speech in stations on ships and by the carrier frequency in shore s ta tio n s...... 231 Articulation reference equivalent ...... 23 D efinition...... 23 Determination...... 25 Reference system for the determination of the — 0 23, 25 Relation with the transmission performance r a t in g ...... 25. 26 — of a local telephone s y s t e m ...... 23 Attenuation: — of a coaxial p a i r ...... 97 — of a repeater section of a loaded cable...... 138 — of a repeater section of a cable with unloaded symmetrical pairs .... 82 — of a repeater section of an open-wire l i n e ...... 67 Composite — of the lines connecting the international exchanges to the local exchanges ...... 24 Composite — between the terminals of the terminating u n i t ...... 146 Composite — of a line transformer...... 140 Crosstalk — (see: Crosstalk) Variation of — of a television circuit as a function of tim e ...... 198 Attenuation constant of loaded c a b le s ...... 138 Attenuation equalization: — in the case of repeaters between cables with different characteristics . . . . 121 Use of pilots to control attenuation equalisation in carrier systems on coaxial p a ir s ...... 87 Attenuation frequency distortion (admissible limits of the variation of the equivalent); — of a line connecting an international exchange to a local exchange . . . 24 — of audio-frequency circuits ...... 113 — between phototelegraph o ffices...... 159 — of circuits for television transmissions...... 2 0 1 — of circuits used for voice-frequency telegraphy in the case of audio-frequency circuits...... 169 in the case of carrier system ch an n els...... 152 — of continental international telephone c ir c u it s ...... 40 — of normal circuits for broadcast programmes...... 184 — of old-type circuits for broadcast program m es...... 174 — of the terminal equipment of carrier system s...... 50 Reduction in quality due to — ...... 27 3 3 2 INDEX

Automatic and semi-automatic circuits: Maintenance (guiding principles)...... 321 , Precautions taken to avoid interruptions...... 243 Automatically switched pads: ...... 37 Balancing: Balance return loss of a national sending or receiving system ...... 35 Balancing of repeater sections of unloaded symmetrical p a irs...... 83 Impedance regularity of a loaded cable ...... 136 Broadcast programmes (relays of) : Use of international telephone cables for relays of broadcast programmes . . 171 Cables: Audio-frequency telephone— ...... 118 — inserted in open-wire lin e s ...... 117 Coaxial pair — ...... 96 General recommendation on loaded — liable to be used for the interna­ tional service ...... 1 2 2 Limitation of the number of test points on international — circuits . . . 39 Model Specifications recommended for the supply of carrier-current — : Star quad — designed to provided 12, 24, 36, 48 or 60 carrier telephone channels on each quad p a i r ...... 79 Repeater sections of — containing unloaded symmetrical pairs suitable for carrier currents with up to 60 telephone channels on each pair . 82 Model Specifications recommended for the supply of a repeater section of an international telephone — and its constituent parts: Specification A.I. — Factory length of loaded telecommunication— . . 123 SpecificationA.il. — Loading coils for loaded telecommunication— . . 132 Specification A.III. — Repeater sections of loaded telecommunication — . 135 Unloaded symmetrical pair — ...... 79 — including coaxial pairs designed to provide a large number of carrier telephone channels ...... : ...... 96 Repeater section of a coaxial — designed to provide a large number of carrier telephone ch a n n els...... 98 Use of international telephone — for relaying broadcast programmes...... 171 Cable lines: — for audio-frequency telephone circuits...... 118, 1 2 2 — coaxial pair ...... 96 — used in old-type circuits for broadcast programm es...... 180 — used in normal circuits for broadcast p rogram m es...... 190 Unloaded symmetrical pair — ...... 79 Capacity: Average capacities of the loading sections of a loaded cable ...... 135 Effective capacity of a length of loaded factory cable ...... 126 Effective capacity of a length of unloaded symmetrical pair factory cable . . 79, 80 Capacity unbalance : Admissible capacity unbalances in a loaded ca b le...... : . . . 127 Admissible capacity unbalances in unloaded symmetrical cable pairs .... 81 Definitions ...... : 129 Carrier currents ; Telephony by carrier current ...... 45 Carrier leak: — at point of zero relative le v e l...... 105 in (1 + 3) systems on open-wire lines — in (12 + 12) cable system s...... 109 — in twelve-circuit open-wire systems ...... 65 — transmitted to line in a carrier sy stem ...... 50 Carrier line link; (symmetrical or coaxial pairs) D efin ition...... 57, 59 Establishment...... 270 INDEX 3 3 3

Carrier systems: Characteristics common to modern s y s te m s ...... 45 Definitions relative to — ...... 57, 98, 267 Establishment and maintenance of — 259, 270, 279 Programme circuits on — : Old-type c ir c u its...... 180 Normal circuits ...... 190 Radio relay systems...... 110 Telephone channels of — used for voice-frequency teleg ra p h y ...... 151 Three-circuit overhead wire line — ...... 104 Twelve-circuit overhead wire line — ...... 59 — (12 + 12) on cables...... 107 — on coaxial c a b le s...... 85 — on unloaded symmetrical-pair cables ...... 68 Channels: Carrier telephone — (see, too, carrier sy s te m s )...... Location of telephone — ...... 261 Use of carrier telephone — for voice-frequency telegraphy ...... 151 Circuits for phototelegraph y...... 158 Circuits for programme transmissions: (see also: Programme transmissions) Characteristics of old-type c ir c u its ...... 172 Characteristics of normal circuits...... 183 Designation ...... 240 Setting-up and maintenance of permanent circuits ...... 303 Circuits for television transmissions: (see also: Television transmissions) Characteristics i ...... 193 Designation...... 240 Maintenance ...... 310 Coaxial pairs: — for television ...... 193 — carrier system s...... 85 — section crossing a fro n tier...... 103 Essential clauses of a specification for the supply of coaxial-pair cables . . . 96 Essential clauses of a specification for the supply of an amplifier section of — 98 Standardized — ...... 96 Coexistence of telegraphy and telephony at audio frequencies : ■ Simultaneous telegraphy and telephony on the same conductors : A. Sub-audio telegraphy ...... 166 B. Super-audio telegraphy ...... 167 C. Simultaneous telegraphy by phantoms...... 167 Telegraphy and telephony coexisting on separate conductors...... 168 Voice-frequency telegraphy at audio freq u en cies...... 168 Combination of international circuits: ...... 115 Continental and intercontinental communications: Admissible limits in a continental communication or on the continental section of an intercontinental communication for: group propagation time ...... 28 phase d is to r tio n ...... 29 Connection: Method of interconnection of two international circuits ...... 37 Continuously-loaded cables: use o f ...... 117 Control circuits:'(in broadcast transmissions) ...... 301 Coordination between radiotelephony and line telephony: Recommendations relating to coordination between radiotelephony and line telephony ...... 215 3 3 4 INDEX

Coupling: Coupling by reaction in circuits used for phototelegraphy 158, 161

Crosstalk: Crosstalk balancing networks ...... 84 Crosstalk between complete circuits Abnormal — conditions in radiotelephone c ir c u its ...... 216 General con d ition s...... 35 — in circuits used for phototelegraph transm issions...... 156 — in circuits used for television transmissions ; . 2 0 0 — in normal circuits used for broadcast p rogram m es...... 185 — in old-type circuits used for broadcast program m es...... 176 — on lines connecting international terminal exchanges to local exchanges...... 35 — on open-wire l i n e s ...... 35 Crosstalk components in carrier-current systems: Harmful out-of-band c o m p o n e n ts...... 52 Intelligible crosstalk components ...... 52 Out-of-band innocuous components . . 52 Possible crosstalk com p on en ts...... 52 Unintelligible crosstalk components...... 52 Crosstalk (due to installations) : — between amplifiers on old-type circuits used for broadcast pro­ grammes 181 — between repeaters (two-wire circuits, audio-frequency telephony) . . 143 — between repeaters (4-wire circuits, audio-frequency telephony) . . . 120, 145 — between repeaters ( 1 2 -channel carrier systems on open-wire lines) . . 6 6 — between repeaters (carrier systems on unloaded symmetrical pairs) . 78 — due to an echo suppressor...... 148 — due to telegraph equipment on phantom circuit...... 167 — in an international exchange...... 35 Intelligible — in the terminal equipment of carrier system s...... 51 — introduced by the cabling in an audio-frequency repeater station . . 150 — introduced by the cabling in a symmetrical pair repeater station . . 78 Precautions taken to reduce the risk of — in the rack cabling of carrier sy stem s...... 46 Crosstalk in a factory length o f unloaded symmetrical-pair c a b le ...... 336 Crosstalk in a repeater section: — of a coaxial pair cable...... 98 — of a loaded c a b le ...... 136 — of an open-wire line used for a 12-channel carrier s y s t e m ...... 67 — of an unloaded symmetrical pair cable ...... 82 Crosstalk (miscellaneous): — between windings of a line transformer and between line transformers 141 — for loading c o i l s ...... 134 Methods of increasing — attenuation between open-wire lines ..... 67 Near-end — between the two transmission d irection s...... 36, 45, 154 Secondary far-end — due to reflection s...... 70 D.C. Component (in television)...... 197, 212

D eloadin g...... 83 Dynamic ratio (of programme transmissions)...... 176, 180, 185, 190

Earthing: — of coaxial-cable p a ir s ...... 101 Echo: Calculation of effects of — for a trunk/toll circuit ...... 30 Effects of — ...... 30 INDEX 3 3 5

Echo suppressor (s): Calculation of echo effects, insertion of — ...... 30 Characteristic times of — 32, 147 Classification of various types of — used in rad iotelep h on y...... 221 Essential clauses of a specification for the supply o f — ...... 147 False operation of reaction suppressors or — connected in an international telephone connection routed on radiotelephone and line circuits .... 217, 225 Influence of — on a private telegraph transmissions...... 157 Operating level and sensitivity o f — 31, 148 Use of — in circuits used for phototelegraphy...... 161 Various types of — : relay type —, rectifier type, intermediate —, terminal — . 31 Equivalent: Admissible — for an international c ir c u it...... 21, 39 Admissible — for a 4-wire circuit used for phototelegraphy transmissions . . 159 Minimum allowable attenuation having regard to echo and stability . 30 Variation of the — in the terminal equipment of carrier s y s te m s ...... 51 Variation, as a function of time, of the — of an international circuit or a trunk/ toll circuit...... 25, 30, 41 Establishment: — of a high-frequency carrier s y s t e m ...... 270 — of international circuits for broadcast program m es...... 303 — of international circuits for voice-frequency telegraphy ...... 285 — of international telephone circuits...... 284 — of supergroup or group lin k s...... 272 — of telephone channels...... ' ...... 278 External voltages: .Protection against —, in overhead-wire line carrier systems (twelve telephone circuits) ...... 67 Factory lengths: Essential clauses of a model specification of general application for the supply of factory lengths of loaded telecommunication cable ...... 123 — of coaxial cable p a ir s ...... 96 — of unloaded pairs...... 79 Fading c o r r e c to r...... 220 Fading o f sig n a ls 215, 216

Faults: Localization of — in international circuits and in carrier systems ...... 238, 269 Localization of — in semi-automatic curcuits 327, 328

Feeding o f repeaters: Power feeding of coaxial-pair cable equipm ent...... 99 Power supply installations for repeaters...... 149

Frequency: Carrier — for the transmission of television signals on l i n e s ...... 210 Reduction in transmission quality due to the limitation of the frequency band effectively transmitted . 27 Checking of frequencies in carrier system s 87, 8 8 , 282 — standards...... 89 Choice of the frequency 800 c/s for measurements ...... 278 Cut-off — of a loaded c a b le ...... 137 — effectively transmitted (see Frequency band) — spectrum in carrier telephone systems: Systems on coaxial pairs...... 85, 95 System on radio relay l i n k s ...... I ll Systems on unloaded symmetrical p a ir s ...... 73 Three-channel systems on open-wire lines . 105, 106 Twelve-channel systems on open-wire lin e s...... 60 (1 2 + 12) cable system s...... 108 3 3 6 INDEX

Frequency (cont.): — stability of virtual carrier frequencies in carrier telephone systems: General recom m endation...... 45, 50, 153 Systems on coaxial pairs ...... 85 Three-channel systems on open-wire l i n e s ...... 105 ■ Twelve-channel telephone systems on open-wire lines ...... 65 — stability of carrier frequencies in carrier telephone systems used for voice-frequency telegraphy...... 153 Frequency band: Frequency band effectively transmitted: — by a normal circuit for broadcast program m es...... 184 — by a radiotelephone circuit...... 216 — by an old-type circuit for broadcast program m es...... 174 — by audiofrequency circuits ...... 113 — by audio-frequency repeaters (2-wire circuits)...... 142 — by audio-frequency repeaters (4-wire circuits) 119, 144 — by each telephone channel in a modern carrier system: Systems on coaxial pairs...... 45 Systems on unloaded symmetrical-cable pairs ...... 45 Systems providing three circuits on open-wire li n e s ...... 104 Systems providing twelve circuits on open-wire lines ..... 59 Systems (12 + 12) on cables ...... 107 — by international telephone circuits 40 — by the lines connecting the international exchanges to the local ex­ changes . . 24 — by the terminal equipment of carrier systems ...... 50 — in a broadcast programme (definition) 174, 184 — in telephony (definition)...... 40 — used for voice-frequency telegraphy on carrier-current telephone channels...... 151 Reduction in transmission quality due to limitation of the — effectively trans­ mitted ...... 27 Widening of the — effectively transmitted by old-type telephone circuits . . . 41, 113 Gain: Amplifier — for broadcast transmissions (old-type circuits) ...... 180 Automatic — equipment on radiotelephone circuits...... 153 Measurement of repeater — for the maintenance of audio-frequency circuits . 293. 294 Repeater — for 2-wire audio-frequency circuits...... 142 Repeater — for 4 -wire audio-frequency circuits 119, 144 Repeater — for 12-channel telephone systems on open-wire lines ...... 6 6 Repeater — for systems on unloaded symmetrical p a ir s ...... 77 Groups (o f telephone channels) : Basic —, in 12-channel systems on open-wire l i n e s ...... 59 Designation and num bering 240, 261 Groups and supergroups in carrier systems on symmetrical unloaded pairs . . 75, 77 Group and supergroup pilots ...... 54 Groups and supergroups in systems on coaxial p a i r s ...... 85 Groups and supergroups in systems on radio relay lin k s ...... I ll Procedure for establishment of group and supergroups l i n k ...... 272 Transfer of groups and supergroups...... 52 Group or supergroup link: D efinition...... 57 Establishment...... 272 Harmonic margin: (see: Non-linear distortion) Harmonic or non-linear distortion: — due to the terminal equipment of carrier sy ste m s...... 51 — in the case of repeaters for 2-wire audio-frequency circuits . 144 — in the case of repeaters for 4-wire audio-frequency circuits ...... 120, 146 — in the case of repeaters for 1 2 -channel carrier systems on open-wire lines . 6 6 — in the case of repeaters for carrier systems on unloaded symmetrical cable p a ir s ...... 78 INDEX 3 3 7

Harmonic or non-linear distortion (cont.) : — of a circuit for television tran sm ission s...... 198 — of an amplifier for old-type circuits used for broadcast programmes . . . 181 — of an audio-frequency circuit...... 113 — of a normal circuit for broadcast program m es...... 186 — of an old-type circuit for broadcast p rogram m es...... 176 Measurement of non-linear distortion of a carrier system ...... 278, 281 Measurement of non-linear distortion of repeaters in a carrier system .... 78 Highest voltage (on programme circuits) : Highest voltage at any point in a c ir c u it...... 176, 185 Hypsogram: — of an audio-frequency telephone c ir c u it 284, 317 — of an old-type circuit for broadcast program m es...... 174 — of a normal circuit for broadcast program m es...... 183 Hysteresis: Additional resistance (of the loading coils) due to — 133 Impairment in transmission performance rating: G eneral...... 22 — due to circuit n o is e ...... 27 — due to limitation of the band actually transmitted...... 27 — due to room n o ise ...... 28 Impedance: Impedance regularity of a loaded c a b l e ...... 136 — of trunk/toll and international circuits...... 38 — characteristic of coaxial pairs ...... 97 — characteristic of loaded cables ...... 138 — characteristic of unloaded symmetrical p a ir s ...... 7 9 — of terminal equipment for carrier systems ...... 51 — in the case of telev isio n ...... 195 — of the repeater in 2-wire circuits (audio-frequency telephony) 143 — of the repeater in 4-wire circuits (audio-frequency telephony) 119, 145 — of the repeater in 1 2 -channel carrier systems on open-wire lines 6 6 — of the amplifiers in programme transmissions (old-type circuits) 181 — of the line transform ers...... 140 — of the terminating units ...... 147 Nominal — of an open-wire l i n e ...... 6 6 Impulse indicators: — used on circuits for programme tran sm ission s...... 178, 189 — used on radiotelephone circuits...... 218 Inductance: Difference in — of the loading coils of the two pairs of the same quad . . . 135 — of loading co ils...... 133 Mutual — in unloaded symmetrical p a i r s ...... 81 Insertion loss — of an echo suppressor ...... 148 — of monitoring equipment, on 4-wire audio-frequency rep eaters...... 120, 145 — of monitoring equipment, on 2 -wire audio-frequency rep eaters...... 143 Insulation: — of cabling in repeater s t a t io n s ...... 150 — of loading co ils...... 134 — resistance of a coaxial-pair c a b le ...... 97 — resistance of a line transformer...... 141 — resistance of a loaded c a b l e ...... 126, 135 — resistance of an unloaded symmetrical pair c a b le ...... 82 Interconnection: — of international and trunk/toll circuits...... 37 — of radio relay links with the general netw ork...... 1 1 0 — of systems on coaxial pairs ...... 1 0 2 — of two radiotelephony circuits by means of a 4-wire metallic circuit . . . 226

22 3 3 8 INDEX

Intercontinental: Admissible limit for the group propagation time on the European section of an — communication...... 29 Admissible limit for the phase distortion on the continental section of an — communication...... : ...... 29 Admissible limit for the phase distortion of a complete connection between subscribers, in the case of an — communication established on a radio­ telephony circuit ...... 217 — radiotelephone c ir c u it s ...... 215 — radiotelephone c o n n e c tio n s...... 215 Intercontinental circuits: General characteristics of a long-distance intercontinental telephone circuit: by land c a b l e ...... 43 on open-wire l i n e s ...... 43 with a long submarine s e c t io n ...... 44 International circuits: Designation ...... 239 International continental circuits: General characteristics of international continental circuits ...... 38 Make-up of long-distance circuits...... 38 International line: (definition in the case of automatic cir c u its)...... 321 International Maintenance C e n tre ...... 323

International programme connection: D efin ition ...... 172, 182 Line-up and supervision...... 177, 305 R e le a se ...... 309 International radiotelephone circuits: General conditions for radiotelephone circuits: Circuits with carrier frequencies above 30 M c /s...... 216 Circuits with carrier frequencies below 30 Mc/s ...... 216 Interconnection of two radiotelephone circuits by means of a 4-wire metallic circuit...... 226 Protection of reaction suppressors on a radiotelephony c ir c u it ...... 225

Leakance: — of a loaded cable ...... 127

Levels: Adjustment of level at a point of transfer of a basic g ro u p ...... 54 Admissible limits for the variations, as a function of frequency, of levels at the output of an old-type programme transmission repeater...... 174 Maximum relative level on audio-frequency telephone circuits (at the output of a four-wire r e p e a te r )...... 146 Operating level of an echo suppressor...... 32, 148 Relative levels in carrier telephone systems (especially at the output of terminal repeaters): three-circuit open-wire system s...... 105 twelve-circuit open-wire s y s te m s ...... 60 unloaded symmetrical-pair cable systems 72, 73, 75 coaxial-pair cable sy ste m s...... 103 Relative power levels at group and supergroup distribution fr a m e s ...... 54, 276

Line transformers: Essential clauses of a specification for — ...... 140

Line period (before programme transmission): Definition and du ration ...... 302 Action during and after th e— ...... 177, 186, 187, 305, 306 INDEX 3 3 9

Line-up record — for a broadcast programme circuit ...... 304, 318 — for a group or supergroup lin k ...... 277, 315, 316 — for a l i n e ...... 272, 311, 312 List of phrases to be used in maintenance services...... 236 Loading: Loading spacing ...... 137 Use of coil-loaded c a b le...... 117 Loading coils: Essential clauses of a model specification of general application for the supply of loading coils for loaded telecommunication c a b le s ...... 132 Loading o f circuits: Notes on the use of different types of lo a d in g 118, 139 Types of loading for circuits used for voice-frequency telegraphy...... 169

Long-distance lines: — in international programme transmission 172, 182 — in international television transmission 193, 195

Maintenance: General Recommendation relative to the — of international circuits...... 233 — Instructions ...... i ...... 235 — of carrier systems...... 279 — of circuits used for voice-frequency telegraphy ...... 291 — of circuits used for telephony ...... 286 — of circuits used for television ...... 310 — of semi-automatic circ u its...... 319 Organization of periodical — m easurements...... 286 Periodical — P rogram m e...... 241 Role of the Ninth Study Group, responsible for — questions...... 2 3 9 Measurements: Types of measurement to be m a d e ...... 269 (setting-up measurements) — on a circuit carrying voice-frequency telegraphy ...... 286 — on a telephone c h a n n e l...... ;...... 278 — on a telephone circuit...... 284 — on a programme cir c u it...... 304 — on group and supergroup connections . . . 273, 275, 277 — on high-frequency l i n e s ...... 270 Maintenance— : definition of — ...... 269 — on a circuit carrying voice-frequency te le g r a p h y ...... 292 — on a group or supergroup connection...... 280 — on a line-regulation section ...... 279 — on an audio-frequency c ir c u i t ...... 293, 294 — on a programme c ir c u it...... 305 — on a telephone circuit...... 287, 290

Measuring apparatus: — for maintenance measurements in carrier system s...... 281 Mixed lines: (general conditions to be fulfilled by mixed lin e s ) ...... 117 Modulation: (of the television signal transmitted on lines): Modulation p o la r it y ...... 210 Modulation with residual sid eb an d ...... 210

Monitoring arrangements: Monitoring arrangements to allow supervision of a broadcast programme . . 181 Silent monitoring arrangements (on audio-frequency repeaters) 120, 143, 145 3 4 0 INDEX

National frequency stan dard ...... , ...... 89 Near-end crosstalk (see: Crosstalk) Noise: Circuit noise: — admissible at the end: of a chain of international and trunk circuits...... 36 of a line connecting an international exchange to a local exchange 37 of a long-distance intercontinental circuit on open-wire lines . . 43 of an international c ir c u it...... 42 of the nominal maximum circuit on coaxial p a ir s...... 94 of the nominal maximum circuit on symmetrical cable pairs , . 70 — admissible in an international call ...... 24 — on carrier-current telephone channels used for voice-frequency tele­ graphy ...... 152 — on circuits used for phototelegraphy ...... 161 — on long-distance lines, in television ...... 199 — on normal circuits for broadcast programmes ...... 185 — on old-type circuits for broadcast programmes ...... 176 Reduction of transmission quality due to circuit n o is e ...... 27 Room — in the measurement of the articulation reference equivalent . . 25, 28 Rom noise, corresponding reduction in transmission q u a lity ..... 28 Room — in places containing telephone receivers...... 28 Noise (miscellaneous) : — background noise of repeaters in carrier systems on symmetrical cable pairs 78 — disturbing noise causing operation of echo su p p resso rs 217, 225 — disturbing noise produced in a telephone circuit by the installation of simultaneous telegraphy by phantom c ir c u it...... 167 — disturbing noise in amplifiers in old-type circuits for broadcast programmes 181 — in radiotelephone circuits caused by atmospherics 215, 216 Nominal maximum circu its...... 47 General d e fin itio n ; ...... 47 Nominal maximum circuits for telephony: in cables ...... 47 in coaxial-pair c a b le s ...... 93 in radio-relay lin k s ...... 47 in symmetrical-pair cables ...... 68 on open-wire l i n e s ...... 47, 59 Nominal maximum circuits for television transmissions . 193, 194 Open-wire lines: Construction of open-wire lines (mechanical, electrical q u a litie s)...... 116 Loading o f — ...... • • • Methods for increasing crosstalk attenuation between — ...... 67 Nominal impedance of — ...... 66 — used at audio-frequencies 113, 115 — used with 3-channel carrier telephone systems ...... 104 — used with 12-channel carrier telephone system s...... 67 Patrolling along international open-wire ro u tes...... 117 Test points on international— ...... 116 Origin of a circuit : For programme transm issions 172, 183 For television ...... 193 Oscillation: — in a radiotelephone circuit...... 216 Patrolling: Patrolling of international open-wire li n e s ...... 117 Peak voltage: Peak voltage during a programme transmission: Specific v a l u e 178, 187, 305, 306 Supervision ...... 180, 189, 307 INDEX 34 1

Periodical Maintenance Program m e...... 241 Phantom circuits: Simultaneous telegraphy by phantom circuits...... 167 Use of phantom circuits on unloaded symmetrical pairs for broadcast trans­ missions ...... 191. Phase distortion: — of a carrier telephone channel used for voice-frequency telegraphy . . . 152 — of a circuit used for phototelegraphy...... 159 — of a connection including a radiotelephone circuit ...... 217 — of a continental telephone circu it...... 41 — of a normal circuit used for broadcast programm es...... 184 — of an old-type circuit used for broadcast programm es...... 174 — of a television circuit . ' ...... 201 Phototelegraphy...... 158

Phrases: List of — to be used by maintenance and fault services and in repeater stations for the maintenance of international telephone communications .... 236

Pilots: Group and supergroup pilots ...... 54, 56 — in three-circuit open-wire system s...... • ...... 105 — in twelve-circuit open-wire sy s te m s...... 60, 65 — in (12 + 12) cable system s...... 108 Line-regulating p ilo t s ...... 87 — in a coaxial pair system for automatic sw itc h in g . . 90 — in multi-purpose s y s t e m s ...... 90 Stability of pilot generators: in three-circuit open-wire systems ...... 105 in twelve-circuit open-wire sy s te m s...... 65 Synchronization and regulation — in carrier systems, in symmetrical-pair unloaded ca b le...... 70 Synchronizing and frequency control pilots ...... 88 Polarity (of television s ig n a ls ) 197, 210

Power: Overload level of — in symmetrical cable p a irs...... 78 Overload level of — in twelve-channel open-wire carrier s y s te m s ...... 66 — at output of a programme transmission amplifier ...... 174, 181, 183 — at output of four-wire circuit (audio frequency) repeater ...... 120, 146 — at output of two-wire circuit (audio frequency) repeater ...... 144 — at output of voice-frequency telegraph repeater ...... 146 — at output of a voice-frequency telegraphy repeater (audio-frequencies) 120 — for continuously-transmitted telegraph marking sig n a l...... 157 — in amplitude-modulation and frequency-modulation phototelegraph transmissions ...... 159, 161 — of signals transmitted in international c ir c u it s ...... 285 — of telegraph currents in voice-frequency telegraphy 168, 151 — transmitted during a programme transmissions .-....■...... 307 Normal-type circuit 183, 187 Old-type c ir c u i t 174, 178 Variation, as a function of frequency, of relative power level at the output of a frontier r ep ea ter ...... 113 Power fed station ...... 98

Power supply voltages: Influence of — on repeater g a i n ...... 119, 143, 145 Variations in — for audio-frequency rep eaters...... 149

Power ratio: — of a line transform er...... 142 3 4 2 INDEX

Preparatory period Definition and duration ...... 303 Measurements during and after the — 177, 187, 306

Programme transmissions: Adjustment and maintenance for international — ...... 297 Circuits fo r — : normal type 171, 183 old ty p e 171, 172 Designation of circuits for — 240 Loading coils for old-type — c ir c u its...... 133 Technical responsibilities during an international — 172, 182, 299 Thermionic valves for — rep ea ters...... 149

Propagation time: Difference between propagation times of picture and s o u n d ...... 200 Group — ...... 28, 41

Propagation velocity: — on loaded c a b le s ...... 137 Use of high-velocity c ir c u its...... 30

Psophometric e.m.f.: (see also: Noises, Psophometric power and Psophometric vol­ tage) Admissible — at the end of an international telephone c ir c u i t ...... 42 — on an old-type circuit for broadcast programmes ...... 176 — on a normal circuit for broadcast program m es...... 185 — on lines connecting international terminal exchanges tb local exchanges 37 — produced by an amplifier for programme transmissions, old type . . 181 — produced by equipment for simultaneous telegraphy by phantoms . 167

Psophometric power: Definition ...... 47 Limits for nominal maximum circuits: on coaxial p a i r s ...... 47, 94 on symmetrical p a i r s ...... 70

Pulse: Radio relay links with pulse position m odulation...... I ll

Quality o f telephone circuits: Improvement of old-type audio-frequency circuits...... 113

Radio relay links (use o f ) ...... 110, 215, 216

Radiotelephone stations: Devices used in ship and coast — ...... 231 Requisite conditions for connections between mobile — and international telephone lin e s ...... 229

Radiotelephony: Radiotelephony at sea: essential characteristics of devices operated by speech and by the carrier frequency...... 231 Recommendations relative to coordination between radiotelephony and tele­ phony ...... 215

Reaction suppressors: Classification of various types of — ...... 221 Protection of — used in radiotelephony...... 225 Use in radiotelephone c ir c u its ...... 217 INDEX 3 4 3

Reference equivalent: Measurement of — ...... 22 Practical limits of the — between two operators or between an operator and a subscriber...... 22 Practical limits of the — of a national sending and of a national receiving system in an international telephone connection...... 21, 24 Practical limits of the total — of an international connection between two sub­ scribers ...... 21 Variations as a function of time and tolerances...... 21, 22

Regularity (of impedance) : Attenuation of regularity in an amplifier section of a coaxial cable used for te le v isio n . . . 212 — of an amplifier section in an unloaded symmetrical-pair cable . . . 83 — of a coaxial-cable s e c t io n ...... 97, 98 — of a coaxial pair used for television 97, 212 — of a length of symmetrical-pair ca b l e ...... 97, 98

Regulated line section: Symmetrical or coaxial p a ir s...... 5

Regulation: Regulation of a programme circuit: Normal-type circuit...... 186 Old-type c i r c u i t ...... 177

Regulators: Volume — (radiotelephony)...... 219

Relays (broadcast programme) : Use of international telephone cables for relaying broadcast programmes . . 171

Repeaters: Cord-circuit — ...... 137 Intermediate — for carrier telephone system s...... 66, 77, 95 Line transformers used with two-wire circuit — 141 Provision of power supplies for — ...... 149 — for audio-frequency circuits: Two-wire ...... 142 F our-w ire 119, 144 — stations, cabling and siting ...... 150, 39 Terminal — associated with p a d s...... 37 Thermionic valves for — (for ordinary telephony or programme transmissions) 149

Repeater stations: Attended — ...... 98 Control and sub-control sta tio n s...... 236 Control and sub-control stations for programme transm issions...... 303 Power-fed — ...... 98 Remotely-regulated — ...... 99 Spacing between power-fed stations and attended sta tio n s...... 101

Reserve: — repeater or line section in coaxial-pair carrier sy stem s...... 90

Resistance: Difference between effective — and direct current resistance of loading coils . 133 Insulation — 134, 135 — unbalance between conductors and pairs of a loaded cable: In a length of c a b le ...... 125 In an amplifier s e c t i o n ...... 135 — unbalance between the loading coils of the two pairs of a quad . . 135 3 4 4 INDEX

Return loss of the national sending or receiving s y s t e m ...... 35 Routing forms: — for group or supergroup l i n k s ...... 273, 313, 314 — for telephone c ir c u its 284, 317 Side-tone path: Influence on the transmission performance ra tin g ...... 26 Influence on the volume of vocal sounds e m itte d ...... 26 Signal-to-noise ratio: for long-distance television l i n e ...... 199

Singing point: (audio-frequency m easurem ent) 294, 295

Speakers (in coaxial pair ca b les)...... 99 Speech-level meter: Use on programme c ir c u it s 178, 189 Use on radio c ir c u its...... 218 Stability: Carrier frequency — ...... 45, 50, 153, 65, 105, 85 Magnetic in loading c o il s ...... 133 Standardization: — in Europe of carrier-system ra ck s...... 46 Symmetrical pair (s) (unloaded cable): Essential clauses in a specification for the provision of an amplifier section in a — cable . . ■...... ’ ...... 82 Essential clauses in a specification for the provision of — c a b le s ...... 79 Intermediate and terminal repeaters for — carrier sy stem s...... 77 — cable section crossing a f r o n t ie r ...... 80 — cable s y s t e m s ...... 68 Types of standardized — c a b l e s ...... 79 Use of phantoms, or of frequencies below 12 kHz, for programme transmissions 191 Synchronization: Coaxial-pair carrier sy s te m s...... 87, 88 Symmetrical-pair carrier systems ...... 72 Telegraph circuits (see also: Telegraphy on telephone circuits or telephone cables): Arrangements for setting-up and changeover for a voice-frequency telegraph circuit . 154, 285 Designation of circuits used for voice-frequency telegraphy...... 239 Maintenance ...... 291 Precautions taken to avoid interruptions 243, 291 Telegraphy on telephone circuits or over telephone cables: Coexistence of telephony and telegraphy...... 166 Coexistent telegraphy and telephony on separate conductors...... 168 Private telegraph transmission on an international leased circuit, alternating with private telephone c a l l s ...... 157 Repeaters for voice-frequency telegraphy, at audio frequencies...... 146 Setting-up and changeover for a voice-frequency telegraph circuit 154, 285 Simultaneous telegraphy and telephony on the same conductors: A. Sub-audio telegraphy...... 166 B. Simultaneous telegraphy by phantoms or double phantoms .... 167 C. Super-audio telegrap h y...... 167 Use of carrier-system telephone channels for voice-frequency telegraphy . . . 151 Voice-frequency telegraphy at audio freq u en cies...... 155 Telephony: Audio-frequency te le p h o n y ...... 113 Multiple carrier telephony...... 45 INDEX 3 4 5

Television: Characteristics of an international television circuit and of a long-distance line 193, 195 Maintenance of television circuits...... 310 Origin and extremity of a — circuit (definition)...... 193 ■— signal transmitted on lin e s...... 210 — standard co n v erter 195, 198 Temperature: Variation in attenuation of a coaxial cable as a function of — ...... 97 Terminal equipment: — for carrier telephone systems: general recom m endations...... 50 — on coaxial pairs ...... 95 — on radio relay lin k s...... 110 — on twelve-channel open-wire lin es...... 65 — on unloaded symmetrical p a ir s ...... 77 Terminating sets: Essential clauses for the specification for the supply of terminating sets . . . 146 Testing o f semi-automatic circu its 325, 326 Test points: On cable circu its...... 39 On open-wire l i n e s ...... 116 Thermionic valves : 1 Indications for the preparation of a specification for the provision of — for repeaters (for ordinary telephony or for programme transmissions) . . . 149 Tests for the rejection of — for audio-frequency repeaters ; 294, 295 Tests for the rejection of — in carrier s y s te m s ...... 281 Through group f i l t e r s ...... 54 Through supergroup f i lt e r s ...... 54 Transfer: Group and supergroup — points (definition; terms not used in the United Kingdom of Great Britain and Northern Ireland)...... 57 Transfer of groups and supergroups...... 52 Transitory state: — characteristics, television ...... 201 — measurements, te le p h o n y ...... 98 Transmission performance rating: Application to national transmission p la n s...... 26 Average — of a local sending or receiving system ...... 23 Average — of a national sending or receiving s y s te m ...... 23 Average — of an exchange ...... 24 Average — of a trunk/toll or international telephone cir c u it...... 24, 25 Average — of an intermediate l i n e ...... 24, 26 Average — of the connection between the local exchange and the international terminal exchange ...... 24 D efin ition ...... 23 Influence of side-tone path ...... 26 Limits of the — of an international communication and of a national sending or receiving sy ste m ...... 25 Relation with the A.E.N...... 25, 26 Transmission: Designation of circuits for phototelegraph — ...... 240 Phototelegraph — ...... 158 Telephone circuits for phototelegraph— 158 Television — ...... 240 3 4 6 INDEX

TrunkI toll circuits: Characteristics of loaded trunk/toll circuits liable to carry international calls . 38 Types of loading: Remarks on the use of different types of lo a d in g , . 118, 139 Unbalance: — due to equipment of simultaneous telegraphy by phantoms...... 167 — of telephone circuits and l i n e s ...... 37 Vibration t e s t s ...... 242, 243 Video signal: Characteristics at the video junction points ...... 196 Voice level: Influence of local e f f e c t ...... 26 Voltage (unweighted) due to noise: At the extremity of a programme circuit: Normal type ...... 185 Old type ...... , . . . - ...... 176 Volume: Influence of local effect on the — of vocal sounds transm itted...... 26 Instrument enabling the special operator at the point of junction of radio and metallic circuits to measure the — ...... 218 Maximum or minimum — transmitted by a programme circuit: Normal kind ...... 189 Old-type ...... 180 Measurement and regulation of— on a radiotelephone circuit . . . . . s. . 217,218 — Regulator for radio signals: Conditions to be met by an automatic — regulator at the junction point of the land telephone network and of a radio link.... 219