Nov. 27, 1951 s. s. H|l_|_v 2,576,115 ARRANCEMENT ECR TRANSMIITINC ELECTRIC sICNALs CCCURYINC A wIDE FREQUENCY BAND CvER NARRow BAND CIRCUITS Filed Feb. 4, 1948

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www@ @waisuwìälu Patented Nov. 27, 1951 ' 2,576,115 UNITED STATES A,lnßxrlszN'r oFFlcE 2,516,115 ARRANGEMENT FOR TRANSMITTING ELEC TRIC SIGNALS OCCUPYING A WIDE FRE QUENCY BAND OVERI NARROW BAND CIR CUITS i Stuart Seymour Hill, London, England, assigner to International Standard Electric Corpora tion, New York, N. Y., a corporation of Dela

Application February 4, 1948, Serial No. 6,316 In Great Britain February 7, 1947 13 Claims.` (Cl. 17H4) 2 The present invention relates to electric signal of one or more sections to the frequency range. transmission systems of the kind in which signals passed by the corresponding channel and meansl occupying a relative wide frequency band are for transmitting the frequency shifted sections. transmitted over two or more circuits each of over the respective channels, characterised in­­ which is capable of transmitting only a relative “ this, that the frequency shifting means in thev ly narrow frequency band. case of at least one section comprises two succes The invention has its principal application to sive frequency changes, one of which employs a broadcasting circuits. fixed carrier frequency and the other a carrier It is frequently necessary to send broadcast ma frequency which is adjusted to a value deter terial to the studios by telephone line. Some 10 mined by the cut-off frequency of the correspond times special broadcast channels designed for a ing channel. j wide frequency band with low noise and distor The invention will be described with reference tion are available, or ordinary open wire lines can to the accompanying drawings, Figs. 1 and 2 ofl perhaps be specially equalised for a given occa which respectively show block schematic cir sion. Very often, however, only narrow band 15 cuit diagrams of the transmitting and receiving commercial telephone channels will be available. arrangements of a system according to the inven Such channels may, for example, pass a band from tion. Terminals l and 2 of Fig. 1 are intended about 300 to 3,000 cycles per second, while the to be connected respectively to terminals 3 and broadcast band which it is desired to transmit 4 of Fig. 2 by separate telephone or other rela may extend from about 100 to 6,000 cycles per 20 tively narrow band circuits or lines. The system second. to be described will be assumed to be designed It has therefore already been proposed to split to meet a particular series of requirements, but4 the wide broadcast band into two sections, which it should be understood that it may be adapted after the necessary frequency shifts are trans for other requirements. None of the frequency mitted separately over two different commercial ` ranges given below are intended to set limitations or other narrow band telephone circuits, and are to the system, and are only `quoted for the sake then recombined at the receiving end after the of example, and may be extended Without any necessary frequency transfers. dimculty. It has been found, however, that in the ar It will be assumed that the broadcast band rangements so far proposed, the transmitter and 30 which it is desired to cover extends from 100 to receiver circuits have _to be specially designed for 6,000 cycles per second, and that the commercial the particular band-width passed by the telephone telephone channels cover a band from about 300 channels which will be used, because the receiver tow'3,000 cycles per second. It will further be` frequency shifts necessary for the proper re assumed that the commercial telephone circuits COmbïIling of the two parts of the band depend es likely to be available may have upper cut-oli? fre on this band-width. In other words, if the cir quencies which may lie anywhere between about cuits are designed for telephone channels with a 2,700 and 3,200 cycles per second. The upper given band-width, they cannot be satisfactorily cut-off frequency of the commercial circuits will used with channels having a different band ` be denoted by fc. width. At the transmitting end of the system the pro . It is therefore the principal object of the pres gramme signals covering the wide band are ampli ent invention to overcome this difficulty, and fied in a preamplifier 5 ,and are then passed to provide an arrangement which can be easily through a clean-up> low pass filter 6 which re adapted for use with telephone channels of vari moves those upper frequencies which the sys ous band-widths. ' 45 tem cannot be expected to transmit. This filter This object is achieved according to the inven will therefore limit the band betweeny 100 and tion by providing an arrangement for transmit about 6,300 cycles per second. The limited band ting an electric signal occupying a relatively wide of frequencies is then supplied in parallel to upper frequency band over a plurality of communication and lower channels through a conventional hy channels each of which is capable of passing only 50 b_rid `coil network l, the place of the balancing a relatively narrow `frequency band, comprising impedance being taken by a synchronising oscil means for dividing the signal band into a plu lator 8 to be described later. This hybrid coil rality of sections of widths corresponding respec network prevents appreciable interaction between tively to the widths of the bands passed by the the two channels. The band is then applied in said channels, means for frequency shifting each the lower channel through'the attenuating pad I 2,576,115 4 Vto a modulator l0 supplied with a carrier wave of might also appear at the output of the second frequency 12 kc./s. from an oscillator Il, the modulator i9 as a band from 3.2 to 6 kc./s. Only lower sideband extending from 5.7_Jtou11191 lic/s.M a Vsmall portion of this 4band is however present being selectedflbyaïñlteíf lf2 which also iatte'nuatesj inthe firstv .lower sideban’diat` ßìl'fito 8.8 läd/s. frequencies up to about’7.5 kc./s. foi1` the purpose The effectof'the` two modulations ’in the upper ' of minimising programme leak through the modu ,channel _has therefore been to remove that por lator. The pad 9 may introduce an attenuai àv"tion of the original programme band which is tion of about 12 decibels, andiisïprovid'ed: in’order dealt with bythe-lower channel, and to shift the to ensure that both channels? are .operated at'fY Yupper~ffrequencies ofthe band downwards by about the same level. In the programme- band»` 2.7 kc/s., the diiîerencebetween the frequencies the middle and lower frequencies predominate, of the two oscillators. 1 At the receiving terminal shown in Fig. 2, the and so without the pad 9 thelflower-,channel would . be at a much higher level thanthenpperchannel Y, ' Aband ofjfrequencies of the lower channel which and there would then be dangerïofzthexintroduc arrive overtheïline connected to terminal ¿l are tion of excessive noise into the upper channel. applied to a first .demodulator 2l supplied with a carrier frequency of 9.35 kc./s. from an oscillator The lower sideband from the‘filter l2 is .applied . 22. " N'ñlter 23»removes the upper side-band and to a second modulator s liedw'ith >al-.c'arrier . upp " l "programme leak, and delivers the lower sideband wave of 12.2 kc./s. from an oscillator I4 the out put of .which is applied through an amplifier 2.15V toA a.second demodulator 24 supplied with a car rier frequency from an oscillator 25 at 9.15 lio/s. to the commercial channel connected to terminal 20 The lower sideband from the demodulator. 2K» 2;.twhichrchannel is îassumed-.to cut-off at about coversïthe‘range lûll'to 3,600'cycles per‘second of 3.2 A»kd/s; nand. `so rselects4 part; `of- . the lower :side band. {The‘highest frequency? cnf-:the sideband the original signal band.. Besides the unwanted appliedï to thesecond modulator .-l 3 (11.9. -kc./s.) upper sideband (a) which covers the Vrange 15 to 25 1749 kcJ/s.; there will be a‘sm'all progra-mmev leal; appears ¿asl-1.300 cycles-in the Jfinally* transformed . (b) inthe range' 5.85 to 8;75 koi/sand also those band. The ­highest frequencyof` the originalwide . frequencies (c). immediately above 3.2 kc./s. input-:band f which1is passed» through thelower. which have not been effectively removed bythe channel is A3À,kc./s.,f `and _thisappears as 9 .kc./S._, filters 6 and l2. 4 Y after. thefi-lrst modulation; 1and.3.2’lbands from! the lower and upper there v mayebe some.. transmission upto about channels arerecombined. The frequencies (c) 4.7 ks.¿/s., correspondingv to.4.5¿kc./s..in the orig may beat vwithV frequencies` legitimately trans inal input -ba/nd. .~ Thus « there `isscme transmis» mitted over the upper channel. An additional ' sionon the frequencies between 3.l and 4.5 lcd/s. 35 simple lowpass ñlter section 21‘is therefore in ofthe signal band. `Howeverfany.frequencies in cludedin the lower branch after the second de the >range 3.2110 4.7 kc./s. which may. be actually modulatorZll for the purpose of attenuating these transmitted .by the-»commercial .channel are ,fur frequencies sufñciently to'render them innocuous. ther. attenuated .at a klater .stagerr in . the receiving The filter 21 also'preventslintermodulations bc 40 'terminal. Y. It wlilllçbeseen ' thatthe' ._ eifectof the. _two mod tweenth‘el high frequency Apro uct-s which may ulations is toraise. all the.frequenciesl in the lower pass between the upper and lower channels­ on part,.of .the'band „by v200 .cycles per> second (the account ofthe imperfections of the hybrid coil network l2 3 which', as` willïv be Y explained presently, difference. between .the frequencies _ of the two o_sìc'illators'L. Frequencies in theupper part of. is: used to combine the two parts of' the pro-` the band have been partlylremovedbyl the line gramme band at the receiving end. j and partly ,byï the high pass >section ofthe filter 'The' upper frequency bandï~300 cycles to 3.290 I2 between the two modulators'll) and'l3. f kc‘l/sl‘which: farrives’ over `the' `'line connected ‘to The modulation process in the upperv channel terminal 3 is shifted tothe'lrange 3 to 5.9 koi/s. by two' successive demoolulations.'V The first demod will now be explained.' The .original wide input 50 programme band isapplied from ’the hybrid coil ulator'zâ is sup-plied with carrier waves at a fre~ network' 1 to aV ñrst modulator' l5 supplied with quency of 9.15 kc;/s. from‘the'oscillator 25 which carrienwaves from an oscillator Hof adjustable supplies the demodulator 24 in the lower channel. frequency. If fc is the cut-off frequency of the> " The lower sideband covering‘the range 5.8‘to 8.65 kc./s. is selected by the band pass filter ßîi yand telephone channels, then 'this oscillator should 56 beset 'to aV frequency of`11.5­1­fc"kc./s. In »the delivered to the second demodulator 3! supplied particular case chosen‘in which f'c is 3.2 kc./s., the fromv an adjustable oscillator 32 providing carrier frequency supplied to the ïi'lrst modulator i6> waves at a frequency of'8._65-ì•fc kc./s., which should therefore be 14.7 kc./s. The lower side for'the present case irl-whichV J‘cI is 3.2 lic/s. will band extends fromv8.4 to 1.4.6 kc./s‘., and is passed 60 be ‘1~l»585'»kc.»/s. . through a ñlter le »having aípass bandV extending The two bands 100 to 3,090 cycles and 35000 from 8.8 to y11.7 isc/s., which-removes the> upper to '5,900 cycles obtained' respectively from the side band, andin addition attenuates frequen filter 21 and demodulatorßl are re-united by the cies up -to 6 kc./s., that is >those frequencies which hybrid coil network 28 already referred to, ar mightappear. owing tov .programme leak. ‘ The 65 ranged so that the upper andV lower channels‘are portiongfofY this lowerlside band .passed by theñlter conjugate, so that intermodulation between these ‘l’ßisfap'pl’ied toa 'second'mo’dulator ‘I9 supplied channels c is substantially> prevented. An at with a carrier wave of .frequency of 12. kc./s. from tenuating pad 32' corresponding to the pad 9 and the oscillator! l which supplies the first modula introducing the same attenuation (e.` g. 12 dec .tor I i! .in the lower. channel. " The lower side 70 ibels) 'is introduced 'in‘itheupper channel be band, coveringthe range '300 td 3,200 cycles per ' tween ‘the demodulation 3l `and the hybrid coil second, is .supplied throughan amplifier` Y29' >to network V28 "to restoreithe 'two sections of the the> secondltelephonejchannel connected Ato ter programme band to th'eproper relative levels. minal.|. ` Frequencies between 6 and l8.8 kc./s., An attenuating’pad 3‘3 introducing for example which yare not much "attenuated by the ñlter i8 about 6.5 decibels maybe included if desired in 2,516,115. 5 6i' the common output channel between the hybrid the `band passed bythe commercial circuits `is coil network 28 and the filter 26 >already men fc-Dß kc./s. and therefore the broadcast band tioned, and the programme band may be delivered must be divided at the frequency s ' -to the output conductor 34 through a suitable amplifier 35. , ` « It will `be understood that‘the adjustable oscil Hence the maximum frequency of the upperpor lators l'l and 32 supplying the first modulator lli` tion `of the broadcast band which can be trans at the transmitting end, Fig. 1 and the second mitted over the upper channel is Zic-0.5 kc./s. demodulator 3l at the receiving end Fig. 2 of the This has to be transferred by the double modula upper channel will be set to provide frequencies 10 tion to fc and so since the oscillator Il is used determined by the cut oiffrequency of the tele to supply a frequency of `12 kc./s. to the modu~ phone channels which will be used. If for ex lator I9, it follows that the frequency of the os ample >this cut olf frequency is only 2.7 kc./s.,.the cillator I1 must be 12­|~fc-0.5=11.5­|­fc kc./s. frequencies of theoscillators will be respectively lThe derivation of the fixed frequency portion 14.2 and 11.35 kc./s. and the programme band 15 of the frequency of the oscillator Il may be fur which can be transmitted over the two telephone ther explained as follows: , channels will be correspondingly reduced tothe Let fb=the frequency defining the lower limit range 100 to 4,900 cycles per second. of the Wide-broadcast band. ‘ ­ Besides adjusting the oscillators in accordance Let fc=the upper cut~oif frequency of the com--` with the cut-off frequency of the telephone chan 20 mercial channel. i » i nels it will also be necessary to adjust the filter Let fl=the frequency defining the lower limit 2,1 (Fig. 2) used for preventing intermodulation of the commercial channel. z' between the upper and lower channels. As this Let 02=the frequency of the-second oscillator filter can be‘quite simple this will cause no diffi of the upper channel. culty. ' i 25 Let X=a fixed frequency. . The oscillators used to supply the modulators should be of a very stable and accurate type, and ` Then in order to avoid dis-tortion it is necessary to X +fc=the frequency of the first oscillator of the ensure that the oscillators at the receiving end upper channel. l l Fig. 2 are `correctly set so that the overall tone 30 X may then be Written in terms of 02 and the translation effect is zero Within a tolerance of frequency defining the upper limit of the Wide perhaps i5 cycles. For this purpose, the oscil broadcast band, as determined by fc. lator `8 (Fig. 1) is provided for transmitting over the system a complex wave at a frequency of, say, i The derivation may be as follows:­ l kc./s. with prominent 2nd and 4th harmonics. 35 fc-fl=width of band passed by commercial The fundamental and second harmonic pass over channel. . ' the lower branch, and the fourth harmonic passes fc-fZ-i-fb=point of divisionbetween upper and over the upper branch. The received complex lower channels of Wide-broadcast band. tone after recombination at the receiver (Fig. 2) (fc-fZ-l-fb) ­|­ (fc-fl) =2fc­l­fb­2fl=The fre is applied to a monitoring device 36 connected to 40 quency defining the upper limit of the wide~ the output of the amplifier 35, which device in broadcast band which can be transmitted over cludes selective circuits which` pick out the three these two channels. Y frequencies and a small oscillograph. The three frequencies are applied in pairs‘to the deflecting Now : plates of the oscillograph, and a stationary figure The frequency of the first oscillator-the lfre-` will be obtained only when the tone translation 45 quency defining the upper limit `of the Wide-, effect is zero.` The oscillators at the receiving end are slightly adjusted to produce this result. broadcast band=intermediate frequency. If desired, ringing and telephone circuits 31 ` And: and 38 may be provided in the upper branch at thetransmitting and receiving ends, between the 50 The frequency of the second oscillator-the in telephone channel and the modulating or de termediate frequency=fc „ , modulating equipment in each case. l These cir cuits permit the operators to communicate with one another when making the initial arrange ments for the broadcast. `These circuits are of known type and should preferably be designed to render ineffective the broadcast equipment of the upper channel during ringing and conversa tion by the operators. By employing the `upper In this equation if l2 kc. is substituted for 02, channel conversation would be possible Without 60 .1 kc. for fb, and .6 kc. for 23‘1, X wil1=11.5, the interrupting a broadcast, the quality `of which value of the fixed frequency indicated. would, however, be temporarily impaired by re The frequency for the oscillator 25 (Fig. 2) .is moval of the upper part of the band. also ,determined by programme leak considera ` The frequencies chosen for the oscillators ll tions. It can be shown that to avoid programme and 25 of Figs. 1 and 2 are not very critical, and 65 leak in the demodulator 3l, the frequency of the other values could be used. The frequency 12 oscillator 25 should be greater than Bío-0.5 kc./~s., kc./s. `for the oscillator Il was selected to be and therefore, in the case chosen, in which the about double the maximum frequency of the maximum value of fc is 3.2 kc./s. the minimum broadcast band to be transmitted,` in order to value for the frequency of the oscillator 25 will minimise programme leak. The frequency 12.2 -70 be 9.1 kc./s. A larger value than this could be kcJ/s. for the oscillator I4 then follows since a used, and the value actually chosen, namely 9.15~ frequency shift of 200 cycles per second is re kc./s. was selected to suit a particular filter which> quired for the lower part of the band. Y The fre was available. The frequencies of 9.35 kc./s. for quency 11.5-l-fc for the oscillator I1 is deter the oscillator 22 and 8.65-l-fc kc./s. for the os mined in the following l way. The width of 75 cillator 32 follow from the value 9.l5«kc./s. se-` andere Y 1 Sì l'é'cted'foir theroscillatcr aannemer-.tc obtain me@ cur-*off frequency lof >said.:wirresponding: channel, ne‘cessaryishifts ».to" restore`A the >sections' of the the-fixed carrier frequency and the‘frequencies broadcast band to‘tlieir'proper positions. ' Y ydefining?thev lower''limits­ of fthe "wide-bandol’ It will be’clear that a broadcast band could frequencies «and-the pass‘band'of the correspond be split intoY more than two sections for separate ing channel. Y - ­ transmission 'overa similar numberv 0f" differ-i “broadband electric signall transmission> entïcominercial circuits, means'being provided system according to' claim 2 further comprising according to similar principles for conveniently areceivingarrangement connected to said chan :adjusting the frequency changing arrangements nels Vcomprising means for frequency shifting iì'rifïaccordance with the pass band ofthe commer eachof said vsections back to the corresponding cialfciréuíts.` While the-arrangement has been frequency range `of said wide frequency band, pri-niarily’ap‘plíedto the transmission of broad and means for "combining the restored sections to cast `progranime signals', it could be used for' trans reform-'said’wide frequency band, said frequency fnïitting any typeïof Signals occupying a wide band shifting means at'the receiver for said one ‘sece over twoßor more' narrowl band circuits, for'any tion ­ comprising A' two'A successive frequency chang-V desired purpose. ' ing‘j'stages,'fone"of said stages having> meansrffor ` What is'claimëd is: generating'ab?ixed carrier­ frequency, the Aother 1. An electric signal‘receiving arrangement for stage'having means for generating a carrier ‘free reëéi'vi-ng signals having frequencies lying within quency >adjustable to­ a value determined by said a wide band and which are transmitted over a 20 cut-off frequency of said corresponding channel. plurality of channels each having a signalV pass 4. Á rsystemV according to claim 2 in which said band narrower than said wide band whereinlsig-L' di'vidingmeans is adapted 4to divide said'wide'fre nalsïhaving" frequencies within at least one band quency> band intoftwo sectionsffor transmissionv portion of said wide band, which portion' is ad over'two Anarrow band channels.' said ‘frequency justable with respect to the pass-band-charac 25 shifting means comprising Vmeans including two Y teristics of said channels, are converted to aband frequency changing stages for shifting the. lower of frequencies capable of being transmitted over frequency "section upwards to occupyithe band, a- selected Y one l of said channels,v comprising passed by "one channel and means including two means for receiving signals transmitted over an frequency-_changing stages‘for shifting the upper other of said> channels and for restoring there 30 frequency section downwards to ­occupy the‘band " céived signals back to the corresponding band passed by the other channel. ' portion of said Wideband, means'for receiving ‘15. A system according to claim> 4 in which both signals transmittedV over said one channel,- means of the two Vlast-mentioned frequency shifting including a first oscillator adjusted to oscillate means for >the two >channels include common at a predetermined frequencyV for restoring said, 35 means for generating a common ñxed carrier fre last mentioned signals back to said one band of OlU-BIICY- " Said wide band, ‘said last-mentioned means com Y' 6. A system according’ to claim 2 in which theV prising means' for converting said last-mentioned signals `occupying the wide band are coupled to signalsfto aV4 new band of frequencies, said con said frequency shifting means througha hybrid verting means including a second oscillator vad 40 coil network. . ' justed to oscillate at a frequency equal-tothe 7. A system/according to claim 2 further com sum of the cut-olf frequency of said one channel prising a reeciving .arrangement connected to plus a fixed frequency which is a function of- the .said two­ narrow band channels comprising two frequency of oscillation of said ñrst oscillator and frequency changing stages for shifting back .the the frequencies defining the lower limits of said 45 lower frequency section to the frequency range wide band of >frequencies and the pass band of originally occupied thereby, and twofurther fre said one channel, and> means for converting the quency ¿changing ,stages> for .shifting back the signals having frequencies within said new band upper frequencyV section to the frequency range to signals having frequencies within said one originallyV occupied. thereby, in which the two band portion, and combining means connected further frequency changing.. stages have means to the output of each of said receiving and re for generatingv respectively carrier frequencies storing means to reform said wide band of fre one of which is fixed andthe other adjustable to a' value'V Adetermined `by the upper cut-off fre quencies. i ` , 2. A broad"ba?d electric signal Vtran'emission quency and in4 which the first mentionedV fre system including a transmitting arrangement'for quency changing stages .both have means for transmitting an electric signal occupying a rela generating ?ñxed carrier frequencies,V the carrier tively wide frequency band over aplurahty of generating means `of oneV of said ñrst mentioned communication channels each of which is ca frequency changing stages constituting the ñxed pable of passing' only 'a relatively’narrow fren frequency carrier generating means ,of one >of quency'band, ysaid »arrangement being adjustable 60 thefurther frequencychanging stages. e for use with channels having different pass-band „ 8'.«A system `according to claim 7 in which .the characteristics, comprising means for dividing transmitting arrangement includes means for at the signal band into' a plurality of sections of tenuating ‘the frequencies `of the lower section widths corresponding >respectively to the vi’dths before transmission over the vcorresponding chan of the bands passed by said chan-nels, means'ifor nel to equalizeV the levels of the bands transmit frequency shifting’at-least one of Said sections to ted over the two channels,­and in which the re the frequency range passed by the corresponding ceiving arrangement includes means for attenu-V channel, means for transmitting the frequency ating the frequencies of the upper section by the shifted section overl the correspondingchannel, ' same amount for -restoring the-‘relatíveflevelsfof th'eîfrequenc'y shifting' means for said one sec 70 the two sections before recombination. tion comprising two successive frequency chang 9. A system according to claim 8 in whichY there ìngstages, one of said stages havingl means for is provided Vat the receiver a hybrid-f coil network generating a fixed carrier frequency, the other connected to the output'of the last Stagesïof both stage-having means Vforfgeneratinig a >carrier free said first/and second mentioned frequency chang quency adjustable ¿to aiyalue determined 'bythe u ing stages, ,whereby :thegîtwo >sections,;.o„f;.the wide 2,576,115

band are recombined after frequency restoration, mentioned signals to a new band of frequencies, and means for connecting the output of said net and means including a second oscillator and a work to a common output channel. pass-band ñlter for converting the signals having 10. A system according to claim 2 further com frequencies within said new band of frequencies prising a receiving arrangement including means to frequencies passed by said second channel, said responsive to frequencies within said two sections first oscillator being adjusted to oscillate at a for comparing the overall frequency shifts pro frequency equal to the sum of the cut-off fre duced by the transmission of the two sections re quency of said second channel and a fixed fre spectively over the said channels. quency which is a function of the frequency of 11. A system according to claim 2 in which the said second oscillator, the frequency defining the said comparing means comprises an oscillator lower limit of said wide band of frequencies, and connected to the input of said frequency shifting the frequency defining the lower limit of the sig means in the transmitting arrangement and nal pass-band of said second channel, whereby adapted to supply a given fundamental frequency the limits of the second portion of said wide band accompanied by strong second `and fourth har 15 will be determined by the pass-band characteris monics chosen so that the fundamental and sec tics of said second channel. ond harmonic lie in the band occupied by the 13. An electric signal transmitting arrange lower section, and so that the fourth harmonic ment according to claim 12 in which the fixed lies in the other band, said receiving arrange frequency portion of the frequency' of said first ment including a monitoring device adapted to 20 oscillator is equal to the frequency of said sec compare the frequencies of the said fundamental ond oscillator plus the frequency defining the frequency and harmonics as received. lower limit of the wide band of signals minus twice 12. An electric signal transmitting arrange the frequency defining the lower limit of the sig ment for transmitting signals having frequencies nal pass band of the second channel. lying within a wide band over a plurality of chan 25 nels each having a signal pass-band narrower STUART SEYMOUR HILL. than said wide band, said arrangement being ad justable for use with channels having different REFERENCES CITED pass-band characteristics, comprising means for The following references are of record in the selecting and transmitting signals having fre 30 ñle of this patent: quencies within a ñrst portion of said wide band over a first of said channels, and means for trans UNITED STATES PATENTS mitting signals having frequencies within a sec Number Name Date and portion of said wide band over a second of 1,595,135 Affel ______Aug. 10, 1926 said channels, said last-mentioned means com 1,812,405 Ives ______. June 30, 1931 prising a ñrst oscillator for converting said last 2,407,213 Tuniek -___-_...__..__- Sept. 3, 1946