. FOR SOUND BROj 3CASTIP STUDIO ENGINEERING FOR SOUND BROADCASTING IN THIS SERIES MICROPHONES(Staff of the BBC Engineering Training Department) SOUNDRECORDING AND REPRODUCTION(J. W. Godfrey and S. W. Amos, B.Sc. (Hons.), A.M.I.E.E.) TELEVISIONENGINEERING PRINCIPLES AND PRACTICE (S. W. Amos, B.Sc. (Hons.), A.M.I.E.E. and D. C. Birkinshaw, M.B.E., M.A., M.I.E.E.) VOL. I: FUNDAMENTALS, CAMERA TUBES, TELEVISION OPTICS, ELECTRON OPnCS VOL.n: VISION-FREQUENCY AMPLIFIERS (In preparation) CONTROL DESK, BRlSTOL (1 954) BBC ENGINEERING TRAINING MANUALS STUDIO ENGINEERING FOR SOUND BROADCASTING By Members of the BBC Engineering Division GENERAL EDITOR : J. W. GODFREY With 108 illustrations in the text and 9 plates ILIFFE & SONS LTD London SEI First published 1955 Published, by arrangement with the BBC, for WIRELESSWORLD by Ilife & Sons, Ltd., Dorset House, Stamford Street, London, S.E.1 Made and printed in England by W. & J. Mackay & Co. Ltd., Chatham, Kent BKS 2182 CONTENTS Chupter Page PREFACE 7 1 DEVELOPMENT OF THE BROADCASTING CHAIN 9 By F. C. BROOKER, A.M.I.E.E. Introduction-Drawing Conventions-Simple Broadcasting Chain -Amplifier Sequence in Broadcasting Chain-Multiple Pro- gramme Sources and Destinations-Broadcasting Chain for Studio Centre 2 TRANSMISSION QUANTITIES 19 By R. 0. ROWLANDS, M.A. Measurement of Level-The Decibel-Measurement of Gain and Loss-Programme Volume-Distortion-Noise 3 ACOUSTICS AND MICROPHONE PLACING 3 1 BY F. W. ALEXANDER. Ph.D. ~ntroduction-~wustics for Broadcasting-Awustic Materials- Structural Resonandound Insulation-Micro~hone Placine- Drama Production ~echnique-~alks-~~n& Range 4 AMPLIFIER EQUIPMENT 54 By H. D. M. ELLIS, M.A. History-Basic Functions-Amplifier Design Considerations- Specific Amplifier TypesStandardisation of Valves-Power Supphes-Spec~alAmpMiers 5 BROADCAST PROGRAMME SOURCES 75 By H. D. M. ELLIS, M.A. Introduction-Basic Sources of Programme-Studio Cubicle and Technical Facilities--Studio Technical Equipment-War-time Studio Equipment--0.B. Facilities--Recorded Programmes- Miscellaneous Programme Sources-Continuity Suites 6 THE CONTROL ROOM By H. D. M. ELLIS, M.A. Functions of the Control Room-Apparatus for Programme Routing-Line terminating and Amplifying Equipment--Opera- tional positions for Control and S.B.-Test Apparatus and Testing --Control Room Layouts-Telephones and Communications- Programme Volume PROGRAMME CIRCUITS ON POST OFFICE LINES By G. STANNARD, B.Sc. Internal and External Lines-General Transmission Problems- Programme Channels in Carrier Cables-Noise Induction- Repeaters-Repeating Coils-Requirements for good Trans- mission on Lines-Eaualisers-BBC Tieline Connections-Test Apparatus 5 Chapter Page 8 MONITORING 162 BY H.-D~M.EUIS, M.A. General ConsiderationsSource Monitoring-Relationship bs tween Aural and Visual Monitoring-The Programme MeterT Technical Monitoring-Automatic Monitor-Sequenbal Mom- toring 9 THE BBC COMMUNICATION SYSTEM By J. S. NORWELL Introduction-General Reouirements-Technical Reauirements-- Design Consideration-&ciples and ~etho~TheThree Channel Carrier System-Signalling-Telegraphy-Telegraph' In- struments INDEX 204 LIST OF PLATES Control Desk, Bristol, 1954 Frontkpiece Plate Opposite Page Progress in Studio Amplifier Design, 1930, 1940, 1954 64 Outside Broadcast Equipment Type OBA/9 96 Equaliser built to order on Standard Chassis 136 Portable Repeater Set 137 Equaliser and Temperature Correction Unit for Main S.B. Circuits 160 A.C. Test Bay 161 Creed Teleprinter No. 7/N3with Keyboard PerforatingAttachment 192 Creed Automatic Teleprinter Transmitter No. 6S/4 193 PREFACE THISmanual has been produced for the purpose of training technical staff in the general principles underlying the operational procedure at BBC studio centres. Some of the information is specific in that it relates to equipment designed to meet BBC requirements. In present- ing this particular information, it has been necessary to trace the history of studio-equipment design, based on pre-war, war-time and post-war periods. It will be appreciated that this has involved many references to equipment and to procedures which have become or are becoming obsolescent, but which may remain in being for some time to come. An attempt has been made to supplement the above information by principles of audio-frequency engineering which have a more general application, such as Transmission Quantities, Chapter 2; Acoustics and Microphone Placing, Chapter 3; Programme Circuits on Post Office Lines, Chapter 7; Communication Circuits for telephony and telegraphy, Chapter 9. The BBC is aware that these subjects are of wide interest and for this reason the manual is being published as a contribution to the training of broadcasting engineers in all parts of the world. The authors of the manual are specialists, each with many years of experience in his respective field, and no effort has been spared to ensure technical accuracy, combined with clear presentation. The technical level is practical rather than academic, and should present no difficulty to the readers for whom it is intended. In this con- nection, it should be emphasised that the manual is intended for both engineering and non-engineering staff at studio centres. DEVELOPMENT OF THE BROADCASTING CHAIN By F. C. BROOKER, A.M.I.E.E. Operations and Maintenance Department 1.1 INTRODUCTION IN modem broadcasting, the transmission of information from its source to the listener involves a chain of acoustic and electrical equipment which varies in complexity according to the size of the broadcasting organisation and the character of the information to be broadcast. For example the broadcasting of a talk by a single trans- mitter can be effectively carried out from a small building, containing a simple studio equipped with a microphone and its associated ampaer, feeding direct into the transmitter. In contrast, a feature programme, involving many sound sources, will require a number of microphones and amplifiers, adequate mixing, control, and switching equipment, and intercommunication between sources; if such a programme is to be broadcast simultaneously by a number of trans- mitters at different geographical locations, telephone cables or radio links will be needed to convey the programme from its sources to the transmitting stations and to provide inter-station telephone communication. For convenience, we shall refer to the information to be broadcast as programme whether it be a talk on "How to grow Cabbages" or a Brahms symphony played by a full orchestra. It should be understood, however, that different types of pro- gramme present different problems related both to acoustic require- ments and to electrical equipment. With all types of programme, it is essential that aural check on quality should be available at various points along the path of transmission. It is also necessary to ensure that the volume of the programme is kept within certain bounds, fixed by the power-handling capacity of the apparatus used on the one hand, and by the necessity to mask unwanted noise on the other. The combined process of aural and visual checking is called moni- toring. It will now be seen that the transmission of a broadcast programme from source to listener involves the use of studios, microphones, amplifiers, control units, telephone cables, transmitters and checking facilities, in addition to switching apparatus for selecting programme 9 STUDIO ENGINEERING FOR SOUND BROADCASTING sources and destinations. These things combine in a logical sequence to form what is known as the broadcasting chain. 1.2 DRAWING CONVENTIONS Before we attempt to describe the broadcasting chain in detail, it is important that the reader should be familiar with certain con- ventions used in block-schematic and circuit drawings relating to broadcasting apparatus. 1.2.1. Block-schematic Drawings This type of drawing is used to indicate the sequence in which various units of apparatus are linked together; it provides an easy means of tracing the path of a programme from one point to another, when consideration of detail is not essential.,Each unit is drawn in geometrical form, for example a rectangle or a circle, inscribed with an identifying legend or symbol, according to specifications recom- mended by the British Standards Institution and embodied in the BBC Drawing Office Practice handbook. Connections between the units are shown as single lines, though normally representing two conductors; when it is necessary to show greater detail, the diagram may be opened up into the conventional two-wire circuit, as illustrated in Fig. 1, where 11 and 12 represent FIG.1-Block schematic diagram of simple chain the terminals of a Post Office line. In prapice the symbols are self- explanatory and bear no legend; a comprehensive list appears in the BBC Drawing Office Practice handbook. It is sometimes desirable to include more information on a diagram than that conveyed by block symbols, for example, the type of microphone or fader. This can be done by using circuit instead of block symbols as in Fig. 2. Comparing the two figures, the first merely shows a microphone and pickup fed into an amplifier through their respective faders, whereas the second indicates a moving-coil microphone and a moving-iron pickup connected to the amplifier through balanced series faders, with a series-shunt gain control between amplifiers. In Figs. 1 and 2, the direction of ampli- fication is indicated by the apex of the triangle. In Fig. 2, the num- DEVELOPMENT OF THE BROADCASTING CHAIN bers represent the programme volume at specific points, referred to a standard called zero volume (p. 166) and expressed in decibels; thus amplifier A raises the volume from -70 to -10, hence its work- ing gain is 60 decibels. 1 2.2. Circuit Drawings A circuit drawing shows the theoretical as distinct from the mechanical construction of an individual unit such as an amplifier, Fig. 3. All the symbols used on this drawing conform to the BBC Ra. 2-Block schematic diagram using circuit symbols Drawing Office Practice handbook, and are based on British Standard 530: 1948, Graphical Symbols for Telecommunications.