ENGINEERING DIVISION TRAINING MANUAL 1942 THE BRITISH BROADCASTING CORPORATION ENGINEERING DIVISION TRAINING MANUAL 1942 THE BRITISH BROADCASTING CORPORATION CONTENTS PAGE FOREWORD 5 LIST OF PLATES 6 CHAPTER I FUNDAMENTAL PRINCIPLES 7 'Where there is much desire to learn, there of necessity Atoms and electric currents. Generating electricity. Effects of a current. Energy, voltage, current, and resistance. Resistances in series and parallel. Potential will be much arguing, much writing, many opinions; difference and potential dividers. Magnetism. Electromagnetism. Electro- for opinion in good men is but knowledge in the making.' magnetic generator. The transformer. Some Practical Applications of Milton's Areopagitica Electromagnetism. Relays. Microphones. Loudspeakers and telephones. Self- induction. Condensers and capacity. The use of graphs. Graphs in practice. CHAPTER II THE NATURE OF SOUND 33 The sense of hearing. The nature of sound. Wave-motion. Wavelength and frequency. Complex waves and harmonics. The study of acoustics. The loudness of sound. A scale of loudness,—the decibel. The control of volume. Measure- ment of volume. CHAPTER III CONVERSION OF SOUND TO ELECTRICITY, AND THE BROADCASTING CHAIN 50 The simple telephone. Separating D.C. and A.C. Development of the 'Broad- casting Chain'. The studio and listening room. The control room. Programme control and monitoring. Sending the programme to line. The receiving end of a line. Transmitting the programme,—` Wireless '. Outside broadcasts. Additional studio facilities. CHAPTER IV PRACTICAL BROADCASTING APPARATUS 63 The carbon microphone. The moving-coil microphone. The ribbon microphone. The decibel again. Microphone mixers. The control potentiometer. The balanced line. The gramophone unit. Control room wiring. Plugs and jacks. 'Normal ' wiring of breakjacks. The chain in detail. Switching arrangements. Private wiring. Alarm circuits. Another type of mixer. CHAPTER V THE THERMIONIC VALVE 90 Historical development. The diode. The triode. The triode as an amplifier. Grid bias. Obtaining greater amplification. Multi-stage amplifiers. Variable gain amplifiers. Trap valves. Practical amplifiers in the broadcasting chain. Mains rectification. The 0.B.A/8. Negative feed-back. The programme meter. Published by the British Broadcasting Corporation, Broadcasting House, London, W.1. Printed by Waterlow & Sons Ltd., London and Dunstable. CHAPTER VI LINKING BY WIRE 109 No. 1681 General requirements of a wire link. Line characteristics. Inductance and loading. Equalization. Harmonic distortion. Noise. Balanced lines. Phantom working. Repeaters. Line testing. D.C. tests by the P.O. 'Clock' Insulation resistance. The Wheatstone bridge. Out-of-balance tests. A.C. Tests. The sending circuit. Measurement of received level,—The amplifier detector. 3 PAGE CHAPTER VII RADIO TRANSMISSION 131 FOREWORD The production of electromagnetic waves. Wave-motion and wavelength. The generation of high frequency currents. The valve oscillator. Receiving the signal. The modulation of high frequency currents. Class B modulation. Bandwidth occupied by modulated waves. Side-bands. Wave-bands available for broad- casting. Reception of electromagnetic waves. Drives. Production of high D.C. THERE are, of course, many text books on electrical engineering and that branch voltages, and other power supplies. High-power transmitting valves. Protective of it known as 'telecommunications ', but up to now there has never been a devices. Limiters. book describing the technical practice and equipment of the BBC. This book has been written by F. C. Brooker, one of the instructors of the CHAPTER VIII THE PROPAGATION OF BBC Engineering School. It has been edited and amplified by L. W. Hayes, 161 ELECTROMAGNETIC WAVES Head of Overseas and Engineering Information Department. It is not intended Medium and long-wave propagation. Service area. Short-wave propagation. to do more than serve as a training manual both for those who have little technical knowledge, and for those who need to brush up the things that 169 CHAPTER IX AERIALS FOR BROADCASTING STATIONS have been forgotten. The task of writing a text book readily understandable General requirements of an aerial. Aerials for medium and long waves. « Aerials by those joining the BBC with more enthusiasm to do ajob of war-work than for short waves. Polar diagrams of aerials. Power distribution diagrams. Short- the technical knowledge of how to do it has not been easy, particularly when wave broadcasting. A method of designating aerials. it is borne in mind that many engineers will be waiting to tear to shreds any CHAPTER X SOUND RECORDING APPLIED TO mis -statements or false analogies I Furthermore, all explanations have been 182 BROADCASTING made without the use of mathematics, covering both the latest technical practice General requirements of a recording system. The Marconi-Stine' steel tape as well as any obsolescent equipment still in use. How well Mr. Brooker has system,—historical. Principles of magnetic recording. Speed control and other succeeded is best left to the judgment of the reader. mechanical features. Disc recording,—historical. The disc. Mechanical arrange- The classic definition of an engineer is someone who can make for twopence ments of the disc machine. The cutter head. Quality of disc reproduction. The reproducing turn-table (TD/7) and editing. 331- r.p.m. Disc recording. • Philips- what it would cost anyone else ashilling to produce. Another way of putting Miller' film recording,—general principles. Film machine mechanism. Recording it is someone who really can make for ashilling what everyone else thinks they and reproducing heads. Film editing. General. can make for twopence. This merely underlines the necessity of practical experience and it is not suggested that those who thoroughly study this book 203 APPENDIXES will automatically become fully trained radio engineers, but by doing so will I. General Letter Symbols of Electrical Quantities have placed themselves in a position to become useful members of the staff II. Standard Symbols used in Drawing of the Engineering Division of the BBC, and should have obtained that sound III. Formulx and Tables. An ABAC basic understanding of the working of the equipment on which further progress IV. Attenuation Curves of Ground-wave Radiation can be built up. V. Modern Control Rooms, and Continuity' Working One of the best ways of understanding how a thing works is to formulate your own theories on the subject and then ask others whether you are right. 221 BIBLIOGRAPHY Your ideas may be wrong, so may his or hers ;but it will probably lead to a healthy argument and an appeal to somebody who really does know. It is 223 INDEX to prompt discussions of this sort that questions have been included at thé end of the chapters. There has been some argument among the departmental heads of the Engineering Division concerning the strictly correct answers to some of these questions, so no junior need feel ashamed to ask help from his S.M.E. or E.i.c. I sincerely hope that anyone who does not understand any part of this book or who cannot answer any of the questions will ask some other h ERRATA member of the staff of his station to explain it more fully, and will go on asking until he really does understand it. In turn, the senior staff of stations P. it Line 1.7 Insert "approx." before 6,000,000,000,000,000,000. have been asked to let us know what parts of this book have not been easily P. 29 Fig. 20 Delete "0" Sn Frequency scale; amend to read "25." understood, so that they can be clarified in future editions. Delete "0" on Frequency scale; amend to read "25." P. 30 Fig. 21 I trust that everyone receiving a copy of this book will make a study of P. 35 Line 20 Delete "Slowly"; amend to read "Quickly." every chapter, even though some of them may not at present appear to have P. 129 Line 36 Delete "Capacity"; amend to read "Capacitances." a direct bearing on their present work, for in this way an interest may be P. 166 Fig.t 123 Delete "i kw. Transmitted" at foot of Diagram. generated in the reader's mind which almost certainly will be of practical use sooner or later. P. 209 Line 27 Insert "Series" before "Circuit." N. ASHBRIDGE rurquAmtri AL CHAPTER I LIST OF PLATES FUNDAMENTAL PRINCIPLES N order to understand how broadcasting works, it is necessary to know PLATE I A typical broadcasting studio for large orchestras the nature of the 'agent' which conveys the intelligence, be it speech or music, over wires and starts it off on its wireless journey from place to PLATE II General view of a Control Room, showing control positions I place. This agent is 'electricity '; it will be met in many different guises— and amplifier bays some perhaps already well known, others not quite so familiar to the man-in- the-street. Therefore, it is proposed to devote some considerable space to the PLATE In Outside Broadcast equipment; showing 4-channel mixer, OBA/8 amplifier, and Mains unit study of fundamental principles. First, then, we want to know the answer to the question .. ' What is PLATE IV Three types of microphone used by the BBC: Reisz, moving electricity ?' Now we are going to consider only the theories accepted by coil, and ribbon modern scientists; for to give acomplete history of the progress of this science and of the work of its pioneers would tend to confuse the reader; besides PLATE V Miscellaneous apparatus used in control rooms; plugs and which, many excellent books treat the subject far more completely than can be jack, potentiometer, relays, and switch attempted here. It will soon be learned that electricity, energy, and matter are intimately PLATE VI (a) Distribution jackfield, and telephone indicators bound up with each other, so let us start by considering the structure of matter. (b) Main distribution frame (MDF) Suppose we take any substance and divide it into extremely small particles.