Radio Broadcast (May 1928-Apr 1929)

CHAIN broadcasting is known to radio listeners as a means whereby a radio program may be transmitted by several or even dozens of radio stations. Regular networks furnish entertainment every day, and on important occasions great extensions are made so that practically the entire United States is covered. The estimated audiences at such times include one fourth to one third of the entire population of the country. More people have thus listened to the voice of one person than ever before in history. The apparatus and methods whereby such important and remarkable results are accomplished are, therefore, interesting subjects to the radio fan. His knowledge of vacuum tubes, audio-frequency amplifiers, and electrical principles will enable him to appreciate various interesting points in the equipment and operation of the wire lines used in chain broadcasting. In addition to the long lines connecting to radio stations in distant cities, there are many shorter lines transmitting programs, such as from studios centrally located in large cities to the powerful radio broadcasting apparatus out beyond the suburbs. Similar circuits are used to broadcast sporting events, banquets, and other occurrences outside the studio, thus greatly extending the range of program features. It is one of the duties of telephone engineers and operating men to plan and supervise both the short and long lines w hich carry radio programs. These connections differ in various respects from regular local and long distance telephone lines, and have, therefore, been given a special name, "program circuits." One difference is that ordinary telephone circuits transmit the voice in both directions (on long circuits "two-way "amplifiers are therefore necessary), but in program circuits it is necessary to transmit only in one direction, that is, from the pick-up microphone to the one or more radio transmitting stations. "One-way" repeaters are therefore sufficient. In the drawing on page 66 arrows indicate the direction of transmission along each program circuit which was used on January 4th, 1928. the date of the first Dodge Brothers program. The regular route of the voice of Will Rogers, acting as master of ceremonies at Los Angeles, may be followed by way' of San Francisco and Denver to Chicago and the East. Also an additional circuit for use in case of emergency is seen passing through southern New Mexico, Dallas, and St. Louis to Chicago. MEETING THE TRANSMISSION REQUIREMENTS A NOTHER important difference between ordinary telephone circuits and program circuits is in the width of the frequency band transmitted. In a telephone conversation, clear, intelligible speech is desired, and it has been found that this can be obtained if frequencies from about 300 cycles per second to about 2000 cycles per second are transmitted, although modern telephone circuits are engineered to carry a somewhat wider frequency range. However, with program circuits, not only satisfactory intelligibility is desired, but also a very high degree of naturalness and faithfulness in the transmission of music and speech when reproduced through loud speakers. To meet these requirements, a much wider band of frequencies is necessary. In the present art it is generally considered desirable to transmit a range of frequencies from about ioo cycles per second to about 5000 cycles per second, and to do this with approximately uniform efficiency. In this way the low, medium By C. E. Dean American Telephone and Telegraph Company THE CHAIN BROADCASTING CONTROL ROOM IN CHICAGO Similar control rooms are located in Boston, Cincinnati, Detroit, St. Louis, Atlanta, San Francisco, and New York, each in charge of a "transmission supervisor." Repeaters, oscillators, equalizers, transmission measuring devices, and other apparatus necessary in the exacting work of transmitting the programs are shown in the illustration. Cone loud speakers are mounted in the protecting frames at the left. During operation, one cone is connected to the Red network, another to the Blue, a third to the Purple; the fourth is a spare. and high pitch ranges of music and other program material are transmitted with a considerable degree of faithfulness. In addition to the wider frequency-range requirements, program circuits are called upon to transmit greater volume variations than ordinary telephone circuits. For example, the music of a symphony orchestra will vary from a very loud intensity, when many instruments are sounding, to a very faint intensity at other times. What Radio Owes to Chain Broadcasting THE Washington air, in and near the halls of Congress, has been full of pointed and often unpleasant comments about chain broadcasting during the recent weeks when the last radio bill was under consideration. Aside from the political aspects which so fascinate our legislators, it can be said without fear or favor that chain broadcasting is responsible almost entirely for the growth of high-grade programs in this country. Chain broadcasting has partially solved the old question: "Who is to pay for broadcasting?" As the use of the wire network, linking stations, has increased, so has the radio audience, and with it the time, money, and effort expended on programs. This article explains some of the technical aspects of ihe accomplishment, much of which appears for the first time. — The Editor. At all times extraneous noise on the circuit must be slight in comparison with the volume of the music. The critical times are during the faint portions of the program, and to transmit these satisfactorily, a very quiet circuit is obviously necessary. The large variations in the volume of orchestral music (which are of the order of 50 TU, an energy ratio of 100.000) are greater than radio stations can transmit without overloading on the loud signals and losing the faint portions in local noise, static, etc., at the receiver. So at the microphone amplifier one of the broadcast station control operators manipulates the amplification control so as to reduce these variations, cutting down somewhat the loud portions and bringing up somewhat the faint portions, taking care to preserve as nearly as possible the naturalness of the music. The program circnits, i. e., wire lines are quiet enough to be able to more than handle all the volume variation which the broadcasting radio stations desire to transmit. Besides the requirements just considered, the program circuits must of course function harmoniously with the other circuits of the telephone plant, so that program transmission will not be overheard on the ordinary circuits, nor vice versa. For short connections in cities and at other places, circuits in cable are usually employed. The attenuation, the loss, introduced by a seven-mile length of 19-gauge cable pair (consisting of No. 19 B & S copper wire), with no loading coils or other apparatus connected, increases considerably with increase of frequency. One TU of loss means a reduction of power to 795 per cent, of its original value, two transmission units means a further reduction to 795 per cent, of what is left, or to .795 x .795 = .63 = 63 per cent, of the original amount. 65