Radio Broadcast (May 1928-Apr 1929)

66 RADIO BROADCAST JUNE, 192S Three TU is a power ratio of 50 per cent., four is 40 per cent., and five TU 32 per cent. Twenty TU is a power reduction to 0.01 or 1 per cent, of its original value, as shown by the bottom line of the chart. (TU are also used to express the amount of amplification, or "gain," of an amplifier, the ratio being the reciprocal of that for loss. Thus, 20 TU gain is .-fa = 100 times, meaning that the output power is 100 times as great as the input power. For furthei information on TU see Martin, Journal. A. 1. E. E. June, 1924). If the cable mentioned were used without any correcting agency there would be a serious reduction in the strength of the high-pitch components which give music its charm and brilliancy. But frequency distortion, if not too great, can be offset by introducing an opposite distortion, a veritable case of two bad elements combined to achieve the desired good result. To correct the frequency characteristic of short cable, special devices called "equalizers" are used. These consist of inductance, capacitance, and resistance, three of the elements forming a parallel resonant circuit, such as is familiar to radio amateurs from its use as a wave-trap. However, here the elements are so chosen that the resonant frequency is far lower, lying a little above the range of frequencies which the circuit transmits. As in a wave-trap, the impedance is high at the resonant frequency, so that here the equalizer introduces little loss since it is shunted across the line. But at lower frequencies the impedance is much less, and by proper adjustment of the two resistances and the equalizer is made to have characteristics just the opposite of those of the cable pair. The resulting curve for the cable with the equalizer is practically horizontal, which is the result desired. The volume is then raised to a higher level by a distortionless amplifier. For the long connections between cities in chain broadcasting, "open wire" circuits are largely used, that is, circuits consisting of wire on insulators supported by cross arms. Most of this wire is hard-drawn copper (No. 6 B. & S.) 0.165 inches in diameter, the most rugged type of open wire line used in the Bell System. The energy loss along this type of line is much less than along an equal length of the cable just considered, but after the current has traveled about two or three hundred miles it must be reinforced. For this purpose an audio-frequency amplifier, called a "repeater," is used. An open wire circuit is similar to cable in that the energy loss is greater at high frequencies, but somewhat different methods are used to make the open-wire frequency characteristic horizontal. Repeaters which introduce greater amplification for the high frequencies are used in conjunction with equalizers. These equalizers are different from the cable equalizer since the conditions are not the same. TELEGRAPH AND AMPLIFIER ARRANGEMENTS PARALLELING every long program circuit * is a telegraph circuit over which reports and instructions are transmitted. With keys and sounders at every repeater station this provides an auxiliary communication channel for the use of those responsible for the program circuit. Other telegraph circuits connect the radio stations on each chain with the key station for the coordination of station announcements and other program details. One of the most interesting features of a program network is the means employed to restrict the effect of an accidental short-circuit of the line at any point. Without the methods used, such a short-circuit, besides preventing any transmission beyond the particular point, would greatly reduce the voltage for a considerable distance back along the line. Now an amplifier, besides its primary purpose, has the important property that a change in the condition of the output circuit (such as a short), has practically no effect on the input circuit. So, wherever a program circuit forks, an amplifier is inserted into each outgoing branch, with the result that a short-circuit across one branch will not affect the transmission along the other branch. This is done regardless of whether or not amplification is needed — the one-way feature of an amplifier is taken advantage of in this way to increase the reliability of the system. For this reason repeaters average about 125 miles spacing in the East, when otherwise two or three hundred miles would be sufficiently close, for there are numerous forking points in this part of the country. The drawing on page 67 illustrates, by a typical case, the manner in which the power decreases along each section of a program circuit and is built up to its original value at the repeater points. For example, at the New York repeater station the incoming power from the radio studio is given a net amplification of 9 TU, and then begins the trip to Troy, New York. Along the circuit the power decreases steadily until at Troy it is only 3 TU above the original input at New York. Here it is amplified again, and continues on toward Syracuse. The maintenance of a horizontal frequency characteristic, the importance of which has already been stated, necessitates the introduction of losses at the repeater points which are offset by amplification; for simplicity these are not indicated, the net gain at each repeater station being shown. The final output power of the circuit at Chicago is seen to be four times greater than the input power at New York. The scale at the right gives for any point the number of TU by which the power at that point exceeds the input to the circuit at New York. The left scale gives the corresponding power ratio. THE AMPLIFIERS OR "REPEATERS" (~\^ COURSE., the transmission of music and ^-^ speech over program circuits is by alternating currents having frequencies the same as those which are present acoustically in the sound at the microphone. So the repeaters in the circuit are audio-frequency amplifiers. At the end of each program circuit in chain broadcasting is a radio transmitting station which sends the program out on the ether at a radio frequency. Special study has been devoted to the design of telephone repeaters, and various types have been developed. Those used in program circuits are two-stage, transformer-coupled amplifiers using 130 volts plate supply. The first main element of the repeater is an input transformer whose secondary is tapped to allow adjustment of the amplification given by the repeater. The tapped voltage from this transformer is applied to a high-mu tube having an amplification factor of about 30, and an output resistance of about 60,000 ohms. From this tube the energy goes through an inter-stage transformer to the second stage. Here there is a tube having an amplification factor of about 6 and an output resistance of about 6000 ohms, similar to the 216-A or 1 12 type tubes which have been used in other amplifiers. There is an output transformer for delivering the amplified energy to the outgoing program circuit. Provisions are made for close adjustment of amplification and for adjustment of the frequency characteristic. The amount of amplification or "gain" in the repeater may beadjusted to any value overa range of 37TU in steps of as little as 0.3 TU so that very accurate setting is possible. At 1000 cycles this adjustment varies the gain from 5 TU to 42 TU, which is the same as varying the power amplification from 3.2 to 1 5,800, or the voltage amplification from 1.8 to 126. TESTING AND OPERATION OF PROGRAM CIRCUITS A FEW years ago the testing and operation of all the program circuits then in use was in charge of one "transmission supervisor" located in New York. Since then, the extent of program circuits has grown by such bounds that it has become necessary to have additional transmission supervisors, and these are now located at Boston, Cincinnati, Detroit, Chicago, St. Louis, Atlanta and San Francisco. Each transmission supervisor is responsible for the program circuits going out from his control point. He, therefore, has charge of hundreds of miles of circuits and a number of repeater stations, through which the circuits pass. At each of the repeater stations there are trained men who are on duty during the hours that the program circuits are being tested or used, and these men make reports to the transmission supervisor, as directed, and adjust their apparatus in accordance with his instructions. It is very important to maintain the prograrr circuits in the best of condition, for many thou