Radio Broadcast (May 1929-Apr 1930)

O A D C A S T ENGINEERING BY CARL DREHER Volume Control in Broadcast Transmission The problem of effective amplitude control in broadcast transmission is an old one which has been discussed from the beginning and which has not yet reached a final solution — although a satisfactory answer appears to be in sight. The necessity for it arises through the great range of variation characteristic of acoustic problems, and the fact that electrical machinery and conditions in general are not readily adapted to such a range. A symphony orchestra which is being broadcast may emit ten million times as much sound energy at one time as another. Even if the range is only one million, it is not readily handled in the transmission chain following the microphone. The maximum amplitude must be set to pass through the various amplifier units without overloading. If, then, the minima are left as they enter the microphone, these portions of the transmission will drop below the inescapable noise level of the equipment itself (tube hiss, etc.), extraneous disturbances in transmission (cross-talk on wire lines, induction, etc.) and noise in reception originating in similar ways. The remedy is to compress the range of variation within such limits that it will fit into the design of the equipment, neither falling to a level where noise becomes objectionable, or overloading any part of the system, while retaining the essential characteristics of the original sound output with its artistic values. The usual method adopted is the use of a manually operated voltage divider, otherwise known as a "gain control" or "volume control." This is inserted at some point in the transmission chain, as between amplifier stages. The total resistance of the potentiometer is usually 400,000-600,000 ohms, and the taps are so arranged that each step corresponds to a change of 2 db, sometimes 3 db. Steps of 2 db correspond to about 25 per cent, voltage changes. The calibration holds only when no current is drawn by the circuit element ahead of the potentiometer. VOLUME INDICATORS Such an amplitude control is usually used in conjunction with a volume indicator. This is simply a tube rectifier acting as a peak voltmeter. It may also be calibrated in db. The task of the broadcast operator is to watch the indicator and to turn down the gain control when the meter indicates over-shooting, while when the volume drops too low he must raise the level. He is assisted, in some cases, by a view of the action at the pick-up point, while on other jobs he may have to work blind. He may have a musician working with him to indicate what is going to happen, so that the operator may be prepared for changes. Rehearsals, of course, are a great help. The job is well done when changes in volume are confined to an irreducible minimum, effected in advance of the instant when they become necessary, and not made so abruptly that the at tention of critical listeners is disturbed. These are difficult requirements. An automatic volume control in place of the manually operated form, or in conjunction with the latter, was proposed some years ago as a solution, and it is probable that the next few years will witness its adoption in high-grade broadcast operation. AUTOMATIC CONTROL The best known form of automatic volume control is the radio-frequency type used to maintain constant output in a radio receiver with varying field intensity, when a relatively distant station is being picked up. Such devices are usually operated by the carrier wave, variations in carrier intensity being compensated for by inverse changes in radio-frequency amplification. The operation of such a system is described by Harold A.Wheeler in a paper on "Automatic Volume Control for Radio Receiving Sets," in the Proceedings of the Institute of Radio Engineers, Vol. 16, No. 1, January, 1928. Mr. Wheeler shows a receiver with four neutralized radio-frequency stages, followed by a two-element rectifier with filter circuits arranged to separate the direct and audio-frequency components of the pulsating rectified voltage. The audio components are led through a manually operated gain control to the audio amplifier, which comprises four stages. The direct component of the rectified voltage is led back to the radio-frequency train as an automatic grid bias. Fig. 1, reproduced from Fig. 2 of Mr. Wheeler's paper, shows the circuit. With the circuit constants shown the rectifier reacts on the grids of the radio-frequency tubes in second, so that the system is almost capable of wiping out the inherent audio Audio Detector Volume Control Tubes Plate Tubes Fig. 1 frequency variations of sound. This time constant is, of course, controllable. ANOTHER AUTOMATIC DEVICE More recently G. L. Beers and W. L. Carson, in a paper on "Developments in Super-heterodyne Receivers," published in the Vol. 17, No. 3, (March, 1929) Proceedings of the Institute of Radio Engineers, describe an automatic volume control, the circuit of which is here reproduced in Fig. 2. In this case the grid of the volume control tube is connected in parallel, through a coupling condenser, to the grid of the second detector of the superheterodyne receiver. The voltage drop across a resistor in the plate circuit of the detector furnishes additional negative bias for the amplifier tubes, reducing the sensitivity of the receiver. The circuit constants are chosen to obtain the desired smoothing out without affecting the audiofrequency quality of reproduction. By means of the manual control on the grid of the volume-adjustment tube the degree of control may be set at any desired value. By increase of the bias on this tube a larger grid swing is permitted in the audio detector before the automatic volume control action comes into play. The general principle of automatic volume controls for broadcast transmission is similar to that of the radio-frequency devices described, but some modifications are required. The method may utilize part of the input to an amplifier, which, after rectification, yields a d.c. component for control purposes either through grid or plate circuits of the amplifier. By means of a manual adjustment the audio gain control may be set to reduce the intensity range instead of smoothing it out altogether, since in broadcast transmission it is not desired to deliver a constant output with varying input, but merely to bring up the low portions, or reduce the peaks, within certain limits. This may be done by setting the automatic gain control so that it does not function except on peaks above the allowable level, or, alternatively, permitting it to operate at a low level to bring up the amplification as required. In either case the time constants must be chosen to give a rapid response, of the order of a tenth of a second. It is probable that the automatic volume control for broadcast transmission will be useful only in conjunction with a carefully handled manual control. One reason for this is that announcements, in good broadcast practice, are transmitted well below the level of peaks in the music, a form of discrimination which cannot be expected from anything but a human operator. There is no reason, however, why the operator cannot throw a key, for instance, to lower the over-all gain 4-6 db for speech, while the advantages of the automatic volume control are utilized during the musical intervals of the broadcast. • JUNE 1929 • • 109