Radio Broadcast (Nov. 1925-Apr 1926)

APRIL, 1926 SIMULTANEOUS USE OF SEVERAL MICROPHONES 677 when people discuss the merits of a broadcasting station — quality being the effect of a great complexity of notes and their faithful reproduction. Behind the change in phrase there is an evolution from the relatively simple to the relatively complex. And the end is not yet. If radio movies become a reality, will radio fans be asking, "How's the visibility?" a decade or two hence? Technical Operation of Broadcasting Stations 4. Multiple Pick-Up FIGURE 2 shows the layout of what is commonly known as a "mixing panel" for combining the outputs of several microphones. The transmitters M„ M2, M3, are of the carbon type, and they are fed in multiple from a single battery B, each having its own resistance Rx, R^, or R3, in series, to limit the d.c. through the microphone to the proper value. Each microphone feeds its audio output into a repeating coil, which is simply a hi transformer, usually with a torodial winding on an iron core, much used in wire telephone practice. This repeating coil is marked RC, with the appropriate numerical suffix. Also, the secondary of each repeating coil is paralleled by a potentiometer, P„ etc. The variable terminals of these potentiometers are connected in series, as shown in Fig. 2. One extreme terminal goes to one terminal of the low side of the input transformer IT ahead of the first tube; the other extreme terminal of the potentiometer chain goes to the other side of the input transformer primary. The secondary winding of the transformer goes to the tube. In the output of this tube there may be an over-all gain control, as described in the March issue, supplementing the individual gains afforded by the potentiometers. The value of the impedances will now be given roughly. For a 6-volt battery, R will be of the order of 200 ohms, which with the 100 ohms impedance offered by each button of the microphone limits the d.c. per button to the appropriate value of 20 milliamperes. Since, for audio frequencies, the buttons are in series, a normal carbon microphone has an output impedance of about 200 ohms. This matches such a repeating coil as the Western Electric 77-A. The impedance does not change in the repeating coil (the windings being alike) hence the potentiometer across each secondary may also be of the order of several hundred ohms total resistance. The primary of the input transformer has an impedance at low frequencies of 500 ohms, stepped up to about 50,000 ohms in the secondary to match the input impedance of the vacuum tube. This transformer has, it will be seen, an impedance ratio of 100:1, corresponding to a turns ratio of 10:1, the impedance varying as the square of the number of turns. It is important that the potentiometers should be smoothly variable, to avoid abrupt changes in the output of the indivi dual microphones, and that a true zero be obtainable, so that any microphone on the panel may be cut out completely if desired. Usually telephone keys are provided, so that after the gain on a transmitter has been set at zero, its circuit may be opened entirely by means of a key or switch. The diagram shows the operation of this system with carbon microphones, but a condenser type may be used equally well provided that its associated amplifier has a step-down transformer whose output matches a carbon microphone approximately (200-500 ohms.) This output is then connected to the two outside posts of one of the sets of three, the middle post, to which the frame or diaphragm terminal of a carbon microphone goes, being left unconnected. The output of the condenser may then be mixed with a carbon microphone output, if desired. In fact, there is no reason why the outputs of several transmitters of differing frequency characteristics may not be mixed in this way, for the purpose of securing an over-all output superior to any of the component pickups. Or, in the more usual situation, several more or less identical microphones have their outputs mixed, combining pickups which differ owing to the physical positions of the respective microphones with reference to the source of sound. Finally, any microphones may be swung in or taken out at will, to suit changing conditions during a concert. All these operations are noiseless, inasmuch as the variation is carried out in a circuit carrying nothing but audio frequency currents.