International photographer (Jan-Dec 1934)

July;1934 T h INTERNATIONA NIOTOGRAI'll E R Thirty-one The volume indicator, which likewise is connected across the hridging hus, is simply a modified vacuum-tube voltmeter that is equipped with a tapped input transformer. A simplified circuit is given in Figure 2. The tap switch for the transformer and a key switch that controls a voltage-dividing resistance in the grid circuit of the tube are calibrated to read directly in decibels of gain or loss relative to zero level (0.006 watt) when the pointer of the volume indicator meter is at mid-scale. Because of its calibration in decibels, the volume indicator may be used to measure the electrical level at different points in the recording circuit when not used by the monitor man as a volume indicator. The operation of the volume indicator is due to the fact that a speech voltage applied to the primary (P) of the input transformer causes another and greater voltage to be induced in the secondary of the transformer. A portion of this secondary voltage, the exact amount being determined by the positions of the tap and key switches, is applied to the grid-filament circuit of the vacuum tube, which is of the voltage amplifier type. In chapter IX it was explained that a three-electrode vacuum tube operates on the principle that the amount of plate current drawn by the tube is controlled (within limits set by the filament and plate voltages and the constructional features of the tube) by the value and polarity of the voltage applied between the grid and filament. The higher the negative potential of the grid with respect to the filament the lower will be the plate current drawn by the tube. If the grid is made positive, the flow of plate current will increase with an increase in the positive potential of the grid. With no a-c. speech voltage applied to the primary of the input transformer the grid-bias voltage, which is obtained from the filament battery, is adjusted by means of the potentiometer, P, to a value that causes the meter needle to point to a red mark at about one-tenth full scale. This adjustment must always be made before using the volume indicator if accurate readings are desired. If an a-c. voltage of fixed amplitude is then applied to the primary of the input transformer, the plate current of the volume indicator tube will be increased by a definite amount that will be dictated by the amplitude of the voltage. Any variation in the a-c. voltage will cause either a further increase or a decrease in the reading of the meter. As long as an a-c. voltage of appreciable amplitude is applied to the volume indicator, the pointer will show a reading above the red index mark, due to the portion of the grid-voltage plate-current characteristic curve on which the tube is operated. When a speech voltage, such as generated by a micro phone during recording, is supplied to the bridging bus, the volume indicator meter pointer deflects, or "kicks", in a degree that is determined by the amplitude of the speech sounds that are causing the generation of the voltage. Since the sounds are continually varying in intensity, the meter registers a continually changing reading. The monitor man watches the meter during recording and varies his mixing controls so that the kicking of the pointer does not pass approximately mid-scale during normal recording. Values higher than that will cause overloading of the system and the introduction of distortion in the sound record. Very loud sounds like explosions may be permitted to exceed this value, however, because distortion would not be apparent in them. The next chapter will consider the monitoring amplifiers and the system of direct and photo-electric cell monitoring. 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