16-mm sound motion pictures : a manual for the professional and the amateur (1953)

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NOISE REDUCTION DESIGN CONSIDERATIONS 283 An initial bias or margin adjustment is also provided. This adjustment may be the resistance variation of a cathode bias resistor on the output tube ; it is customarily set so that when no modulation is present, the d.-c. output current through the biasing shutter or galvanometer will provide a transparent track width of about 0.003 in. to 0.005 in. as seen on a positive film. Margin is provided to offset the starting-time delay of the bias amplifier when an actuating signal appears in the system. Ordinarily, once the milliammeter reading for the desired margin is determined, it is merely necessary to set the margin control for the correct current; readjustment is rarely required. In the design of bias amplifiers, some engineers prefer half-wave rectification of the audio signal, while others prefer full-wave rectification. Since it is well known among design engineers concerned with sound film recording that the pressure wave of the average sound wave shows a much sharper rate of pressure rise that the rarefaction wave, half -wave rectification is theoretically preferable despite a more difficult filtering problem, because of the more rapid rate of rise in bias current that can be obtained when the microphone and the amplifiers are properly poled throughout the recording system. Other engineers take the view that such poling is difficult if not impracticable, and therefore prefer the fullwave rectifier with a simpler filter; this viewpoint is valid when, as is often the case, the phase distortion in the system is quite large. The shortcomings of bias amplifier designs were appreciated early, and many efforts have been made to overcome them. Generally speaking, the improvements have been made in two general directions, one to speed up the action of the bias amplifier attack and the other to compensate in some fashion for the delay that inherently exists. The improvement in the former direction has led to more complex filters, to better regulation of the plate voltage supply energizing the bias amplifier circuits, and to the use of an ultrasonic carrier that is modulated by the applied signal. The use of the ultrasonic carrier reduces the size and the time constants of the filter needed; other advantages, such as altering the slope of the signal current bias voltage curve at the starting threshold can be derived from the modulating and demodulating functions. No doubt the future will see the application to this problem of the gate and the pulse techniques, principles that proved so valuable to the performance of radar equipment during the recent war. There is one fundamental method to compensate for bias delay: that is to eliminate the effect of the delay either by (a) delaying the