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282 IX. SOUND-RECORDING EQUIPMENT AND ARRANGEMENT
If a negative sound track is to be recorded, the transmittance of the sound track must be made a maximum for zero modulation ; this is accomplished by moving the bias key or switch in the direction opposite to that used for direct-positive recording in order to reverse current polarity, and by changing to the negative aperture in the recording machine.
Noise Reduction Design Considerations. When a signal is suddenly applied to a recording system, the amplified signal appears at the galvanometer before the bias current appears. The time of transmission of the bias is longer than the time of transmission of the signal proper because of the delay introduced by the bias amplifier.
The starting-time delay is known as the bias opening, attack time, or bias unlocking time. Although this time delay may be the source of audible distortion for steep wavefront sounds (such as pistol shots), commercial equipment used to record conventional sounds will show little audible distortion. The attack time of a bias amplifier depends upon the designer 's choice ; wide differences of opinion exist among designers as to the optimum time. One commercial design has an attack time of 20 milliseconds ; many designers think this time rather long.
When the signal actuating a bias amplifier ceases, the bias current decays. To avoid audible distortion, the bias closing (or decay) time is made quite long compared with the opening time ; it is 70 milliseconds for the unit whose attack time is 20 milliseconds. Since the closing time for a noise-reduction system is not critical, it is not made adjustable.
The amplifier portion of the * 'front end" of a bias amplifier is customarily designed with a maximum gain in excess of that actually required. A gain control in the form of a screwdriver slot arrangement, is provided to reduce the gain to the desired amount. The attack time of the bias amplifier depends upon the slope of the signal voltage -bias current curve and upon the gain of the ' ' front end ' ' of the bias amplifier. The slope of the signal voltage bias current curve depends upon the design constants of the unit; the gain of the amplifier is usually set by means of the screwdriver slot gain control so that full unlocking is effected when a sine wave tone of 60% modulation at 1000 cps is applied. The relationship of bias voltage to applied signal voltage is quite linear up to complete unlocking. Ordinarily, the gain-setting adjustment is made during bias amplifier manufacture and need not be altered during use, since the amplifiers are quite stable in operation and the recording machines with which they are used are quite consistent in sensitivity of both the galvanometer and of the cooperating shutters — if used.