Motion picture sound engineering (1938)

34 MOTION PICTURE SOUND ENGINEERING In Class A recording, half the total signal energy is recorded on each half-track with the/ result that they are alike but out-of-phase as discussed above. (See Figure 16.) In Class B recording, one half-track receives the positive signal energy and the other half-track receives the negative signal energy. The adjustments necessary to produce each of these tracks are accomplished by proper external connections to the light valve in the case of variable density, and by the inclusion of the proper aperture in th: variable area optical recording system. As explained in the chapter on Noise Reduction, Class B track is not suitable, at least at the present time, for general theatre release, because of the necessity of maintaining an accurate sensitivity balance in the push-pull reproducer. Inequalities resulting from the processing of the film also add to the reproduced distortion. The push-pull, Class A, variable area recording is accomplished by converting the optical system by change of apertures, using the same shutter in both cases. (See Figure 16.) As the noise reduction signals are cancelled out by the reproducer circuit, increased speed of operation of the shutter, if desired, may be secured. In this case, as in reproduction from a variable density pushpull record, a considerable reduction in even harmonic distortion is obtained and distortions due to audible variations in average print transmission are cancelled out. Figure 1 7 — Four-ribbon, push-pull light valve. Class AB recording consists in adjusting the recording mechanism to record Class A for low volume signals and Class B whenever th? signal volume exceeds a given amount. The point of separation between the Class A recording and Class B recording is determined by the setting