Movietone Bulletin (November 1928)

MOVIETONE BULLETIN Published | in the interest of the Movietone Projectionist by Fox CASE CORPORATION 460 WEST 54™ ST., NEW YORK VOLUME I, No. 46 NOV. 17, 1928 The Motor Control Box TO IlOV. DCc.POWER SUPPLY SERIES WINOING —— 0.c. MOTOR INDUCTOR TUNED CIRCUIT WINDING INDUCTOR GENERATOR RECTIFIER TUBES AMPLIFIER TUBE GRIDS FILAMENTS PLATES Fig. 1. Simplified diagram, showing action of D. C. motor and control box. NVERYBODY who has atK tempted to develop a system for synchronizing sound with pictures has found, before long, that the recording and reproducing of the sound accompaniment, and its synchronization with the picture, were not the whole of the problem. A very important question, it was soon realized, was the driving of the mechanism at the correct speed. In this connection, three things are essential: 1. Not only must the sound be reproduced in synchronism with the picture —this can be done comparatively easily by driving both the projector and the reproducing mechanism with the same motor—but also the speed of the reproducing equipment must be exactly the same as the speed at which the picture was taken and recorded. The reason for this is that if a sound record is played at a faster or slower speed than it was recorded, there will be a distortion of the voice or music, which makes it sound unnatural. This you can easily verify by a little experimenting with a record on a good phonograph, one where departures from high quality reproduction are readily noticeable. The standard film speed for all Western Electric disc and film recording and reproducing is 90 feet per minute, or 18 inches per second. The drive gearing used is such that this requires a motor speed of 1200 R.P.M. Not only must the reproducing speed be the same at which the picture was taken and recorded, but this speed must be kept very steady indeed— it cannot be allowed to vary by more than avery slight percentage, despite all ordinary changes in load and in supply voltage. Otherwise fluctuations in pitch will be constantly noticed in the reproduced sound, causing the voice to seem shaky and the music out of tune. Here again, the effect referred to has at some time been noticed by nearly everyone, in listening to a phonograph which did not have a very good governor. In addition to the foregoing requirements it is also necessary, of course, to be able to regulate the motor speed in the usual manner when silent pictures are being shown. The funetion of the motor control box and special motor used with Western Electric reproducing equipment is to meet the three requirements just stated. Readers with some knowledge of electric motors may here ask why the constant speed requirements could not be made by using a compound wound motor on D.C. or a synchronous motor on A.C., as these are usually considered to be constant speed machines. With regard to the compound D.C. motor, the answer is that while for ordinary industrial purposes this is true, yet under common fluctuations of load and voltage, the speed regulation of such motors is not close enough to meet the very exact require