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CHAPTER XXX DRIVES
( 1 ) To move the sound film, or the disc turntable, two classes of mechanism are required. One is the driving motor. The other is the transmission, consisting of gears, chains or pulleys, or combinations of all three, by means of which the motion of the motor is conveyed to the disc turntable or to the sprockets that move the film.
It will be logical to postpone discussion of the transmission mechanism until the motors that drive it have been considered.
(2) Four things are required of a modern projector drive motor, (a) It must be sufficiently powerful to perform the work, (b) It must possess moderately strong starting torque (torque is rotary force) in order to bring the projector up to speed promptly and avoid sloppy changeovers. On the. other hand, the starting torque must not be so great that the mechanism is started with a jerk that might strip or weaken the teeth of the gears, or tear the film, (c) It must revolve at precisely the same speed as the motor with which the recording was made, neither slower nor faster, otherwise music will be off key and voices out of proper pitch, (d) It must impart an absolutely steady and unwavering speed to the film or record.
Slight fluctuations of motor speed are distinctly audible, and spoil sound completely. Since the pitch of reproduced sound depends entirely on the speed of the motor (Page 486), fluctuation of motor speed will make a steady note sound like a rapid succession of two different notes, one somewhat lower or higher than the other. (3) If the motor speed fluctuates rapidly this condition is called "flutter/' If the change in speed takes place
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