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

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472 XIII. PROJECTION AND PROJECTORS wheel using a rubber-tired pressure roller that keeps the film ki contact on the overhung drum when the pressure roller tends to stick or when the rubber tire is flat because it wras permitted to rest on the drum when the machine was not in use. Figure 113 shows the RCA Model 400 projector, a post-war design that has many good features. If a machine has a very good motion with very little flutter, the 3000cycle tone, when reproduced by the loudspeaker from an American Fig. 113. RCA model 400 — a modern 16-mm sound projector. Standard Z22.43 test film, will sound as if the tone were produced by an audio-frequency oscillator. For even a critical listener the amount of flutter found in the recorded film is so small that switching from an oscillator to reproduction from this film from a grindstone* on an A-B testf will show little if any difference other than a difference in the hiss * A grindstone is a specially built sound reproducer that has a very small amount of film motion irregularity. The machine used in one case has a sound drum that rotates twice per second with an eccentricity of only 0.0002 in. The flywheel which Is rigidly coupled to the drum shaft weighs over 60 lb. and rotates on a single ball bearing. t An A-B test is a direct-comparison listening test made by electrically switching from one arrangement to the other by snapping a changeover switch.