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268 PRINCIPLES OF CINEMATOGRAPHY
heavily built and acts as a flywheel. It is solidly connected to a second shaft, on which is mounted the speed control roller.
The combined action of the flywheel drum and the viscous couple to the directly driven section of the mechanism make the control roller very resistant to changes in speed. In consequence almost all the small variations in driving speed, caused by the electrical supply, inaccuracies in cutting the gear teeth or irregular frictional loads in the bearings, are eliminated from the control roller and the film is drawn at a constant speed through the scanning gate.
It should be noted from Figure 122 that a considerable mechanism, occupying a relatively large container, is necessary to produce the required constant film speed at the scanning point. The film itself is required to pass through two completely opposite mechanical conditions. Firstly, when passing through the picture gate, it must move forward in a series of rapid intermittent motions, interrupted by periods when it is absolutely stationary. Before the film reaches the sound scanning point this intermittent movement must be completely eliminated and, apart from that, any inaccuracies in the projector gearing must also be removed.
Obviously, the varying load on the Maltese Cross intermittent mechanism will cause corresponding variations in the friction between the teeth of the gear wheels which operate this cross. In turn, these changes in gear loading will be transmitted to some extent throughout the entire machine. Modern equipment and design have produced gears cut to exceptional accuracy and so shaped that these small changes in loading pass almost unnoticed. Further assistance is given by the heavy duty motor and flywheel system.
However, it has been found that, to completely eliminate such small variations, and also to eliminate the large intermittent movement of the film just below the picture gate, a film path of considerable length must exist between the picture and sound apertures. To ensure that all films may be reproduced in synchronism on all projectors, a standard film distance of 20 frames has been specified between these two apertures. This means that, when a given sound is located in the sound gate, the picture corresponding to this sound is printed 20 frames further into the roll of film; that is, the sound is printed ahead of the picture.
Some people have questioned this practice, and have reasoned that a further adjustment should be made to compensate for the relative speeds of light and sound. Light travels at a speed of approximately 186,000 miles per second, whereas sound only