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Nov., 1937]
NEW MOTION PICTURE APPARATUS
545
aged perforation without loss of loop. However, if two successive perforations are damaged, the loop will be lost. It has been claimed that a tandem claw distributes pressure on the film, reduces the wear, and increases the life of the film. Experience has shown that the life of film is determined not by the number of times it passes through the projector, but by accidental damage in handling it.
The seriousness of losing a loop is the resulting damage to the film and the difficulty of rethreading. The Sound Kodascope film-handling mechanism is so designed that it is practically impossible to damage film by loss of loop. Furthermore, the loop may be reestablished most easily by turning the operating knob off and returning it to the operating position.
As stated earlier, the film motion through the scanning beam is generally governed by some type of mechanical filter whose main element is a flywheel. The function of this filter is to isolate the film from various disturbances that would otherwise act upon it. After studying several such devices, the one shown in
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FIG. 8. Overall frequency characteristic from film to acoustic pressure, film loss not included.
Fig. 7 was chosen as being the most practicable at the present time. The soundtrack is scanned on film drum 1, whose motion governs that of the film at this point. This film drum is mounted rigidly on a shaft which also carries a flywheel 2 and a second drum 3. This system is driven by a viscous coupling between the drum 3 and a hollow shel| 4, which, in turn, is driven by the projector mechanism. The mechanical filter as a whole is designed so that the viscous medium is provided from the same oil bath as that used for lubricating and silencing the projector mechanism.
Under ideal or specified conditions of operation, the performance of this drive approaches perfection. While the change of viscosity of the oil with temperature, and the change of perforation pitch with film shrinkage, in extreme cases, would interfere with the performance of the filter, these extreme cases are not normally encountered in practice.
As stated previously, the power amplifier is mounted in the loud speaker cabinet ; while the pre-amplifier is mounted at the bottom of the front side of the projector and can readily be detached for servicing. The photoelectric cell is re