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136 G. PULLER [J. S. M. P. E.
fully adjusted to obtain a line of light of the proper width on the film. A light beam of greater width would result in suppressing the higher frequencies, which are essential to good articulation and brilliance in music. An opening in the aperture plate limits the length of the light beam to 0.080 inch.
The lens surfaces should at all times be kept free from oil deposits or dirt, as this materially reduces the amount of light transmitted to the film, and results in a reduction in volume. To insure smooth passage of the film by the light source, the aperture plate should be inspected frequently, and any accumulation of dirt or other foreign substance adhering to the guide surfaces should be carefully removed.
A telescoping sound gate with a self-adjusting pressure pad keeps the film in proper contact with the aperture plate when the system is in operation. The gate is arranged to be opened for threading the film by the aperture plate. A loose loop of film is allowed between the lower take-up sprocket in the projector mechanism and the point where it enters the sound gate, which prevents the irregular movement of the film in its passage through the projector mechanism from affecting the uniform velocity of the film as it passes through the sound gate. The sound gate with the lens tube in position can be seen in Fig. 3. The film, after leaving the sound gate, passes over a sound sprocket which draws the film through the gate at a uniform rate.
The film next passes over a hold-back sprocket before finally entering the take-up magazine. A slack of about two sprocket holes should be allowed in the film between the sound sprocket and the hold-back sprocket, the purpose of which is to prevent any uneven pull on the film due to the take-up mechanism from reacting on the uniform velocity of the film as it passes over the sound sprocket.
A film guide roller mounted directly above the sound gate provides an adjustable means for guiding the film edgewise through the gate and retains the sound track of the film in its proper path with respect to the center of the lens tube assembly. Improper adjustment of this guide roller may cause the light beam to intercept a portion of the film having no sound track, and in the event of the light beam intercepting the sprocket holes serious disturbances in the reproduced sound would result. Fig. 5 is a diagrammatic illustration showing the path of the film through the film compartment.
The photo-electric cell compartment houses a Western Electric photo-electric cell which converts the light beam intercepted by the