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MODERN PROJECTION
PROJECTION • SOUND REPRODUCTION
ACOUSTICS
The Soundhead of the New Western Electric Sound System
O Examining the operating principles and design of the pickup mechanism of equipment just introduced for theatres, in the first of two articles
the NEW Western Electric standard reproducer set ("Mirrophonic") contains but two moving assemblies. Chains and couplings have been completely eliminated. There are no sprockets. There is no gear noise because there are no gears. The film glides over two simple rollers. The film speed control mechanism is equipped with an inertia or flywheel, controlled scanning drum, called a kinetic scanner. It is mounted on precision ball bearings to reduce friction to a minimum and is housed in a single compartment frame casting (See Figure 1).
Extra precautions have been taken to seal the ball bearings effectively against the entrance of dust, since this would interfere with the smooth rotation of the scanning drum and would introduce disturbances in the reproduced sound. The
construction is such that the lubrication is entirely automatic. The kinetic scanner and its associated parts are accurately balanced to insure uniform travel of the film past the scanning point and to prevent the film from weaving in and out of its proper focal plane.
Constant speed of the scanning drum is maintained by means of the specially designed double flywheel of the kinetic scanner. One flywheel and the scanning drum are solidly locked together by a common shaft, preventing high frequency oscillations or extremely rapid changes in film speed. The other flywheel is free-floating and is coupled to the first one by means of a mechanical filter which suppresses slow oscillations.
The kinetic scanner is so stabilized or steadied that a sudden change in the film
Figure. I. The kinetic scanner. The disc-like flywheel connected to the scanning roller, and the deflecting prism, are shown at center. See text.
Figure. 2. The film bath. This picture indicates the method of threading the kinetic scanner. Separate exciter lamp housing is shown at extreme left. Note focus screws on lens tube casting.
speed or a disturbance in the film loop, such as the passage of a film splice, has no appreciable effect on its unformity of rotation. During the acceleration period of the scanning drum, the two flywheels are coupled together by means of a centrifugal clutch. This development was made through the application of special test equipment by means of which minute variations in film speed may be accurately determined, as a percentage, based on the entire absence of velocity irregularities. The new test equipment is known as a flutter bridge.
The film is threaded into the projector mechanism (See Figure 2) in the usual manner up to the intermittent sprocket, from which it passes directly through a highly polished and hardened film chute into the sound reproducer, which fits between the projector mechanism and the lower magazine. Here it is accurately guided by a pivoted guide roller onto the scanning drum of the kinetic scanner, around which the film is wrapped. From this point it passes up to the lower feed sprocket in the projector mechanism and thence down to the lower film magazine. The extremely low friction of the kinetic
September 19, 1936
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