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June, 1946
FILM-DRIVE FILTER MECHANISM
457
(c) A value of compliance largely independent of the varying quality or bent condition of the film,2
(d) Firm contact between film and drum.
Problems in the utilization of considerable film tension without a corresponding reduction in loop compliance have been overcome by the design of units which produce low cutoff frequencies with flywheels of reasonable size. The drum and flywheel are propelled by film which is deflected from a straight path by a spring-tensioned idler arm and roller with which a damper is associated. This tensioned path of film
SOFT ROLLER
R2
FLYWHEEL AND MAGNETIC DRAG
SOUND DRUM AND FLYWHEEL
SOUND SPROCKET
•INTERNAL FILTER CIRCUIT
FIG. 1. Single-arm filter.
together with the idler arm and roller constitutes a compliance as in the case of the 5-shaped loop and will be referred to as a film loop although the two appear considerably different.
Since substantially equal amounts of tension in some form must exist at either side of the translation point, tension must be introduced to offset that of the lower film loop. Two methods have been developed to accomplish this for 2 different types of application. The first type utilizes the eddy current drag created by the use of permanent magnets mounted near the flywheel periphery. This will be referred to as the single-arm type. The second type utilizes an idler arm and roller on the incoming film loop similar to that in the outgoing film loop. This will be referred to as the double-arm type.