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FILM GUIDING 241
Film Guiding
Theoretically, when a film is guided in a sound recorder, it should move with absolutely uniform velocity in a fixed plane past the exposing light beam; it should be properly located with respect to dimensions in its relation to the light beam, moving without side weave. By moving in such a manner only the sound represented by the light beam variations will be recorded. If the plane of the film is not fixed, and the film moves in a direction along the optical axis, there will be an in-and-out-of -focus effect produced, since the depth of focus of the objective lens is usually small. If the plane of the film is not fixed, and the film moves in a direction perpendicular to the optical axis, small variations in distortion may occur due to the slight nonuniformity of the intensity of the exposure light beam and due to the slight nonuniformity of the sensitivity of the film. Large variations in distortion may occur if there is excessive weave, since the tops of the recorded wave peaks may be cut off when the print from the film is run through a sound projector.
As in the sound projector, the design engineer is faced with a dilemma, since the requirements for ideal film motion and the requirements for ideal film guiding are mutually contradictory in a good sound-recording machine. For accurately guiding the film with respect to the scanning beam, the film guide should be placed where the light beam impinges upon the film. For best motion, the film guide should be placed as far away as possible from the point where the light beam impinges upon the film. The result actually achieved represents the success of the chosen compromise of the design engineer; it is measured by the amount of flutter and by the amount of weave or misplacement present in the film.
35-mm sound recorders have been designed to propel film in only one direction ; 16-mm machines, which recognize the commercial need for the recording at will of sound records for either standard or nonstandard emulsion position, are designed to propel film through the machine in either direction. In machines that provide for guiding in only one direction, a guide located ahead of the recording drum represents, theoretically, a good design compromise. Inasmuch as the 16-mm film is propelled by a single row of sprocket holes, propulsion thrust occurs which acts in the manner of a lateral-pressure edge guide. In practice, this design can be relied upon to guide satisfactorily when the machine is up to speed and running smoothly ; track location may be inaccurate when the film is accelerating or decelerating. Appreciable difficulty in track