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330 J. J KUHN [J. S. M. P. E.
In the process of dubbing, it is frequently necessary to add noise effects or other sounds already recorded on a film. Where noise effects are of a more or less indiscriminate type, such as the clatter of horses' hoofs, wind noise, machine-gun fire, etc., they are usually recorded on short lengths of film, the ends of which are cemented together to form an endless loop. To accommodate such loops in the re-recording machine, a set of guide rollers is provided on top of the film magazine.
Film Passage. — The film coming from the supply reel is held in engagement with the pull-down sprocket by a double pressure roller. It then passes around an idler roller to a large drag roller.
The film then passes upward through a roller type of sound aperture to the sound sprocket, thence to the combination pull-down and hold-back sprocket. On leaving the hold-back sprocket the film passes by a roller which is associated with a signaling device and onto the take-up reel in the film magazine. Excluding the pressure roller, all sprockets, guide-rollers, etc., in contact with the film are constructed of stainless steel.
The roller type of sound aperture guides the film past the scanning lens which projects the image of a brightly illuminated slit upon it, the variations in the transmission of the film causing variations of the current through the photoelectric cell mounted behind it in accordance with the pattern on the sound track.
Light for illuminating the slit of the lens system is obtained from a special ribbon filament lamp mounted in an adjustable holder.
Drag Roller. — The purpose of the drag roller is to impart a uniform tension to the portion of the film between the drag roller and the sound sprocket. It is constructed of stainless steel and is approximately l3/4 inches in diameter. Since the uniformity of the tension is determined by the constancy of rotation of the drag roller, it is important that the latter run absolutely true. Flanges are provided on each side of the roller to guide the film through the sound aperture.
The drag roller is prevented from rotating freely by friction disks placed against the sides of the roller. These consist of a special alloy of bronze and graphite. The drag can be varied by adjusting the pressure of the compression spring on the outer face of the drag roller.
The film is held against the drag roller by means of a pressure roller. This roller is constructed of impregnated fabric, and is held under constant tension by means of a flat steel spring.