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SOUND REDUCTION PRINTING 331
size. Film passes from the supply spool 'A' over a constant speed feeding sprocket 'B' and is then caused to assume the curvature of the stationary gate 'K' by means of four rollers, two of which are mounted above and below the gate and are indicated at positions *H' and 'J'; the remaining pair being mounted to pivot about the centres at 'F' and 'G' and so to accommodate the free loops between the gate and the feeding and hold-back sprockets. The film then passes via the lower hold-back sprocket 'D' to the take-up spool *E\
The counterweight applied to rollers 'F' and 'G' may either be of a positive or negative form. At position 'F' it is assumed that the counter-weight will cause the roller arm to pivot about point 'L' and so increase the effective weight of the roller on the film that is, the counter-weight will be added to the weight of the guide roller itself. At position 'G' the counterweight will cause rotation of the roller arm about position 'M' and will serve to increase the effective tension on the film between the lower gate roller 'J' and the hold-back sprocket 'D\
It will be realised that the radius of curvature of the stationary gate *K' must be slightly greater than that of the central sprocket 'C so that the sprocket teeth just project through the slot cut in the gate channel. Since the film is therefore only driven through the exposing gate by teeth engaged with one set of perforations it is necessary to apply some form of edge-guides to position the film laterally and to prevent 'film weave' or movement across the exposing aperture. This is usually accomplished by applying a spring loaded shoe to one side of the film and locating the opposite side of the film against an adjustable stationary guide. This adjustable guide may be moved into or away from the film channel by rotating a calibrated hand wheel controlling a gear mechanism used to locate the guide shoe. This arrangement makes it possible to place any sound track centrally with respect to the printing aperture, regardless of the fact that it may have been originally recorded some distance from the standard position as specified by the Society of Motion Picture Engineers.
It is therefore seen that the film is held under considerable side tension as it passes through the gate but only under very slight longitudinal tension, controlled by the weighted rollers 'F' and *G' to be just sufficient to cause it to remain in engagement with the sprocket teeth. Those who have had experience with machines in which film passes over a stationary surface will appreciate that the continual passage of millions of feet of film will cause considerable wear to the surfaces and, inevitably, scratches will