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SOUND REDUCTION PRINTING 317
flutter' to the sprocket wheels and, thirdly, the pitch and size of the sprocket teeth may not be accurately computed to accommodate films which may be old or shrunk, The most objectionable error in this system is that a momentary increase in the speed of the 35-mm film can occur when the speed of the 16-mm film is momentarily retarded this condition would be caused if inaccuracies in the 35-mm worm gear forced this train forward at a speed higher than normal and, at the same instant, similar inaccuracies in the 16-mm worm gear retarded that train to a speed below normal.
The arrangement shown in sketch 'B', Figure 139, is favoured by many designers since variations in gearing will affect both the 35-mm and the 16-mm sprockets in a similar direction at any given instant. In this arrangement drive from the main motor is transmitted by a single worm gear to a common shaft upon which both 16-mm and 35-mm sprockets may be mounted. Obviously, any variation in the speed at which this shaft rotates will cause equal changes in the angular velocity of both sprocket wheels. Such variations would, therefore, only cause changes in exposure and not optical slippage between the negative and the relative position of the reduced image on the 16-mm film. However, it must be remembered that any changes in the relative positions of the sprocket teeth, due to inaccurate workmanship, would cause optical slippage although it would probably be less noticeable in a machine of this type than if it occurred in a machine of the type shown in design 'A'.
It will be realised that design 'A' has the great advantage that the optical layout will be in one straight line path whereas, in design 'B', it is necessary to transfer the image from the 35-mm film to the 16-mm film by a path as indicated by the 'arrowed' line. Because of this attention was turned to the possibility of modifying design 'A' to eliminate some of the possible sources of film speed variation.
One of the most successful systems designed to impart a constant speed to film was originally used by the R.C.A. Company and developed as a drive for their 35-mm sound recording equipment. Until now we have considered it necessary for the film to be supported on a rotating sprocket wheel as it passes the scanning point and, in consequence, we have been forced to recognise several sources of error which may be attributed to this method of traction. Quite obviously, passing the film over a smooth drum would eliminate any errors present in the sprocket itself and, if such a drum were perfectly concentric with an accurately mounted shaft, it should be possible to cause the film to travel at a speed equal to that of the drum surface providing some means could be employed to