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SOUND REDUCTION PRINTING 325
tooth immediately contact is made with that tooth under these conditions the film will always travel forward at a constant speed, namely the 'slip back' speed and, therefore, at a speed slower than that of the sprocket itself.
It may be thought that this last arrangement would produce a solution to the problem, but it must be remembered that the chances of obtaining a film which obeyed these conditions precisely are exceptionally remote and although both this arrangement and that in which the film is of a pitch exactly equal to that of the sprocket, each produce a constant speed, they can only be considered as theoretical examples and would only be met with in practice on very rare occasions. The practical performance of any sprocket and film must inevitably be lower than the theoretical standard since, amongst other considerations, the shape of the sprocket teeth will only be absolutely correct for a very short period during their useful life and, even under such conditions, would only operate at maximum efficiency if their working faces were glass-smooth and accurately reproduced from one tooth to the next.
The foregoing considerations have merely shown that, even under ideal conditions, it is not practical to assume that perfectly uniform film motion may be obtained at the arc of contact between the film and its driving sprocket. However, this conclusion does not necessarily indicate that those errors which remain will be of sufficient magnitude to materially affect the reproduction of the final sound track whether it is produced by a direct recording or by reduction printing. Obviously, the variations in film speed will be less in a sound recording instrument using raw film stock than they will be in a sound reduction printer using a processed negative film. The normal variations from true pitch which are likely to be met with in original recording are of the order of 0-5% (this figure includes the variations which may exist in the original perforating and those which may occur if the film is stored for some time under adverse conditions). Under these conditions any change in the amount of film moved forward by a perfect and imperfect engagement between perforations and sprocket teeth would be a little over 0-001 -inch per engagement. Since 35-mm film normally moves at an average speed of 18inches per second, and assuming a 9,000-cycle note were recorded, this change in constant speed would be equivalent to one half cycle at that frequency and would only cause a very slight flutter in the reproduction.
It will be apparent that, since a duplicate negative also includes