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318 PRINCIPLES OF CINEMATOGRAPHY
ensure that the film did not slip relative to the drum. The first requirement that the drum shall remain true or concentric merely calls for good workmanship and careful fitting. The second requirement that the film must travel on the drum without slipping can be ensured by using a drum with a somewhat dull finish and by applying pressure to the film at the beginning and end of the contact path between it and the drum.
Assuming that such a drum can be made it is, of course, still necessary to feed the film onto and away from this drum by passing it over sprocket wheels. Quite apart from the fact that such sprockets would isolate the drum from the feed and takeup spooltensions, it is obviously the number of perforations which pass a given point every second, and not the linear feet of film, which must be equal in both the sound and picture negatives if synchronism is to be maintained. At first it may be difficult to realise that a considerable difference can exist between these two factors but, when it is remembered that a film shrinkage of only \ of 1 % would cause a reduction of 5-feet in a 1,000-ft roll of 35-mm film, it is obvious that the more fixed reference base, namely that four perforations regardless of their actual pitch are provided for each 35-mm picture height, is the controlling factor. Since constant speed is to be obtained by passing the film over a smooth drum, it must be threaded with free loops between the sprockets and this drum to provide reservoirs of film from which the drum may draw a constant supply.
Since, under these conditions, the sprocket speed is fixed and will only mesh perfectly with one particular film of known shrinkage, the drum speed must be variable since it is required to rotate at a speed not dependent upon the sprockets but upon the film itself and, more particularly, upon the condition of shrinkage within any given film. The drum must not be positively driven by gears, belts or similar mechanisms unless some means is provided to vary the ratio between the drum and the sprocket speeds. This ratio must be capable of continuous variation and must be automatically controlled by the film itself preferably by the changes in loop length between the sprockets and drum which would take place as the sprocket teeth or gear inaccuracies came into play.
Magnetic Drum Drives
Although the simplest method of driving such a drum is by allowing the film to act as a belt and automatically cause rotation when the actual speed of rotation can then be modified by an idler roller bearing on a loop between the drum and sprockets