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ELECTRICAL PRINTING
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aperture, finally reaching the positive emulsion through the soundtrack negative. In 16-mm printing the same principles are employed, the only essential difference being the use of a sprocket wheel having twenty 16-mm type sprocket teeth. The linear speed in printing varies over wide ranges to as high as 180 ft/min for 35-mm film.
Since the positive film lies outside the negative, it must of necessity ride higher on the printing sprocket teeth. If we assume for the moment that the sprocket-hole pitch of the two films is identical, they will present an uneven match to the sprocket-tooth pitch. Thus, if the pitch of the developed negative should be a perfect fit to the sprocket teeth the same pitch on the positive would in this case be underpitched as compared to the sprocket teeth. This, as is evident from Fig. 1-A, would result in the positive film being driven only by
FILM UNDERSIZE
"FILM CORRECT SIZE
Fig. 1. Effect of film pitch on sprocket-tooth propulsion of film.
the leaving tooth. On the other hand, if the pitch of the positive film is a perfect match for the sprocket teeth, then an equally pitched negative will be overpitched with regard to the sprocket. This results in the negative film being driven by the entering tooth as indicated in Fig. 1-C. In both cases discussed above, the films, negative' in the first, positive in the second, are perfectly matched to the teeth and the film is driven by all the meshed teeth as in Fig. 1-B.
In both these instances slippage of one film surface against the other takes place upon disengagement of the driving sprocket tooth. This results in (a) a velocity or flutter disturbance at the sprockethole frequency (96 cycles for 35-mm, 24 cycles for 16-mm), and (b) an amplitude modulation of the signal at these respective rates. The latter effect has been studied in great detail by Crabtree.1