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FILM PERFORATION AND 96-CYCLE FREQUENCY MODULATION IN SOUND-FILM RECORDS*
J. CRABTREE AND W. HERRIOTT**
Summary. — When motion picture film is flexed around a cylinder the film in the region of the sprocket holes does not follow a smooth curve. In a sound record this leads to frequency distortion of perforation frequency.
If a length of motion picture film is bent into an arc of short radius, it is apparent from visual inspection that the film surface departs from a smooth curve in the region of the sprocket holes and assumes a polygonal profile with the angles of the polygon situated at the sprocket holes. This tendency to a polygonal profile of a curved perforated film exists even where the arc is of large radius, and is readily apparent when suitable methods of observation are used.
Fig. 1 shows the image of a parallel-line screen reflected from the emulsion surface of a length of 35-mm. sound-recording film, curved around a cylindrical form of 2-inch radius. Distortion of the line images results from distortion of the reflecting surface, the degree of distortion indicating the order of departure from the average condition. From an examination of Fig. 1 it is evident that distortion of the film extends well into the sound-track area, although the picture area in general conforms well to the radius. The widening of the linescreen pattern shown between sprocket holes in Fig. 1 is evidence of a lengthening of the radius of curvature or flattening of the film in this area. Distortion of the opposite sign, i. e., shortening of the radius of curvature, is shown adjacent to the sides of the perforation that are parallel to the direction of film travel.
That the degree of distortion increases as the radius of curvature of form decreases is seen in Fig. 2, where are shown reflected images of a series of parallel lines from film surfaces curved over cylindrical forms of different radii. It is to be noted that in some cases the distortions extend well across the sound-track areas.
* Presented at the Fall, 1937, Meeting at New York, N. Y. ; received October 14, 1937.
** Bell Telephone Laboratories, New York, N. Y.
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