American cinematographer (Jan-Dec 1934)

114 American Cinematographer • July 1934 Sound-on-Film Technique Recording for16nnm. by Dr.-lng. Hans Atorf Research Engineer, Tobis-Klangfillm, G.m.b.H., Berlin THE long-desired substandard sound-on-film talking picture, to serve as a companion to theatrical films, but for Industrial, Educational and Personal uses, Is at last materializing. In Germany, as elsewhere, the most popular substandard film size is 16mm. wide, with perforations on both edges. This format is retained for sound-on-film use; but by perforating only one side, we hove left an area, hitherto utilized by the perforations, suitable for carrying a sound-track 1 .65mm. wide. This immediately offers an improvement over professional sound-on-film practice, which could not sacrifice the perforations, and was forced to diminish the picture-area to accommodate the sound-track. In the professional standard, it will be recalled, the film is 35mm. wide overall, with 0 sound-track 2.45mm. in width at one side of the picture, between it and the perforations. Both of the methods professionally used for sound-onfilm recording may be used for substandard recording. Both the Variable Area and the Variable Density types have been tried, but in Germany the Variable Density method has been given preference. (Note: This is exactly opposite to American practice, where Variable Area recording dominates the 16mm. field. Ed.) With either type of recording, the film-speed used in substandard practice is lower than is the case with standard film, being only 18.3 cm. per second against 45.6 cm. per second with 35mm. As a result, the striotions comprising the recorded sound waves are placed much closer together than in the larger size. A note of 437 cycles per second ("A") for instance, recorded on standard film gives a wave length (or separation between alternate dark or light striotions) of: 45.6 437 cm. 1.05 mm. while the same sound recorded on 16mm. has a wave length of only : 8.3 == .42 437 cm. ■■'■Translation from "Filmtcchnik," Berlin. 2000 *eoo tooo sooo Fig. 1: Influence of slit width. Curve 1; slit width of recorder, .0023m. Curve 2; slit width of recorder and of projector, .0023m. Curve 3; slit width of recorder, 10023m., of projector, .0030. fi-vcl sifi/er'cr ^g/j i.e _ Huryt fi" normal film o ■' Schn^rx I f' t ^ Fig. 2: Influence of Deviation of the slit from a straight position. •rO% too fooo fnCO ^^r/z Fig. 3: Revolving power of film material relative to frequency range. To express it more simply, the distance between two striotions recorded on 16 mm. film is the same for 8000 cycles OS it is on 35 mm. film for 2000 cycles. This naturally results in a considerable series of difficulties. Both recording and reproduction of sound-on-film reccords is done through a slit of definite width. With increasing frequencies, the slit width causes a linear distortion of the recorded amplitude, in such a way that, eventually, the slit width may become equal to the "wavelength" between two striotions, thereby cancelling out the sound impulse; in such a case, if recording, no sound would record, or in reproducing, nothing would come from the loud-speakers. For example, if the slit width in a recorder were equal to .0023mm., a frequency of 8000 cycles could not be recorded. (See Fig. 1, Curve 1.) For both optical and physical reasons, it is impossible to use a slit smaller than .001 to .0012mm., and even such a Continued on Page 1 19