Sound motion pictures : from the laboratory to their presentation (1929)

Record Details:

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42 SOUND MOTION PICTURES The film and the needle for playing the record are each set at starting points determined by marks made prior to recording the sound picture. The motor is then started and the film and the record are operated, in mechanical synchronization. The mechanical vibrations of the needle, as in the talking machine, are transformed into minute electric currents which are then amplified and finally transformed back into sound by the horns behind the screen. The Movietone, or sound-on-film system, is photo- graphed by means of light variations on the side of the film. The film sound record, as indicated, consists of a ladder-like band of microscopic lines or striations running along one side of the film. This sound track is i.io inches wide. The process consists in photographing variations in light intensity on the film. Differences or changes in inten- sity of sound are represented by differences in the density of light and shade, while pitch is represented by the num- ber of changes from dark to light and back again in a given length of track. The sounds to be recorded are picked up by microphones which convert sound vibrations into electrical vibrations. These electrical vibrations are am- plified and in turn vary the intensity of a recording light. The sound record is converted into a corresponding electric current by arranging that a narrow, high-intensity beam of light shall pass through it and fall upon a photo- electric cell. The arrangement is shown in Figure 2, page 43. The light from the bright filament of the exciting lamp is focussed as a very narrow line upon the film by passing through a system of lenses and aperture plates. This recording light is contained in a tube that is inserted in the back of a camera in such a way that the variations in light intensity fall directly upon a narrow edge of the negative film on which the motion picture is also simul- taneously being recorded. Aside from the fact that the