Journal of the Society of Motion Picture Engineers (1930-1949)

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Variable-Area LightValve Modulator* BY LEWIS B. BROWDER WESTERN ELECTRIC COMPANY, HOLLYWOOD, CALIFORNIA Summary — A variable-area modulator is described which employs a ribbon light valve as the basic modulating element. Double-width push-pull, ' variable-area sound track or standard width dulateral sound track may be recorded at will by inserting the appropriate light valve into the modulator. The light valve is registered in place in the modulator by indexing dowels and securely locked by means of lever-controlled clamping springs. The light-valve ribbons are oriented so as to be parallel to the direction of motion of the film. The ribbon edges are projected at ten times magnification onto the film to define the amplitude co-ordinate of the recording image while the image height is determined by a narrow rectangular stop which is imaged onto the film at a 70:1 reduction in height by a cylindrical lens system. The modulator is a completely self-contained unit embodying the basic components for the recording optical system, an optical system for rear projecting an enlarged image of the ribbon aperture onto a viewing screen, a photoelectric monitoring system, and an exposure meter. INTRODUCTION THE WESTERN ELECTRIC variable-area sound-on-film recorder to be described represents a reduction to motion picture studio practice of the recording apparatus employed in the stereophonicsound-film system demonstrated by the Bell Telephone Laboratories before the Society in 194 1.1 In particular, the ribbon or light-valve type of modulator and the basic recording optical arrangement as used in that system have been carried over into the new recorder. As used in the variable-area modulator, the light-valve ribbons are supported so that their edges are parallel to the direction of motion of the film. The ribbons thus serve to define the vertical edges of an illuminated aperture, an enlarged image of which is projected onto the sensitive surface of the film. These aperture edges move toward and away from each other in response to speech and noise-reduction currents to vary the lateral extent of the illuminated image on the film. The height of the recording image in the direction of film travel is fixed, being defined by a horizontal slit located on the * Presented May 17, 1948, at the SMPE Convention in Santa Monica. NOVEMBER, 1948 JOURNAL OF THE SMPE VOLUME 51 521