The RCA Recording System and its Adaptation to Various Types of Sound-Track*

Abstract

The photographic recording of sound is accomplished by modulating a narrow beam of light and projecting it upon a strip of moving film. There are three ways in which the amount of exposing light may be varied. A light-beam of fixed dimensions may have its intensity varied; a beam of constant intensity and length may have its width varied; or a beam of constant intensity and width may have its length varied. The first two types of modulation produce variable-density soundtracks, while the third type produces variable-width tracks. — The recording optical system can be made to modulate either the intensity or the length of the light-beam. The unit consists essentially of an incandescent lamp to produce the light, a system of lenses to direct the light, an aperture and slit to limit the light, and a reflecting mirror galvanometer to modulate the light. A magnetic shutter for ground-noise reduction is also part of the standard variable-width recording unit. A system of mirrors and lenses intercepts a small portion of the recording light and projects it upon an external card. This system magnifies the deflection of the galvanometer and shutter to such extent that the degree of modulation and the zero settings can be observed easily by the unaided eye. — Many different types of sound-track can be made with the recording optical system without sacrificing any of its advantages as a light modulator. By the use of the appropriate condenser and aperture assembly, the system will record standard bilateral variable-width track, standard variable-density track, push-pull class B variable-width track, push-pull class A variable-density track, and push-pull class A variable-width track. The manner in which each of these systems functions is shown and described in detail.