Sound Recording with the Light Valve

The light valve developed by Bell Telephone Laboratories is an electromagnetic shutter consisting of a loop of duralumin tape formed into a slit at right angles to a magnetic field. Sound currents from the microphone and amplifier flow in this loop causing it to open and close in accordance with the current variations. The slit is focussed by a lens on the sound negative film. An incandescent ribbon filament is focussed on the light valve, and the light passed by the undisturbed slit appears on the film as a line at right angles to the direction of the film travel. As the valve aperture is modulated by sound currents, the film receives a varying exposure and a sound record of the variable density type is obtained. For talking pictures such a sound film is made on a separate recording machine synchronized with the camera and is printed alongside the picture on the finished positive. The prints are displaced so that the sound is advanced over the corresponding picture. This is in order that the sound may be projected at a point of continuous film motion below the picture gate.