Sound Recording with the Light Valve ^ 



By DONALD MACKENZIE 



Synopsis: 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. 



THE sound records I am about to describe are of the variable 

 density type, and the method of making them is that developed 

 by Bell Telephone Laboratories. 



It is not difficult to specify the requirements of this type of sound 

 film. So far as possible the exposure of the negative must be kept 

 within the straight line portion of the Hurter and Driffield curve for 

 the emulsion chosen, and the print must be timed with the same 

 restriction. The development of the negative and of the print must 

 result in a positive where the transmission of each element of length 

 is proportional to the exposure of the corresponding element of the 

 negative. The light modulator must be supplied with undistorted 

 power from the recording microphone and amplifier. When the 

 positive is projected, the striations of the sound track must be enabled 

 to modulate the illumination of a photo-sensitive cell to retranslate 

 the photographic effect into electrical current which shall be a fair 

 copy of the microphone current generated by the original sound. 

 From this point on the problem is the familiar one of sound re- 

 enforcement, the film and cell having taken the places of the sound 

 source and microphone. 



Fig. 1 shows a photograph of the light valve, invented in 1922 by 



E. C. Wente of the Bell Telephone Laboratories. Essentially, it 



consists of a loop of duralumin tape suspended in a plane at right 



1 Presented before Society of Motion Picture Engineers at Lake Placid, New 

 York, September 25, 1928. 



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