SOUND RECORDING WITH THE LIGHT VALVE 



175 



plished. The natural frequency of the valve is set by adjusting the 

 tension applied by the pulley B; for reasons which involve many 

 considerations the valve is tuned to 7,000 cycles per second. Under 

 these circumstances about 10 milliwatts of A.C. power are required 

 for full modulation at a frequency remote from resonance; about one 

 one-hundredth of this power at the resonant frequency. The im- 

 pedance of the valve with protecting fuse is about 12 ohms. 



PLANE OF 

 RIBBON OF 



LIGHT 



PLANE OF 

 VALVE 

 RIBBONS 

 ('0.002" X 0.256"' 

 ^ SLIT 



PLANE OF 



IMAGE ON 



FILM 



('0.001" X 0.I28"^ 



^ IMAGE J 



CONDENSING 

 LENS SYSTEM 



OBJECTIVE 

 LENS SYSTEM 



Fig. 2 — Diagram of the optical system for studio recording. 



If this appliance is interposed between a light source and a photo- 

 graphic film we have a camera shutter of unconventional design. 

 Fig. 2 shows a diagram of the optical system for studio recording. 

 At the left is a light source, a ribbon filament 18 ampere projection 

 lamp, which is focussed on the plane of the valve. The light passed 

 by the valve is then focussed with a 2 to 1 reduction on the photo- 

 graphic film at the right. A simple achromat is used to form the 

 image of the filament at the valve plane, but a more complicated 

 lens, designed to exacting specifications by Bausch and Lomb, is 

 required for focussing the valve on the film. The undisturbed valve 

 opening appears on the film as a line 1 mil by 128 mils, its length at 

 right angles to the direction of film travel. The width of this line 

 varies with the sound currents supplied to the valve, so that the film 

 receives a varying exposure: light of fixed specific intensity through a 

 varying slit. 



Fig. 3 shows a studio recording" machine with the door of the exposure 

 chamber open. In this machine the film travels at 90 feet per minute, 

 and the sound track is made at the edge away from the observer. 

 The line of light, the image of the valve, overruns the perforations 

 by 6 mils, extending toward the center of the film 122 mils inside the 

 perforation line. The right-hand sprocket serves to draw film from 

 the feed magazine above and to feed it to the take-up magazine below; 

 this sprocket is driven from the motor shaft through a worm and 

 worm-wheel. The left-hand sprocket engages 20 perforations and is 

 driven through a mechanical filter from a worm and worm-wheel 



