SOUND RECORDING WITH THE LIGHT VALVE 111 



be developed and draw the Hurter and Driffield curve for this contrast 

 for the emulsion chosen for the negative sound record. The maximum 

 exposure should correspond to the beginning of over-exposure, the 

 average should be half this. The Hurter and Driffield curve will give 

 the density of the over-exposure point for the chosen contrast and the 

 density for half this exposure. Let the machine be run to expose 

 film to light through the unmodulated valve for several values of 

 the lamp current. Develop the film and measure the densities due 

 to the various values of lamp current. Select, by interpolation if 

 necessary, the lamp current which corresponds to half o\'er-exposure. 

 With this current in the lamp the machine is ready to make a sound 

 record, since the focussing of the valve has already been done and 

 manufacturing specifications insure that the line of illumination shall 

 lie, within 3 minutes of arc, at right angles to the direction of film 

 travel. 



Consider at this point the procedure in the recording studio. Adding 

 sound to the picture introduces no complication of technique other 

 than to require sufficient rehearsing to make sure of satisfactory 

 pick-up of the sound: microphone placement must be established and 

 amplifiers adjusted to feed the light valve currents which just drive 

 it to the edge of overload in the fortissimo passages of music or the 

 loudest utterances of speakers. 



In Fig. 3 the photograph shows a photoelectric cell mounted 

 inside the left-hand sprocket, which carries the film past the line of 

 exposure. Fresh film transmits some 4 per cent of the light falling 

 on it, and modulation of this light during the record is appreciated by 

 the cell inside the sprocket. This cell is connected to a preliminary 

 amplifier mounted below the exposure chamber, and with suitable 

 further amplification the operator may hear from the loud speaker the 

 record as it is actually being shot on the film. Full modulation of 

 the valve implies complete closing of the slit by one side of the wave 

 of current; this modulation should not be exceeded or photographic 

 overload will abound. 



One or more cameras and one or more sound recording machines 

 are driven by motors electrically synchronized from a common 

 distributor. Speed control and synchronization of these motors are 

 described in Mr. Stoller's paper. At the beginning of the day's work 

 a check is made of the operation of the driving motors, and the tuning- 

 and-spacing of the valves is verified. 



Fig. 4 is a schematic diagram of the studio equipment for sound 

 recording. Provision is made for combining if desired the contri- 

 butions of several microphones on the set. This combination is 



