SOUND RECORDING WITH THE LIGHT VALVE 179 



circuit, the output of the recording amplifier is applied to the light 

 valves and the monitoring horns are connected to the photo cell 

 amplifiers on the recording machines. With no film in the machine 

 and at a convenient lamp current a complete rehearsal is made to 

 verify the operation of the valves at the proper level. Film is then 

 loaded, cameras and sound recorders are interlocked and starting 

 marks made on all films by punches or light flashes. 



A light signal from the recording room warns the studio, which 

 after lighting up signals back its readiness to start. The machine 

 operator starts the cameras and sound recorders, brings up the lamp 

 current to the proper value, and when the machines are up to speed 

 signals the studio to start. During the recording, the mixer operator 

 monitors the record through the light valves, thereby assuring himself 

 that no record is lost. 



In the choice of emulsion for the sound negative, the usual designa- 

 tion of speed may be disregarded, because it is desired to make the 

 exposure of the unmodulated track many times the under-exposure 

 of the emulsion used. The advantages of positive emulsion for the 

 sound negative have come to be generally recognized; positive has 

 been used by Bell Telephone Laboratories since 1924. The scale of 

 Eastman positive film is about 20 to 1 ; we adjust the recording lamp 

 current to give an illumination on the film for the unmodulated track 

 of 10 times the under-exposure. After one lamp has been calibrated 

 as described before it may be replaced when necessary by another 

 in which the wattage in the ribbon filament is the same; the light 

 emission is very closely correlated with the wattage. Where the 

 unmodulated or average exposure is ten times the under-exposure 

 minimum, 90 per cent modulation of the light can be permitted with- 

 out running into under-exposure on the faint side of the wave. For 

 sound currents reaching 100 per cent modulation of the light, 90 

 per cent of the wave is free from distortion; if the average light were 

 halved, still 80 per cent would be free from distortion. There is 

 therefore considerable latitude in the average exposure, and the 

 negative is satisfactory if the transmission of the unmodulated track 

 lies between fairly wide limits. 



The choice of the negative sound gamma is determined by the 

 practice of the laboratory in regard to picture development. It is 

 usual to see on the screen pictures whose overall gamma considerably 

 exceeds unity. On the sound track the overall gamma should equal 

 unity, and the development of sound negatives should be uniform, 

 though that of picture negatives is left to the judgment of the finisher. 



Theoretically, it should be immaterial what combination of reciprocal 

 values is chosen for the negative and positive sound gammas. Prac- 



