SOUND RECORDING WITH THE LIGHT VALVE 



183 



in the projector by a line of light 80 mils long, 1 mil wide, centered on 

 the striations. This gives a margin of 10 mils at each end of the 

 reproducing line, an allowance for lateral shifting of the film on the 

 sprocket teeth. 



In conclusion, let me estimate the quality of the sound record to 

 be expected. Assume that the recording lamp current has been set 

 to within 5 per cent of the theoretical optimum, the overall gamma 

 held between 0.8 and 1.2, and the final average positive transmission 

 is between 32 per cent Snd 38 per cent. Then the distortion of wave 

 form due to photographic handling is so small that the ear can not 

 distinguish the record from a theoretically perfect one. The fre- 

 quency-amplitude characteristic of the reproduced sound remains to 

 be stated. 



FREQUENCY - C.P.S. " 2 



Fig. 6 — Characteristic curves of sound recording. 



Due to the fact that the element of illumination, both in recording 

 and in reproducing, is 1 mil wide instead of infinitely narrow, the 

 final print will not reproduce the higher frequencies as efificiently as 

 the lower. For example, at the standard speed of 90 feet per minute, 

 the line of illumination covers on the film an entire cycle length of the 

 frequency of 18,000 cycles. This frequency is therefore extinguished 

 completely. The drooping characteristic resulting from this effect, 

 called the film transfer loss, may be largely offset by judicious choice 

 of electrical characteristics and by taking advantage of the mechanical 

 tuning of the light valve. 



In Fig. 6 is shown in curve A the light modulation by the valve in 

 recording for constant sound pressure of various frequencies at the 

 transmitter; in curve B the overall characteristic of the reproduction 

 in terms of electrical power delivered to the loud speaker for constant 

 sound pressure at the transmitter in the studio. Experience shows 

 that curve B is close enough to flat; the success of the record, as of 

 the picture, depends on the director. 



