SOUND RECORDING WITH THE LIGHT VALVE 181 



for a given reproducing light source, varies directly with the average 

 transmission and the per cent modulation of this average. This 

 average density should be on the straight line portion of the positive 

 Hurter and Driffield curve, far enough to keep the denser negative 

 portions from reaching the under-exposure region. For Eastman 

 positive film a suitable transmission of the unmodulated portion of 

 the sound print is 35 per cent, referred to air, for the usual values of 

 positive gamma: 1.4 to 1.8. At this average transmission only the 

 peaks of the recorded sound will encroach on the region of under- 

 exposure. For the reciprocally developed negative track the region 

 of under-exposure will have been reached by occasional peaks on the 

 other side of the wave, and such photographic distortion as exists will 

 be balanced between positive and negative. 



Here we appropriately consider the photographic distortion as it 

 occurs in variable density records. If the entire negative exposure 

 has been confined to the under-exposure region of the emulsion chosen, 

 a huskiness w^ill result in the reproduction which can not be corrected 

 by any known technique. But if the unmodulated negative trans- 

 mission, for a gamma of 0.55, is about 16 per cent referred to air, 

 90 per cent of the wave will be clear of under-exposure, and experience 

 shows that the ear detects no distortion. In telephonic terms, 

 everything at a level 1 TU below full modulation will be free from 

 distortion, and the peaks will be substantially perfect. The same 

 may be said of the positive printed to an average transmission of 

 35 per cent, provided the o\'erall gamma approximates unity. 



It has been calculated that if the overall gamma departs from 

 unity by 0.2 in either direction, a harmonic of 5 per cent amplitude 

 of the fundamental will be introduced. Experimentation has shown 

 that a 5 per cent harmonic is the least detectible. We state then the 

 tolerance on the overall gamma for the sound track as 0.8 to 1.2. 

 Variation of corresponding amount in the contrast of a picture print 

 is intolerable; therefore greater latitude in contrast is permissible in 

 the sound record than could be tolerated in the accompanying picture. 



In printing these sound negatives in combination with pictures for 

 projection in the theater, it is customary at the present time to print 

 one negative, masking the space needed for the other, then run the 

 positive again through the printer with the other negative, masking 

 now the space already printed. In printing the picture negative, 

 light changes are made as usual; for the sound negative the light is 

 regulated to result in 35 per cent transmission of the unmodulated 

 track after positive development. Provision of suitable masks in the 

 camera has been made to show in the finder and expose on the film 

 only the portion which will be available for picture projection. 



