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BELL SYSTEM TECHNICAL JOURNAL 



similar to that driving the feed sprocket. The mechanical filter 

 enforces uniform angular velocity of the left-hand sprocket which 

 carries the film past the line of exposure: the focussed image of the 

 valve; balancing of the flywheel which forms part of this mechanical 

 filter holds the angular velocity constant to one-tenth of one per cent, 

 despite the imperfections of the driving gears. 



Fig. 3 — Studio recording machine. 



So far we have provided a means for driving the film and a means 

 for modulating the light thereon, but we have not chosen the average 

 illumination about which the modulation is to take place. The 

 maximum exposure corresponds to the maximum opening of the 

 valve and is therefore double the average. 



Choose now the contrast to which the negative sound record is to 



