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374 CINEMATOGRAPHIC ANNUAL
sity records, and the transmission of the positive print at every point should be proportional to the exposure of the negative at the corresponding point. If that can be accomplished, then we deliver to the photoelectric cell a light the same as it would receive had there been no record interposed.
Fig. 3 shows a studio recording machine with the door of the exposure chamber open. In this machine the film travels at 90 feet per minute, and the sound track is made at the edge away from the observer. The line of light, the image of the valve, overruns the perforations by 6 mils, extending toward the center of the film 122 mils inside of the perforation line. The righthand sprocket serves to draw film from the feed magazine above and to feed it to the take-up magazine below; this sprocket is driven from the motor shaft through a worm and worm-wheel. The left-hand sprocket engages 20 perforations and is driven through a mechanical filter from a worm and worm-wheel 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.
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.
Fig. U is a skeleton diagram of the studio recording channel, showing the recording amplifiers and the direct and photocell monitoring circuits.
It is my purpose here to describe the procedure necessary to render the film as nearly perfect as possible, and produce a satisfactory delay circuit. We ask of the film or any other recording device that it should take the current fed to it and reproduce that