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PRINCIPLES OF CINEMATOGRAPHY
PUSH-PULL PHOTOCELL WITH TWO CATHODES t TWO ANODES
SINGLE PRISM E. TWO CYLINDRICAL LENSES
ATHODES
DOUBLE PRISM /CONDENSER LENS 'CYLINDRICAL LENS
PUSH-PULL SOUND TRACK
NOTE^ OUTSIDE HALF OF TRACK IS REPRODUCED BY LOWER ANOOE t CATHODE ; INNER HALF BY UPPER ANODE € CATHOOE
Fig. 121. — Push-pull sound reproduction.
sets of electrodes, as seen in Figure 121. It will be noticed that, by using specially designed optical components, it is possible to feed each half of the sound track to its respective section in the photocell. As explained previously, the current from the cell is fed to two valves, coupled in parallel, so that more than double the volume range previously obtained by a single cell and sound track will be reproduced.
Film Drive at the Scanning Point
Obviously, the film must be driven past the light beam at a perfectly uniform speed to ensure that the pitch of the reproduced sounds is free from variations or 'wowing'. It must be remembered that the photo-cells previously described are able to respond to changes in light intensity as rapid as 9,000 times every second. The film speed past the scanning point is 18-inches (45-7-cms) per second; a variation in speed of 1% would cause a constant 6,000 cycle frequency on the sound track to be reproduced with a rise and fall between 6,060 and 5,940 cycles per second, and would be detected by the audience.
One of the many arrangements which have been devised in order to maintain a constant film speed past the scanning point is seen in Figure 122. The left-hand diagram shows a side elevation of that part of the projector mechanism between the picture gate and the final take-up spool. As we have seen previously, the film is drawn intermittently through the picture gate by a sprocket wheel solidly connected to a Maltese Cross mechanism. The film is then formed into a free loop between the intermittent sprocket