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July, 1939] MOTION PICTURE FILM AND TELEVISION
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(2) The intermittent mechanism is designed for the three-to-two ratio of pull-down periods required in using 24-frame film for 30-frame television.
(5) A special synchronous driving motor is used to assure that the projector mechanism always "locks-in" in proper time relation with the synchronizing pulses.
(4) An additional film gate with light-source and photoelectric cell is included near the picture gate for deriving a control potential which varies with the average density of the film.
In the projector shown in Fig. 5, it was impracticable to locate the shutter between the light-source and the film. The shutter was, therefore, mounted just beyond the projection lens. Sufficient clearance between the shutter and lens was provided to permit limited movement of the lens for focusing. The time during which the image
VIDEO AMPLIFIER
LINE AMPLIFIER
DEFLECTION AMPLIFIERS
POWER SUPPLY RECTIFIERS
TO TRANSMITTER
FIG. 8.
Essential elements of a television film transmission system.
may be projected, onto the photoemissive mosaic of the Iconoscope is limited to the vertical return time of the scanning beam. With present television standards this is not more than 10% of Veo second or Veoo second.
In order to make efficient use of the projection lens, it is necessary that the aperture in the shutter be at least as wide as the diameter of the lens. A large-diameter shutter (23 inches) is necessary to meet this requirement. This shutter rotates at 3600 rpm and has a peripheral speed of approximately 4y4 miles per minute. The shutter is enclosed in the circular housing shown at the extreme right-hand side of Fig. 5. In the shutter housing opposite the projection lens is a window through which the picture is projected. The shutter disk is made of two overlapping sections of thin metal. These two sections can be rotated with respect to each other through a small angle in