International projectionist (Jan-Dec 1955)

times, or 1728 degrees. During this time the shutter will expose the film frame 8 times. When the driving gear has turned 144 degrees, the yoke will shift the cam to the right so that the left pin will engage the star and move the film to the next frame. Then the cam will shift back to neutral again and remain there for another period of time. This time the cam will revolve more than 7 times, or 2592 degrees, and the shutter will expose the film frame 12 times. Thus the film frames are exposed alternately, one frame 8 times and the next 12 times, while the film passes through the projector at the rate of only 6 frames per second. The twowing shutter revolves at 1800 r.p.m. giving the 60 exposures per second for television. Chart 2A is a condensed form of chart 2 and shows 8 frames of film timed at 2-3. Chart 2-B shows that of slow-motion projection of only 2 frames which coincide exactly with that of the 8 frames above, each of LOOP_ which represent screen. Ys second on the Speed, Flicker, Motion With the high-speed shutter there is no flicker, and the image shift UPPER FEED SPROCKET "FILM SHOE 'SPONGE RUBBER PAD STANDARD FILM GATE INTERMITTENT SPROCKET .FILM SHOE FiGURE 4 from frame to frame is very smooth on normal subject action, no jerk or jump is noticeable. If the subject action is fast, such as a car traveling broadside past the camera at high speed, then there is naturally some noticeable image shift. This slow-motion principle need not necessarily employ the high-speed Geneva cam but can be worked out at standard cam speed of 1440 r.p.m., which of course calls for wider shutter blades. It can also be attained with the cam pins at 180 degrees and at various cam and shutter speeds; also at any desired number of frames per-second projection speed. Thus the slow-motion principle is quite flexible. The use of this would apply to education or industry where it is desirable to retard normal screen action for close examination. As mentioned previously, the life test on a loop of film was more than 4,000 runs. This long film life was obtained by the use of a special filmbraking system which allowed the film gate tension to be reduced to a minimum, with only sufficient pressure to keep the film flat at the aperture. A sponge rubber pad is placed within the upper loop (Fig. 4). At the beginning of pull-down the loop is full and does not touch the rubber pad. With very little tension at the gate, there is a minimum of strain on the film during the acceleration period. As the film slows down and comes near the end of pull-down, the upper loop comes in contact with the sponge rubber pad which applies the braking action to prevent the film from overshooting the aperture. The braking action of the rubber pad acts in two ways: first, by friction on the surface of the film; and second, by pressure within the entire (Continued on page 24) PICTORIAL REPRESENTATION OF MOTIOGRAPH PROJECTOR PROGRESS THROUGH THE YEARS Motiograph was ready for sound with the Model "H" in 1926. This model had the first double shutter, and introduced Monograph's sound system which reproduced sound on film and Vitaphone records. Model F (1921) enclosed mechanism for first time; shutter had two blades and a timing device for increased operating efficiency. Monograph's postwar projector; the Model AA, latest of the line, was brought out in 1946. It embodied radical improvements in performance and operation. It is shown here on an S base with 25 inch magazines. 10 INTERNATIONAL PROJECTIONIST • FEBRUARY 1955