International projectionist (Jan-Dec 1949)

r^i A a PULL DOWN CLAW film. Since motion picture technique has already established a 24-frame rate, it was necessary to develop a method of using the 24-frame projector with the 30-frame Tv system. Alternate Scanning Frequency In the RCA 16-mm projector this method is often referred to as 2-3-2-3 scanning. This method permits the film camera to scan the first film frame twice, the second three times, the third twice, the fourth three times, etc. The average rate, then, is 21/2 scannings per film frame, which, when multiplied by 24 film frames per second, provides 60 scanned fields per second, which, when interlaced, is 30 Tv frames per second. Thus is accomplished the use of a 24-frame projector with a 30frame Tv system. Figure 2 reveals how this type of scanning is accomplished. Line D shows a standard Tv film camera cycle. Tivo successive scans of 1/60 second duration tCA 3Smm TELEVISION PROJECTOR OR RCA 14mm PROJECTOR RCA Multiplexer — For uninterrupted projection of multireel films where two projectors are needed. This ingenious device eliminates the need for an additional film camera. It consists of a V-shaped mirror for reflecting images from either projector to film camera, and a slide film projector for inserting station breaks, commercials, etc. FIG. 4. Diagramatical representation of the optical system and film-feed arrangement of the RCA 16-mm Tv projector. are required to make one complete Tv frame, which, of course, results in the 30-frame rate mentioned previously. The spaces shown between the scanning fields are known as "blanking FIG. 5. Graphical comparison between standard theatre and Tv 35-mm intermittent movements. THEATRE periods." It is during this blanking period that bursts of light containing picture information are transmitted from FIGURE 6 How it Works — Line A shows the pulldown timing of a standard 35-mm film projector (no lighting during 90° pull-down). Line B shows pull-down timing of the RCA 35-mm television projector — and the duration and repetition rate of the short intervals during which light passes through the film. Line C shows the projector lighting interval of the 35-mm television projector. The "light-on" intervals are produced by a pulsecontrolled camera lamp that produces an 360° (1/24 SECOND) FRAME No. 1 -90° 288° (1/30 SECOND) the projector through the film to the film camera. The Iconoscope used in the film camera has a storage-of-memory characteristic which permits it to retain the information received from the light burst for a period long enough to scan one field. The source of light is a 1000watt, 115-volt lamp. The duration of these light bursts is controlled by the size of the opening in the rotating shutter (Fig. 3). It is now evident that these bursts of light will deposit intelligent picture information on the mosaic of the Iconoscope only if the motion picture film is stationary in the aperture. Shortened Pull-Down Period If pull-down periods of standard duration were used as in Line A (Fig. 2), the light bursts would transmit information appearing in the aperture during the pull-down period. In view of this, TELEVISION it is necessary to shorten the pull-down period to the point where it will not disturb the transmission of the picture 800-microsecond flash every l/60th second. The picture images are projected onto the film camera pick-up tube during the retrace (blanking) interval of its scanning beam. The "storage" property of the tube permits scanning during the unlighted interval between flashes. Scanning releases the picture charge — converts it into a video signal. A synchronizing generator keeps the projector and film camera in phase. FRAME No. 2 K 1 /60 SECOND -H TELEVISION FIELD 5% LIGHTING INTERVAL TELEVISION FIELD TELEVISION FIELD TELEVISION FIELD TELEVISION FIELD TELEVISION FIELD UNLIGHTED INTERVAL '.FILM CAMERA SCAN) TELEVISION FRAME 10 INTERNATIONAL PROJECTIONIST • May 1949