International projectionist (Jan-Dec 1947)

24-Frame/p.s. Projection vs. 30-Frame Video Rate Difference in standards is reflected in mechanical changes made in new RCA TP-16A 16-mm tele film projector. RCA RESEARCH LABORATORIES, PRINCETON, N. J. THE many projectionists who through having taken special courses to fit them for active participation in the theatre television field, no less than their fellow craftsmen who have adopted self-training methods through intensive reading and the like, will be especially interested in the technical data relative to the new RCA TP-16A 16-mm film projector designed for operation with a television camera and facilitating the use of newsreels, short film subjects and film commercials for low-cost television programming. Projectionists will find much of interest in these data, not only from the standpoint of their application to the video art but also as a highly interesting comparison with the more familiar standard types of 16-mm projectors. This new TP-16A projector, an adaptation of the well-known standard RCA 16-mm mechanism, has been modified to project motion pictures onto the mosaic of a pickup tube in a television camera where the varying light values of the film are translated into video signals for transmission. But, it may be asked, don't motion picture projection standards differ from those applying to television? The answer is that they certainly do — and therein lies the reason for this article. Accelerated Pull-Down Rate Television standards and proper synchronization require transmission of 60 fields (30 interlaced frames) per second. Since motion picture film is made for projection at 24 frames per second, means have been provided for conversion from the one rate to the other. This is done in the TP-16A by scanning the first frame twice, the second frame three times, the third twice, the fourth three times, and so on. The average rate, then, is 2% scannings per frame, which, at a film speed of 24 frames per second, provides 60 scanned fields per second. To successfully employ the 2-3-2-3 scanning sequence, the pull-down time for each frame must be shorter than that employed in standard projectors. The pull-down interval in a standard projector is about one-sixth the total "frame cycle." If this interval were used for television projection, alternate pulldowns would slightly overlap the scanning cycle and would cause annoying travel-ghosts. To avoid this, the spur gears ordinarily used in a projector have been replaced in the TP-16A with a set of elliptical gears, which cause the claw mechanism to travel about 50 per cent faster in the downward direction. As a result the pull-down time is reduced to about oneeighth the "frame cycle," the film remains in the film gate for seven-eighths of the cycle, and the pull-down cannot coincide with the projection interval. The optical projection system of the TP-16A consists of a 1000-watt air-blastcooled incandescent lamp, a silver-coated Pyrex glass reflector, a large two-element aspheric condenser lens, and a coated F:2 projection lens. This system provides plenty of illumination on the mosaic of the camera pickup tube and is much simpler than systems using switched or pulsed light sources. A number of unusual features are incorporated in the sound system of the new television projector, where the density or area variations of the film sound track are translated into electrical impulses, amplified, and used to modulate the sound portion of the television transmitter. One of these features is the use of radio-frequency voltage on the exciter lamp filament. This type of power source insures a constant beam of light which prevents hum and noise from being introduced by the lamp itself. To insure permanently accurate alignment, the exciter lamp mounting the sound carriage are die-cast in one piece. Another feature of the projector is the use of the well-known RCA-developed rotary stabilizer on the sound drive. This device maintains smooth, uniform film speed for sound take-off, and insures sound reproduction at virtually the originally recorded pitch. Precise Sync Necessary A fundamental requirement of a television film projector is that it must synchronize with the television system. This is assured in the TP-16A by using a common source of power for both the television synchronizing generator, which drives the scanning beam in the camera pickup tube, and for the motor which drives the projector shutter. To make sure that the shutter ■ will be in-step at all times, the motor used is a speciallly wound three-phase synchronous type with D.C. field excitation, always "locking" in phase at exactly 3600 r.p.m. Controls mounted on the projector provide maximum flexibility of operation for the unit. These include stand-by, emergency run, start, stop, and remote controls. When the stand-by control is operated, low voltage is applied to the projection lamp, the blower motor operates, and the pre-amplifier is warmed, as this is the normal warm-up position. When the remote switch is closed, "start" and "stop" controls at the remote location may be used to control operation. These circuits operate through relays and a master contactor mounted on the pedestal of the projector. Also mounted on the pedestal are 115-volt, single-phase, and 220-volt, three-phase, circuit breakers. The TP-16A projector is entirely selfcontained and, with the exception of the film feed arrangement, is entirely enclosed. The projector is mounted on a heavy cast-base frame. This frame in turn is mounted by means of leveling screws on a lightweight pedestal, which provides a convenient place for mounting the controls and field supply for the special, three-phase motor. Questions stemming from this discussion of the difference between present theatre projection standards and those applying to video are invited. RCA 16-mm VIDEO FILM PROJECTOR This unit will facilitate the use of newsreels, short film subjects, and film commercials, contributing to low-cost television programming. It projects regular 16-mm motion pictures onto the light-sensitive element of the electronic pick-up tube in a television camera. 16 INTERNATIONAL PROJECTIONIST • April 1947