Journal of the Society of Motion Picture Engineers (1930-1949)

aperture of the Schmidt optical system used is//0.85. _ focal length effective diameter 6.6 in. 7.77 0.85 Light-meter readings taken at the surface of the corrector plate are 160 ft-c. The optical barrel provides for adjustments of vertical and horizontal centering, conveniently accessible from the balcony by swiveling and tilting the barrel, and also for overall top-tobottom optical focus. The convenience of these adjustments can be readily appreciated by any user who has attempted to make these adjustments on other types of television projection barrels. It will be noted in Fig. 5A that the vertical and horizontal focus adjustments are accomplished by moving the "dish," which is the curved-front surfaced reflector in the optical barrel, vertically or horizontally by screwoperated mechanisms controlled by the two knobs shown for each direction. Similarly, as in Fig. 5B, the optical focus control has been brought forward by mechanical means so that a simple knob within easy reach of the balcony is all that need be controlled. In installations having the optical system higher than the center of the theater screen it becomes necessary, of course, to tilt the barrels downward in order to throw the picture within the dimensions of the screen. In doing this it is found that, if the optical focusing adjustment is carefully set for the center of the screen, the top and the bottom of the picture are somewhat out of focus. By adjusting the vertical and horizontal focus adjustment knobs in the rear of the barrel, the picture can be brought into good overall focus without having to tilt the screen. A high degree of "practical" engineering has gone into the Trad barrel as a result of many years of acrobatic hanging by one foot from theater roofs, perching on the top of fire-engine ladders or chinning oneself on a trapeze while using the teeth for adjustment purposes. So, here now is a barrel which doesn't require an acrobat, or any unusual courage or skill to operate. It may be noted in passing that there is symmetry about the center of the dual unit, with the left-hand and right-hand units being mirror images. Each of them independently provides all of the necessary adjustments in equally accessible form. There are three unique technical features worthy of attention: (1) the high-voltage multiplier supply; (2) the automatic brightness control; and (3) the video amplifier response characteristics. Taking these in order: The high-voltage tripler operates on a very interesting principle. Figure 6 is a simplified schematic showing the operation of the voltage multipliers. It will be noted that the 6BG6 tube shown in the diagram is one of the horizontal deflection amplifiers which supplies the proper waveform of current to the horizontal deflection coils, which cause the electron beam to scan the face of the cathode-ray tube horizontally during operation. At the end of each horizontal line, the 6BG6 plate current is "cut off" after the incoming horizontal sweep signal drops to zero. A positive pulse of voltage appears at point A as a result of the collapsing field of the horizontal deflection coil (this is a kickback or flyback voltage). These positive pulses are first rectified by VI which is a diode vacuum tube, and the capacitor Cl is found to be charged to a value very near the peak value of the original pulse. Since the cathode of VI is connected to the plate of V2 through Rl , then C2 will charge up to the same voltage as Cl. The charge on C2 is thus added to the oncoming pulse and tube V2 rectifies the sum of these voltages, thus charging capacitor C3 to double the original pulse 128 August 1952 Journal of the SMPTE Vol. 59