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Fig. 12. Automatic brightness control circuit.
The use of a manual brightness control would satisfy the necessary conditions for optimum picture performance, but the addition of another control for the operator to set tends to make the operation of the unit a bit more complicated. The ideal condition is to eliminate the use of manually operated controls and obtain optimum performance settings automatically. The "Trad" automatic brightness control circuit described above accomplishes this with a high degree of efficiency. This system, in addition to being an automatic brightness control, is also an automatic high-voltage regulator keeping the voltage safe at the varying input signal levels.
The graphs in Fig. 13 reveal the action of the automatic brightness control with varying video inputs. Also, shown in Fig. 14 is the interrelationship of average beam current resulting from the effect of the automatic brightness control as the video signal is varied. These graphs were made using a flyingspot scanner picture generator, and varying the output signal voltage, going from no picture, through all the intermediate stages, to a condition of maximum contrast.
One of the important design features of the Trad theater projection television unit is the picture quality, which is the result of critical peaking of the video
amplifier circuits to produce a picture of maximum crispness without ringing or smearing. It will be noted from the curve in Fig. 1 5 that the usable response has been extended to approximately 7 me, being more than adequate for clear and crisp picture reproduction from either closed-circuit or off-the-air operation.
No less important to the theater owner and operator is the low cost and relative ease of installation of this equipment. Surveys of many theaters were made before design decisions were arrived at, with the result that much less special work need be done in the theater during the installation of this equipment than many others currently offered.
It will be noted, in referring to Fig. 1, that associated dual video amplifier, sweep, and high and low-voltage supply chassis are installed in a convenientsized housing which is supported at the fore part of the balcony on a single fabricated bracket which is supplied by Trad. This bracket is mounted by means of bolts through the structural support in the balcony. When mounted in this position, it is out of harm's way and yet is readily accessible for routine maintenance and adjustment.
Figure 16 shows the simplicity of the complete electrical wiring of the system,
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August 1952 Journal of the SMPTE VoL 59