Journal of the Society of Motion Picture and Television Engineers (1950-1954)

H. Black-level reference in the vidicon is clean-cut and definite since the output resistor signal voltage, even on a d-c basis, is a function only of light on the raster. Thus, the zero signal or black reference is obtained directly, merely by blanking the scanning beam during the horizontal return interval. Standard clamping techniques can thus be used for automatic d-c set or black-level control. I. Nonsynchronous operation of the projector with respect to the synchronous generator is a desirable attribute of a film-reproduction chain. In smooth network operation, it is often necessary to insert commercials or local film material in station-break intervals. Present techniques call for: fading to black; dropping network synchronizing signal; switching to local synchronous generator, which is locked to and properly phased with the local a-c power supply; and operating the iconoscope film chain conventionally. All of this is essential because of the necessity for exposing the iconoscope during the vertical blanking interval. Misphasing or nonsynchronous operation produces the well-known iconoscope application bar whose amplitude may be 10 to 20 times the useful normal video signal. Several synchronous projector drives, providing driving power with frequency controlled by the synchronous generator, have met with some success in solving this problem. By comparison, the vidicon behaves beautifully under nonsynchronous projector conditions. With a projector light exposure pulse of 7% of vertical field time, standard iconoscope exposure conditions, the "application pulse" signal is perceptible to a critical viewer, but is not particularly annoying. With longer application times, 30 to 65%, available with present 3-2 television projectors, such as the RCA TP-6A, the transition from "Light On" to "Light Off" is not detectable even to the most critical viewer. Longapplication time also cuts down the peak illumination requirements. This means either smaller projector lamps or increase in projector lamp life by a factor of 10, or even more. Inserts in network program can thus be made merely by operating the projector from the local power supply with the local synchronous generator tied to network through a Genlock or similar device. The importance of such a feature will increase as network-to-local operation techniques are refined. J. The "unattended operation" possibilities of the vidicon camera appear unusually attractive. Tests with a wide range of film material have shown that it is practically unnecessary to ride video gain. Black-level control is completely automatic, and there are no shading knobs provided or required. The controls are inherently stable and simple. In principle, the only two variables which require adjustment are wall voltage, which determines electrical scanning-beam focus and, therefore, picture resolution; and beam bias, controlling the number of electrons available for discharging the target. Even this last adjustment is noncritical in that the top beam-current requirement can easily be set by simple operational procedures, and any excess produces only secondary deteriorations in resolution due to increased scanningspot size at lower grid biases. The vidicon is far less critical to set up and operate than the image-orthicon tube. That this is so follows from the fact that in the vidicon the useful video signal is generated only by the electrons flowing through the photoresistive signal electrode, while in the image orthicon, the video signal is obtained from the "return" electrons of the scanning beam, making it essential to adjust beam current very carefully to maintain "percentage modulation" at a high value. Extensive tests show that the inherent stability of the vidicon tube and camera circuits is sufficiently high to make "unattended operation," using only a 156 February 1954 Journal of the SMPTE Vol. 62