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

trol and indication means. The control relays are both normally closed and, consequently, no power is applied to the motor. If either relay is shorted by means of the switch, the relay opens and causes motor power to be applied. The motor rotates in a clockwise direction if the upper relay is shorted, or in a counterclockwise direction if the lower relay is shorted. Since both B-f and ground must be brought to the camera for other purposes, the only additional lead required to bring control to the camera control unit is the one common to both relays. The iris-indicating meters read the voltage from the arm of the indicator potentiometer to ground. The camera viewfinder is essentially a high-quality picture monitor. The sweep circuits provide excellent linearity, and a pulse-type high-voltage supply synchronized at horizontal rate provides a source of accelerating potential which is independent of the horizontal-sweep circuit. The grid of the picture tube is clamped, providing excellent black-level control. A type 5FP4A picture tube is used in conjunction with a magnifying lens which serves not only to provide a larger picture, but to allow more comfortable viewing on the part of the operator. A block diagram of the camera control unit is shown in Fig. 7. Besides the main video amplifier chain, it contains picture and waveform monitors with associated circuitry, the image-orthicon vertical-sweep circuit, and the intercommunication amplifier. A block diagram of the video amplifier chain is shown in Fig. 8. The first stage is a straightforward video amplifier. The second stage provides compensation for high-frequency losses in the camera cable. Variation in compensation is accomplished with a six-position switch, each position corresponding to 200 ft of cable, enabling compensation for up to 1000 ft of camera cable. A second section of this switch causes a 10% increase in the a-c voltage supplied to the camera cable when cable lengths of 600 ft or longer are used. The third stage provides control of video gain by means of bias control of a remote cutoff tube. The output of this stage is clamped to remove hum which may have been picked up on the signal and also to provide a black-level reference for the insertion of blanking and for • white-peak clipper action. The clamp pulse is flattopped and of about 2 /xsec duration. It is derived from the complete synchronizing signal by means of a shorted delay line differentiating circuit. This allows the clamping signal to be set in the proper phase relationship with the incoming camera signal which may be delayed up to 3.3 /xsec, depending upon camera cable length. Control over the delay of this pulse is effected by means of the second section on the cable compensation switch. The clamp pulse has been made as wide as possible to eliminate the random streaking effects caused by clamping on noise peaks. Blanking and vertical shading are added to the signal at the plate of the clamped stage. Insertion of blanking in the plate of the clamped tube ensures constancy of "setup" with picture-content variations. Because of this, there is usually no need to touch the "setup" control during a program. Following blanking insertion, the signal is clipped and fed to the peak-white limiter. The peak-white limiter helps to prevent excessive modulation of the transmitter or overloading of external video line amplifiers. The control is usually set to clip peaks which exceed 1.1 v for a normal white level of 1.0 v. After passing through the peak-white limiter, synchronizing signal is added to the video signal if desired, and the complete signal is fed to the video output stage which feeds the 75-ohm output line. Both picture and waveform monitors are normally fed from the output line. However, a "transmit" relay is provided which allows the output video to be cut off, in which case the pick-off point for 172 February 1953 Journal of the SMPTE Vol. 60