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Fig. 15. Video switcher.
The video mixing unit, or switcher, is illustrated in Fig. 15. It is designed to provide studio switching facilities in a field package. The switch panel is shown in the operating position. When not in use or when in transit, it may be swung back into the main unit. The box containing the switching facilities may be easily removed and can be operated at distances up to five feet from the main unit. The switcher will accept inputs from as many as five local and two remote signal sources. A preview buss, two effects busses and an instantaneous switching buss are provided. The instantaneous buss and the two effects busses with their associated faders allow cuts, superimpositions. fades, etc. The preview buss provides for the previewing of any input signal or the output of the effects busses, thus allowing an effect to be properly set up before transmission. A "transmit" button on this buss provides means for monitoring the output line. The main output provides a 75-ohm source impedance for proper matching to telephone company lines.
A block diagram of the video mixing
unit is shown in Fig. 16. It may be seen that black level clamping is provided to eliminate switching transients. The output line is monitored directly, but through a pad arrangement which isolates the monitor from the hum and tilt usually introduced by a telephone company line connection. Besides the line and master monitor outputs, an effects monitor output and a separate line monitor output are provided. The effects monitor output is used to monitor the output of the effects busses, and the separate line monitor output may be used to drive a studio or announce monitor.
The switcher intercommunication circuits provide intercommunication facilities for the director and allow transmission of program audio to other intercommunications positions if desired.
The power supply for the video switching unit is self-contained.
The master monitor (Fig. 17) is designed to work in conjunction with the video switcher or as a general-purpose monitor. It includes both an 8^-in. picture monitor and a 3-in. waveform
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February 1953 Journal of the SMPTE Vol.60