Combination 8 channel multiplexer and amplifier (IC1) is the heart of the remote multiplexer box in the single coax video system. The MUX baseband video output is coupled by C11 to the coax and the video is decoupled by L1 from dc power arriving on the same line. All circuitry in the multiplexer box is supplied by this power (about 30 mA at 10 V).
Channel-select signals generated at the interface box (11 pulse for channel 0, 8 pulses for channel7) pulse the 10-V supply to 8.8 V and back at a 10-Hz rate. These pulses is converted by Q1 and associated components in the remote multiplexer box to 5V logic levels, which block the 4 bit counter IC2. In turn, the desired multiplexer channel is selected by IC2. Channel 0 is selected by the first pulse of a burst. Subsequent pulses, arriving before the discharge of timeout network R13/C13 advance IC2 by one count each. Thus, channel 0 appears almost instantly and when selected, channel 1 appears near the end of a 0.8s burst. [Circuit’s schematic diagram source: seekic.com]