Posted on Nov 8, 2012 4150
This 1-cable system carries composite video (NTSC, PAL, or SECAM), power, and channelselect signals. The interfa
ce end of circuit deliver 10 V down the cable, pulses the supply voltage to transmit channel-change connnands, and buffers the received video signal. The multiplexer circuit in Fig. 96-2(a) receives power and control signals over the coaxial cable, while driving the cable with the currently selected video signal. The interface circuit delivers 10 V to the cable and pulses the supply voltage to select one of 8 channels. When the send button is depressed, a digital burst of 1.2 V amplitude (negative) is superimposed on the 12-V line (as a voltage drop). This does not affect the video signal. The multiplexer circuit (Fig. 96-2 (c)) consists of a multiplexer and an amplifier. The multiplexer is a Maxim MAX455. The digital code on the supply line is picked off by Al, IC3A, and interfaced to counter IC2, which drives the multiplexer to select the desired video channel.