video on telephone line wires

Posted on Feb 18, 2013

The Videowire converts a baseband video signal into a low impedance differential signal that can be sent over ordinary four conductor telephone wire. A send board mounted close to the camera generates the differential signal and a receive board near the monitor converts it back into single wire video that is fed to a monitor. The video signal is sent over two of the four conductors in the telephone wire and power for both the send board and the camera are carried over the other two conductors. The telephone wire connects to the boards with standard RJ11 modular plugs. The send board can be attached to the back of a camera board with pieces of double sided tape or insulated with some electrical tape and tucked anywhere in the camera enclosure. The receive board can be housed in a small metal enclosure. Figure 1 is a block diagram showing power and signal flow.

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The send board uses a MAX435 dual output wideband transconductance amplifier to convert the NTSC video from the camera into a differential signal. Each output line is terminated in 50 ohms and connects to the red and green center wires of the flat telephone wire. Twelve volt DC power for the amplifier and camera is supplied by the receive board and is carried over the outer yellow and black wires. The MAX435 is designed to operate on - 5 volts, ground and + 5 volts. However, in this application it operates on ground, +5 volts and +10 volts. Circuit components that would normally connect to ground are connected to +5 volts instead. Two 78L05 voltage regulator ICs generate the +5 and +10 volts to power the MAX435, while a 12 volt Zener diode regulates power to the camera. The 12volt DC power that is regulated by D1 and powers the camera also connects to the inputs of IC2 and IC3. IC3 provides a +5 volt output. The ground terminal of IC2 connects to +5 volts instead of ground, causing its output to be +10 volts. Pulldown resistor R7 sinks the quiescent current of IC2 and the output drive current of IC1 to keep positive current flowing out of IC3 at all times. The video signal from the camera is terminated by R1 and coupled to the amplifier input through C1. R2 provides a +5 reference for the input, R3 sets the amplifier gain at 1 and R4 sets its operating current. Two 78L05 voltage...

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