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Guide to Closed Circuit TV (CCTV)

So would you like to play "Spot the CCTV Camera?". Eagle eyed visitors may recognise the profusion of street furniture, surrounding a single heritage dome camera keeping watch over part  of Londons' prestigious Oxford Street shopping area.

Everything you need to
control a CCTV system


In purely practical terms, Relay Interface control systems are in fact another form of hard wired controller. Whereas we’ve already looked at direct driving a PTZ camera down a chunky multicore cable, the Relay Interface approach is subtly different.

With Hardwired systems, the voltages sent from the control unit are at the correct level to drive the appropriate piece of equipment at the other end. Relay Interface is specifically designed for larger multicamera system operation, over much greater distances.

Although quite widely used twenty five or more years ago, the technology appears so primitive now, that no self respecting CCTV professional would even contemplate this ‘bits of string’ approach to controlling a modern camera set up.

The idea behind Relay Interface is that when a button is pressed on the control unit, a low voltage supply is sent down a relatively cheap basic grade multicore cable, directly to a small electrical relay (an electromagnetic switch) which then supplies the correct driving voltage to the piece of equipment.

With this approach, the cameras can be controlled over fairly long distances (thousands of metres rather than just hundreds) using basic control cable.

As the distance increases between camera and controller, so the control voltage can be increased to make sure that even allowing for voltage drop over the long cable run, the correct voltage will still arrive at the relay (for example, launch at 28v DC over a long cable, and it arrives at the 12v relay at around 12.8v, well within the operating range of 10.8 - 13.2v DC).

With larger multiple camera systems, it was normal practice to send the switching voltages to every camera, but the selection of an individual Relay Interface Unit was achieved simply by ‘engaging’ a single location specific relay to enable the ‘Neutral’ (for the P/T Head) and the 0v DC (for the lens) connections, to be enabled. Like the dinosaurs, this once prevelant method of control, is now all but extinct in our thoroughly modern ‘Digital Age’.

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