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If a video signal in this case from a surveillance camera, is linked directly to another device perhaps a monitor or recorder, using a simple copper or fibre optic cable, then it's described as being a 'Closed Circuit' simply because it is not available for anyone else to view (in practice, it is of course possible to tap in to a CCTV system, but then that might be a wee bit naughty!).
Although "CCTV" has become something of a generic term for any visual security system using video cameras, in a practical sense, there are now a number of situations where the terminology isn't really accurate; for example, where a system uses a wireless radio transmitter or the pictures are transmitted through a computer network or the internet (known as "IP Video"), then this type of set up could perhaps be more accurately described as an 'Open Circuit Restricted Access' ("OCRA") system.
Here in the U.K., where pictures are transmitted using wireless equipment, the options are limited to either short range 'licence exempt' microwave transmitters which are only available for a handful of channels (very limiting), or 'network' transmission which works in the same way as using a laptop computer on a WiFi network.
In both cases, it is not safe to assume that the signals are secure, as anyone with the right equipment and knowledge, could in most situations pick up the pictures failry easily.
CCTV = Closed Circuit Television - A discreet system of video components linked togethor by either copper or fibre optic cable.
OCRA = Open Circuit Restricted Access - A "broadcast" system where signals are transmitted either by using radio (In the U.K., there are only limited bands of microwave frequencies available) or wireless 'IP Video' (Internet Protocol) techniques, which without the appropriate network security in place, can easily be intercepted by others using the correct equipment and knowledge.
IP Video = The 'next generation' of digital surveillance systems, using computer systems, computer networks or the internet to transmit and process images.
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