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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.

Time Lapse, VCR's, DVR's, NVR's ...
it's time to go on the record...

Digital Video Recorders (DVR)

Unlike the time honoured ‘Analogue’ VCR (Video Cassette Recorder), a Digital Video Recorder (DVR) is a very different and often totally misunderstood beast.

Firstly in terms of conventional CCTV, with only a few exceptions, the camera signals are being input into the recorder via a standard composite video (CVBS) analogue input (i.e. a normal BNC connector).

This means that in order for the recorder to do its stuff, it must first convert the analogue waveform signal into a ‘Digital’ stream, which can then be recorded as a binary code ( for more details on exactly what 'digital' means see here ). You may already be aware that computers, and of course DVR’s can process digital coding at very high speed, but the inescapable fact is that in order to gobble up the vast amount of raw data associated with a very high resolution colour signal, certain things have to be made to happen first.

During the signal conversion process, the individual images aka fields or frames [a frame comprises of two interlaced fields] are reduced in size (compressed) using a standard recognised protocol, such as JPEG, MPEG, MPEG-2, MPEG-4, M-JPEG, H.264 or Wavelet, which are unique and totally incompatible compression formats, that allow large data rich images to be stored as a stream of ‘slimmed down’ coded files.

One of the standard methods for shedding unwanted bulky information (data) from a stream of images, is loosely based on an older established technique called Conditional Refreshment. This same technique was originally used in "Fast Scan" transmission systems, and is now also used in most telephone or mobile (cellphone) image transmission systems.

Basically, an image is captured in its entirety and then converted to an appropriate format. With subsequent images, bits of the picture which are unchanging (e.g. sky or roadway, or walls) are deliberately omitted from the process for a certain number of images, with only the changes being encoded for recording.

Every ‘x’ number of frames, the whole process is repeated to keep the recording refreshed and up to date; bare in mind this whole process is happening many times per second.

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