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Guide to Closed Circuit TV (CCTV)

TRUSTED - Target Recognition Using Surveillance Technology for Evidence and Detection - A campaign to improve the effectiveness of existing video surveillance security systems.

CCTV Camera Operation - in a little more detail

CCTV Camera Scanning Techniques

So why are pictures made up from two interlaced 'fields'?

Well, half the pictures vertical resolution is captured in 1/50th of a second (CCIR, PAL and SECAM) or 1/60th of a second (EIAJ and NTSC) by the cameras circuitry snatching all the odd lines, 1,3,5,7,9,11,13 etc., immediately followed by the second field of even lines 2,4,6,8,10,12,14 etc. which produces a complete maximum resolution picture in 1/25th or 1/30th of a second depending on the local standard.

In other words, the cameras internal circuits are 'line locked' to capture a 'field' , using the mains frequency as an absolute reference. Quite obviously, trying to use a camera designed for 50Hz, on a 60Hz. supply, just aint gonna work.

The 12v DC camera is significantly different in that it generates its own 50 or 60 Hz. reference internally, in a seperate section of the circuitry. This technique is referred to as 'internal sync'.

External Syncronisation

Historically, on more expensive system cameras, there was also the option of synchronising all the cameras in the system (usually larger installations), to an externally generated pulse signal, although with advances in modern system design, this feature is virtually redundant for security purposes.

Just for the record, with cameras that do provide this facility, connections are usually made using standard BNC connectors, to sockets offering VS (Vertical Synchronisation), VBS (Vertical Blanking Signal), HS (Horizontal Sync), or Gen (Genlock). A seperate Sync. Pulse Generator (SPG) possibly linked to a Pulse Distribution Amplifier (PDA), would previously have provided all the synchronising signals for the entire CCTV installation.


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