Welcome to the
"Technical Section"
of
Doktor Jons
Guide to Closed Circuit TV (CCTV)

CCTV Camera Operation - in a little more detail

CCTV Camera Power Supplies - continued

Doktor Jon has already mentioned that 12v DC is a very flexible option, simply because in certain key applications, it's very straightforward to power the camera from a range of mains derived supplies (240v or 110v AC for example) using a suitable adaptor; or of course from a battery source.

This can be particulalrly useful as a 'back up' power supply (for example in locations subject to regular black outs), or where a camera is to be installed on a temporary basis, or indeed used covertly with no other power supply available.

In addition, it is relatively easy to configure rechargeable 'cells' as a constant emergency back up, to maintain operation if the existing supply is deliberately disabled.

With the 'line powered' cameras, this option may not apply, although 'mains' derived from a battery driven 'invertor', may actually prove to be a more satisfactory option, in certain circumstances.
Whilst 'line powered' cameras are generally the most straightforward to install, they are significantly more expensive, and somewhat more limiting when it comes to using alternative methods of signal transmission, but more about that .... later!

Picture Scanning

Having briefly considered the various power supply options, we now have to look in a little more detail at their relevance in relation to the cameras picture scanning.

Remembering that CCIR was 25 frames per second, and EIAJ was 30 frames per second, there is actually a fairly
simple explanation for this.

If a camera is used in the UK, our AC (Alternating Current) mains voltage is 50Hz., so if the cameras scanning circuitry is 'locked' to the mains frequency, this means that 50 fields (or pictures) are snatched each second. A high quality image is then constructed from 2 'fields', which are overlaid or to be more accurate 'interlaced' to produce one single 'frame'.

This process is then repeated 25 times every second.

Continued>

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