Useful Information

VMS do not install CCTV systems, however the following notes are intended to give guidance to anyone considering a new CCTV system or upgrading an existing one.

The objective of most CCTV schemes is to display and record images for purposes of Safety and Security.

Surveillance is the term used to describe the process of viewing live CCTV images to identify and react to incidents that may be of threat.

Images may also be recorded for review after an incident has been recognised to have taken place.

Images used for investigation & evidence must be fit for purpose, the following factors are of paramount importance;

ARCHIVE – how long images are held for. A common standard is 30 days although a shorter archive may be appropriate and far more cost effective, think about how long it may be before you are likely to become aware that an incident may have occurred. An archive longer than 30 days can be established in compliance with the Data Protection Act provided that you can reasonably justify your reasons for doing so.
 
IMAGE SIZE – expressed as a percentage of screen size occupied by the subject.

RECORDING QUALITY – 4 CIF (704 x 576 pixels) is the ultimate choice for high resolution cameras. 2CIF is however an effective standard for high resolution cameras that compares favourably with the previous analogue Super VHS standard.

RECORDING RATE – expressed as ‘pictures per second’ (pps) per camera. The human eye recognises 25pps as a fluid moving image indistinguishable from real life.
This rate is unlikely to be cost effective, a reasonable compromise would be to settle on a rate between 3 and 6pps per camera, lower recording rates are generally unsatisfactory due to the delay between pictures allowing activity to be missed.

HARD DRIVE SIZE – All of the forgoing factors have an impact on the amount of data that needs to be stored. Typically a single camera recording a 30 day archive at 2CIF resolution and 6pps will require 280 Gbytes of disc space. Disc size may be reduced by application of Video Movement Detection triggered recording which only records if a moving image is detected, this function is available on most DVR’s.

RECORDING MEDIA – The common standard is to record images to Hard Drive and in the event that an incident has taken place, downloaded to either a *Non-Rewritable CD or DVD, it is important that this can be played back directly on a DVD player or DVD enabled PC. It is the accepted norm to download two copies, one as the ‘working copy’ for issue to the Police, the other as an ‘archive copy’ held securely on site.
* Rewritable media including memory sticks etc. are not suitable for evidence that may be submitted in court.

WATERMARKING – Some form of electronic watermarking must be applied by the recording equipment in order that any attempt at tampering with the recording becomes permanently evident.

CAMERAS – The quality of images recovered from the recording equipment is entirely dependant on the input from the camera normally expressed as TVL (TV Lines) ranging from 330 to 600 although a choice in excess of 450 is recommended in order to achieve evidential quality recording.  

COLOUR RENDERING – Accurate colour rendering is fundamental to achieving recognition, most cameras will achieve reasonable colour rendering in daylight (blue end of the spectrum), but a low specification camera will not give accurate colour rendering in some types of artificial light at the yellow end of the spectrum, colours can change completely or look washed out.

LOW LIGHT – Consideration should be given to the effect that low levels of light have on image quality, day/night cameras are often specified, these will switch from colour to monochrome at low ambient light levels and may require supplemental lighting in order to provide useful images. The ultimate is to achieve full colour images although this will require high levels of lighting that may conflict with environmental concerns relating to light pollution and energy costs.

DATA PROTECTION ACT – It is a legally enforceable that CCTV systems comply with the Data Protection Act and that, where appropriate, the Information Commissioners Office is notified that CCTV is operated by an individual or organisation.

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