Choosing the Right DVR
Until 2009 you could simply evaluate the recognition standard of a CCTV system based on what percentage of the screen a person filled (see previous section). But with the latest standard from the Home Office things are slightly more complicated. These percentages are based on the PAL televisions standard which was used for most systems. Now, however, there are a number of recording resolution formats available and the above sizes depend upon the quality of each format. The following table based on the Home Office Operational Requirements Manual for CCTV – page 10 outlines some of the most popular formats.
So if you are using the PAL system and wish to Identify an Unknown Person they need to be 100% of screen height and to Recognise a Known Person then must be a minimum of 50% of the height of the screen etc. However if you have a recorder that produces pictures in 4CIF or D1, because the resolution is better than PAL to Identify An Unknown Person they only need to be 70% of screen height and to Recognise a Known Person they must be a minimum of 35% of the height of the screen.
The problem comes if you purchase a unit that only records in 2CIF or CIF. Now a person has to be 70% height of screen before they are recognisable and it is almost impossible to meet the Identify standard as they have to fill 139% of the screen. This also means that the area you can cover if attempting to recognise a person drops from a screen width (the width of the area viewed on the screen at the point that the target person is standing) drops from 4.5m to 3.2m.
The Home Office Operational Requirements Manual for CCTV – page 14 also states that while recording one picture per second per camera is adequate where there is little activity, 6 to 12 pictures per second per camera should be recorded where there is busy activity such as doorways.
We recommend that you purchase a Digital Video Recorder that can record in D1/C4 format at 6 pictures per second for the length of time you wish to keep the recordings.
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