CCTV: back to basics (part three) | Archive Titles

Table 1
Camera selection	Considerations
Sequential switcher	This is carried out sequentially. The switcher gives high quality resolution at low cost but is at a disadvantage in that it can only record the camera being viewed so can miss a critical event.
Displays two or more cameras by switching between them for a pre-set period.The rate of switching between cameras is programmed by the installer.	The sequential switcher is not to be confused with the microcontroller matrix switching used in large security applications, which are capable of controlling many hundreds of camera inputs.
Quad splitter	Screen display is formed into four quadrants. Resolution loss can be a problem when recording due to information being compressed for the four pictures.
Allows four cameras to be displayed simultaneously.	Simplex. Can record or playback. Is limited to recording full screen images when viewing live pictures in a screen layout.
Multiplexer (MUX)	Duplex. A double unit with two mother boards it can record and display a multi screen. Using two VCRs it is able to record and replay at the same time.
Offer a range of screen displays.If a camera signal is lost a warning prompt can be given on screen.	Triplex. A triple unit that can record and display multiscreen and playback at the same time.

Table 2
Recording equipment	Considerations
Time lapse VCR These machines operate continuously but in a slow time lapse mode until an alarm input is received at which time it will switch to real time (3 hour) recording.	Typically these machines will operate in 3, 15, 27, 48, 96 through to 960 hours. The object of time lapse recording is to enable a standard E180 VHS tape to be able to record a longer period of time. The longer the tape must last the less information will be stored, ie. the recorder in a 72 hour time lapse setting will display a picture only once every second instead of 25 times per second (3 hour mode).
Event recorder This VCR type does not continuously record but awaits a signal to start. In general they are true 3 hour machines.	A conventional VCR has a pick up time of 8-10 seconds before the first picture is laid onto the tape. Also if a tape is placed into a machine it could take some 45 seconds before any actual taping takes place. The event recorder overcomes the problem of bringing the head up to speed, synchronising and then recording correctly.

Table 3
Recording equipment	Considerations
Monitor - 5 per cent	The target for monitoring is generally 5 per cent of the monitor screen. Used for monitoring human or vehicle traffic flows.
Detect - 10 per cent	To obtain best results for electronic detection purposes, the picture size should be no less than 10 per cent of the screen size.
Recognise - 50 per cent	For recognition, the picture size on the monitor should be no less than 50 per cent of the screen. This value can be used to recognise a target such as a 'known person'.
Identify - 120 per cent	In order to gain a picture (on tape, disk or hard copy) of evidential quality, the target size should be 120 per cent of the screen size. This means, for example, the face and body of a grown human to the knees, but not the full height inclusive of legs!