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The early ‘analogue’ Video Motion Detectors (VMD) were actually rather useful tools, although primarily only reliable when used indoors under controlled stable lighting conditions. In operation, the cameras signal was first fed to the VMD unit and then the output displayed on a suitable monitor. The VMD actually generates a number of (movement) sensitive electronic patches that could be positioned anywhere on the picture; the sensitivity and threshold for triggering an alarm condition being manually set using adjustable ‘pots’ (either finger controlled sliders or 'trim tool' variable resistors). The idea was that as a target moved into one of the sensitive areas, the actual light level of the target area would change, and if rapid or extreme enough, would set off an audible and visual alarm, and if required switch on a connected video tape recorder. These units (apart from being great fun to play with) provided a serious and controllable alternative to other types of hardwired alarm sensor, such as PIR’s, door contacts and pressure mats, and of course there is no additional external equipment required to be set up, as all the processing takes place inside the magic box!. As a secondary benefit, using a VMD meant that there was a Video Loss Alarm function as well, so if anyone cut the cameras cables, or attempted to disable the camera in any way, this would also initiate an alarm status. The only major problem with these units was that their apparent sensitivity to movement made it extremely difficult to use them effectively with external cameras, under daylight conditions. The modern day ‘Digital’ VMD is an altogethor different beast. Rather than simply looking for specific changes in the threshold portion of the video signal, these latest designs convert the analogue video signal into a digital data stream, which is then compared against known movement algorithms, to decide whether the change is caused by a two or four legged prowler. | ||||||||||||||||||||||||||||||
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