A video multiplexer is a digital device used
for manipulating the images from CCTV cameras. Its primary function
allows the recording of a number of cameras onto a single VCR.
There are three main functions of a video multiplexer:
· Encoding – taking a single picture
from each camera in turn and feeding it out to a VCR in rapid
succession.
· Decoding – playing back previously encoded recordings.
· Multi-screen – a screen comprised of two or more
camera images displayed simultaneously. They can be displayed
in many different formats including Picture-In-Picture (PIP),
Quad, 7, 9, 10, 13 and 16 ways.
A ‘Simplex’ video multiplexer is
capable of performing any one of the above functions at any one
time, a ‘Duplex’ machine is capable of performing
two functions simultaneously including encoding of cameras and
generating a live multiscreen picture of the camera images.
With a Duplex machine it is possible to encode
all the cameras whilst decoding a previously recorded tape. This
does, however, require the use of two VCRs.
Video multiplexers are available in a variety
of models to suit various applications. Including ‘Simplex’
or ‘Duplex’, monochrome or colour with 4 to 16 camera
inputs.
Most units come equipped with digital activity
or motion detection, automatically taking more pictures from cameras
where changes in the video signal have been detected.
‘Triplex’ units allow simultaneous
recording and playback of cameras and multiscreen display with
both live and play back images. Allowing an operator to continue
monitoring the site whilst reviewing previously recorded images.
The more cameras that are encoded to a single
VCR, the greater the delay between updated images from any one
camera. The theoretical maximum rate a multiplexer can encode
camera pictures, is 50 fields per second.
If a multiplexer is encoding 16 cameras, the
minimum gap between successive picture updates from any one camera
is 16/50 second, approximately 1/3 second.
When a video multiplexer is combined with a
time-lapse VCR the delays between updates further increase. In
24-hour mode a VCR will record one image in eight presented to
it. The minimum gap between successive picture updates increases
to (8x16)/50 seconds, approximately 2.5 seconds. This does not
appear to be a long time but it is clear that a considerable proportion
of the available information from any camera is not recorded and
the probability of obtaining optimum recorded images of an incident
are considerably reduced.
In order to reduce the delays between picture
updates it is recommended that a maximum of 8 cameras are recorded
onto a single VCR. This is a standard that is followed in the
majority of town centre surveillance systems.
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