Larger CCTV systems have multiple cameras, which
can be viewed and controlled from a number of monitoring positions.
These systems are usually implemented by connecting
all the cameras and monitors to a central video switching matrix
unit. This is a large-capacity video switcher, which allows any
camera input signal to be routed to any number of monitor outputs.
This is often referred to as an ‘active cross-point matrix’.
A typical matrix has a minimum of 32 video inputs and 8 monitor
outputs.
Monitor outputs will have the option of time,
date and camera identification information superimposed on the
CCTV picture using ‘On-Screen Display’ (OSD).
Multiple operator positions will be supported
with a priority system to define what happens in the incident
of two operators requesting control of the same camera simultaneously.
Normally there is no limit to how many operators can view a picture
from the same camera. However, it is possible to define which
system cameras an operator can see and control.
The matrix can also be equipped with a
number of auxiliary control ports, which multiplexers and video
recorders, for example can be connected.
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When designing or working with CCTV systems
always remember the video signal coming from the camera is only
1 volt peak-to-peak. As a result it is easily interfered with
by external signals, easily lost or attenuated resulting in poor
picture quality and noisy pictures.
In order to obtain good quality pictures there
are a number of standard rules to follow:
Don’t exceed the following
distances when installing the coaxial cable. Acceptable distances
for common types of coaxial cable:
Cable Type |
Distance (m) |
URM70 |
200 |
RG59 |
250 |
CT100/RG6 |
350 |
CT125 |
450 |
CT167/RG11 |
750 |
Always ensure each video signal
is terminated only once and the terminator resistor is at the
end of the cable. This will maintain the signal amplitude at 1V
and minimise ghosting as a result of reflections.
Most of the time, signals are terminated
automatically. However, take care when a signal is leading to
more than one piece of equipment for instance a telemetry transmitter
and video multiplexer. When ‘looping through’, always
ensure the termination resistors (often denoted as ‘Hi-Z’)
are switched off.
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