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Which Monitor?
A control room operator may be required to view images on a bank of
monitors for prolonged periods. As the effectiveness of the surveillance
system is crucially dependent upon the degree to which the operator can
remain alert and avoid fatigue, correct monitor selection is vital.
The monitor converts the video signal generated by the camera back into a visual
image. It consists of a cathode ray tube (CRT) and a number of signal processing
circuits.
These circuits separate the ‘luminance’ part of the signal from the ‘sync signals’.
The luminance signal is routed to an amplifier, which boosts the signal to a level
that can be used by the CRT.The sync signals are split onto horizontal (H) pulses
and vertical (V) pulses. These in turn are amplified and modified so that they can
control the vertical and horizontal deflection of the final picture.
Where the video signal is being looped through the monitor to another piece of
equipment (for example, another monitor or video recorder), it is important to
ensure that the system is correctly terminated. Some monitors will do this
automatically, but others have a switch located at the rear.This should be set
according to whether the signal is being looped out of the monitor – High
Impedance – or is terminating, in which case it should set to 75 ohms.
The last item of equipment in the system should be set to 75 ohms if it does not
auto terminate. Video signals usually travel over coaxial cable, which has
impedance of 75 ohms. The best and most efficient transfer of the video signal
from camera to monitor is when the camera, cable and monitor all have all the
same impedance values. Contrast levels can be improved where a DC clamp facility
is incorporated into the monitor.This can usually be selected via a switch at the
rear of the unit.
Monitor sizes
Monitor size is designated by the length of a diagonal line measured from one
corner of the viewing screen to the opposite corner.Thus a 9-inch monitor refers to
a 9-inch diagonal picture tube. Sizes typically used in CCTV include 9, 12, 14, 17,
21 and 27 inches with different manufacturers offering different variants.
The choice of size depends in part on the distance between the viewer and
monitor.
18
The Sony Guide to CCTV
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Issue 3