CCTV monitors are used to view the pictures
from CCTV cameras, converting the electrical signal into an optical
image to be presented to the system operator.
They’re similar to domestic televisions
but differ in two respects. Firstly, a monitor has no radio frequency
decoding circuitry. Its signal input is ‘base-band’
video as opposed to the aerial input of a television. Secondly,
CCTV monitor tubes have a higher resolution than a domestic television.
A good quality monochrome CCTV monitor will have a resolution
in excess of 800 lines at the centre of the tube; a television
is unlikely to exceed 400 lines.
Monitors are defined by their tube size, which,
like televisions is their approximate diagonal dimension. Common
monitor sizes:
· Monochrome 9”, 12”, 15”,
17” and 20”
· Colour 10”, 14”, 20 and 21”
When working out which size monitor to use,
it is generally regarded that the optimum size for a particular
application is between 1/3 and 1/5 of the distance from the screen
to the operators position.
CCTV monitors are usually equipped with
BNC rear-panel mounted connectors for video signal inputs. Brightness
and contrast controls are located on the front panel.
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Colour monitors have an additional colour brightness
control. This should be set to give a true representation of the
coloured objects in the scene.
Once a monitors controls have been set, it should
not be necessary for the operator to further adjust them either
when switching between cameras in a multi-camera system or at
different times of the day on a single camera. This indicates
that there are problems with the camera signal inputs. It could
be that termination is incorrect, iris levels have been incorrectly
adjusted or video signal levels have attenuated excessively, either
by coaxial cable runs being too long or poorly made joints.
Later models use TFT LCD displays. These have
a number of advantages over conventional CRT monitors:
· They’re smaller and lighter.
· They’re often available with low voltage power
input, typically 12V DC.
· They’re higher in resolution, typically 1024 x
768 pixels and as a result show more detail. The resolution is
consistent across the whole picture, CRT pictures loose definition
at the edges.
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