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Access
control system:
an electronic system that allows, restricts and tracks
the movement of people through entry/exit points in
a site. Programmable electronic keys, cards etc. with
readers are used to achieve this. |
AGC
Automatic Gain Control:
is circuitry in a camera which boosts the video signal
(including the noise). It becomes operational when the
light level is low. |
Alarm
interface:
is a facility which allows external alarms like PIR's,
reed switches, vibration sensors etc. to be connected
to the CCTV switcher so that a nominated camera is switched
on when an alarm signal is received. |
ALC
Automatic Light Compensation:
is a setting in an auto iris lens to control the iris
opening. It can be adjusted between peak and average
to respond to the bright part of the scene (peak setting)
or the average value of the video signal (average setting).
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Algorithms:
are complex mathematical formulae or rules used to solve
problems. In CCTV, they are used to achieve digital
compression of a video picture. |
Alkaline
battery:
is a type of DC power source with a longer life compared
to the standard battery. It cannot be recharged. |
Amplitude:
refers to the strength of the video signal at a point
and is measured in volts. |
Angle
of refraction:
is the angle of bending of light when it travels from
one medium into another. |
Angle
of view:
represents the area of the scene (maximum horizontal
and vertical angle) that can be seen through a lens.
It is measured in degrees. |
Aspect
ratio:
is the ratio between the horizontal and the vertical
lengths of the video picture. The aspect ratio for NTSC
and PAL systems is 4:3. |
Audible
detector:
is a device which detects sound. In CCTV, it can be
interfaced with a switcher so that a nominated camera
is switched on when the detector is activated. |
Audio
switching:
is a device which can switch between various audio inputs
either manually or sequentially. Many matrix switchers
incorporate this feature. |
Auto
balance:
is a system of detecting and automatically correcting
errors in the amplitude of colour signals. |
Auto
iris:
is a diaphragm which is inbuilt in a lens to automatically
control the amount of light falling on the chip. The
tiny motors in the lens open or close the diaphragm,
depending upon the amplitude of the video signal. |
Automatic
pan:
is a feature of a pan tilt or pan only head which allows
the head to continuously move (pan) left and right between
two fixed points. These fixed points can be set by adjusting
the limit switches in the head. |
Attenuation:
is a reduction in the strength of light or an electrical
signal and is usually measured in decibels. |
Average
video signal:
represents the average light level of the whole picture
used to open or close the automatic iris in the lens.
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