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). |
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. |
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Auto or manual 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. Generally we tend to use auto iris lenses externally where there are variations in the lighting levels. Manual iris lenses are used normally for internal applications where the light level remains constant. However, with the introduction of electronic iris cameras it is now possible to use manual iris lenses in varying light conditions and the camera should electronically compensate. There are several considerations to this option though: the setting of the F stop becomes critical; if the iris is opened fully to allow the camera to work at night, the depth of field will be very small and it may be more difficult to achieve sharp focus even during the day. The camera can maintain normal video levels, but it cannot affect the depth of field. If the iris is closed to increase the depth of field, the low light performance of the camera will be reduced.
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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. |
