Siemens TFT - Manual (Page 203)

Glossary
16-1 © Siemens Building Technologies Group
Aberration
Is a term from optics that refers to anything affecting the quality of
the image in regards to the original scene.
Absorption
Parts of a video signal are absorbed by the material, e.g. by a fibre
cable. Therefore the transmission distance is not infinite.
AC
Alternating current. - Mains power is alternating current. Current
voltages are 24 V AC, 115 V AC or 230 V AC.
Activity Detection
Is a feature built into equipment for detecting movement within the
camera field of view, which is then used to improve the cameras
recording update rate.
ADC
Analogue to digital conversion is usually the first stage of an elec-
tronic device that processes signals into a digital format. The signal
can be video, audio, control output or similar.
AES
Automatic electronic shutter - See “Shutter”.
AGC
Automatic Gain Control is the electronic circuitry in a camera that is
used to increase the video signal and becomes operational when the
light level is low. This usually introduces 'noise' in the picture giving a
grainy appearance.
AI
Automatic iris - See “Auto iris”.
Alarm Input
Is a equipment connection, which allows external alarms like PIRs,
reed switches, etc to be connected to CCTV equipment so that a
nominated camera is switched on when an alarm input signal is
received by the received.
ALC
Automatic Light Compensation is a setting in an auto iris lens to con-
trol 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.
Ampere
Unit symbol A. Is the unit to measure electric currents.
Amplitude
The amplitude refers to the level of the video signal at a point and is
measured in volts.
Analogue Signal
In video, the analogue signal represents the camera scene in the
form of varying voltage levels in the video signal, the voltage being
directly proportional to the light level.
Angle of Refraction
The angle of refraction is the deflection 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.
Aperture
The lens aperture dictates the amount of light reaching the surface
of the CCD. The size of the aperture is controlled by the iris adjust-
ment, by increasing the f-stop number (F1.4, F1.8 etc) less light is
passed by the lens to the CCD.
Archive
The term archive is used for long term off line storage. In digital sys-
tems, pictures are generally archived onto some form of hard disk.
ASCII
This is an internationally acknowledged character set standard.
Aspect Ratio
This is the ratio between the horizontal and the vertical lengths of
the image. The aspect ratio for NTSC and PAL systems is 4:3, which
means 4 horizontal units by 3 vertical units.
Aspherical Lens
Using an aspherical surface these types of lenses are more difficult
and more expensive to manufacturer. They are, however, fewer lens
aberrations to correct for with an aspherical lens.
Asynchronous data transmission
Stands for a transmission technique that uses control bits in the
video line to ensure correct transmission.
Attenuation
The decrease or loss in magnitude of a wave, or a signal, as it travels
through a medium or an electrical system. Attenuation is measured
in decibels (dB).
Auto Balance
Is a system of detecting and automatically correcting errors in the
amplitude of colour signals.
Auto Iris (AI)
A lens with an auto iris can automatically adjust to correct the
amount of light that is passed to CCD of a camera. There is a tiny
motor and amplifier built into the lens body, which receives a control
signal from the camera to maintain a constant one volt peak to peak
video level. There are two manual controls on the lens to compen-
sate for varying conditions of 'peak' and 'average' light.
Auto 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.