Aberration. A term from optics that refers to anything affecting the accuracy of the image
when compared with the original scene.
AC. Alternating Current.
Activity detection. Refers to a method built into some multiplexers for detecting movement
within the camera's field of view (connected to the multiplexer), which is then used to improve
camera recording update rate. It should not be confused with video motion detection.
AC/DC. Usually applied where a power supply can be either Alternating Current or Direct Current.
A/D (AD). Usually refers to analog to digital conversion.
ADC. Analog to digital conversion. This is usually the very first stage of an electronic device
that processes signals into digital format, ie converting a constantly time varing signal into a binary
signal. The signal can be video, audio, control output and similar.
ADSL. Asymmetric Digital Subscriber Line: Modems attached to twisted pair copper wiring
that transmit up to 8 Mbps downstream (to the subscriber) and up to 1Mbps upstream (from the
subscriber), depending on line distance, loop plant condition, electrical noise, and technology and
implementation options. It is known as Asymmetric because of the different data rates upstream
and down stream.
AGC. Automatic Gain Control. A section in an electronic circuit that has feedback and
regulates the output voltage level to fall within predetermined margins. Often used in a camera to
give a constant one volt peak-to-peak video output, when light levels reduce after the lens' auto
iris is fully open.
AIT. Advanced Intelligent Tape (AIT) a type of tape drive sometimes used for storage of digital
ALC. Automatic Level Control. A section of the electronics of an automatic iris lens that sets the
operating level, about which the lens will operate.
Aliasing. An occurrence of interference where an analogue signal has been digitised. This can
occur in images from a CCD chip camera where it is looking at a lots of vertical lies spaced closely
together (high frequency), it is also known as Moiré patterning.
Alphanumeric video generator (also text inserter or character generator). A device for providing
additional information,normally superimposed on the picture being displayed; this can range from
one or two charactersto full-screen alphanumeric text. Such generators use the incoming video
signal sync pulses as areference point for the textinsertion position, which means if the video
signal is of poor quality,the text stability will alsobe of poor quality.
Amplitude. The instantaneous value of a varying waveform.
Analog signal. Representation of data by continuously varying quantities, with respect to time.
An analog electrical signal has a different value of volts or amperes, or phases of either, for
electrical representation of the original excitement (sound, light) within the dynamic range of the system.
ANSI. American National Standards Institute.
Anti-aliasing. A procedure employed to eliminate or reduce (by smoothing and filtering) the
APD. Avalanche Photo Diode. A type of detector used in fibre optic receivers.
Aperture. The opening of a lens that controls the amount of light reaching the surface of the
pickup device. The size of the aperture is controlled by the iris adjustment. By increasing the
F-stop number (F/1.4, F/1.8, F/2.8, etc.) less light is permitted to pass to the pickup device.
Apostilb. A photometric unit for measuring luminance where, instead of candelas, lumens are
used to measure the luminous flux of a source. One Apostilb = 0.318309886183791 candela/m2.
Archive. Long-term off-line storage. In digital systems, pictures are generally archived onto
some form of hard disc, magnetic tape, floppy disk or DAT cartridge, CDROM or DVD.
Artefacts. Undesirable elements or defects in a video picture. These most commonly occur after
an image has been compressed and re-constituted.
ASCII. American Standard Code for Information Interchange. A 128-character set thatincludes
the upper case and lower-case English alphabet, numerals, special symbols and 32 control codes.
A 7-bit binary number represents each character. Therefore, one ASCII-encoded character
can be stored in one byte of computer memory.
Aspect ratio. This is the ratio between the width and height of a television or cinema picture
display. The present aspect ratio of the normal television screen is 4 : 3, which means four units
wide by three units high. Such aspect ratio was elected in the early days of television, when the
majority of movies were of the same format. The new, high definition television format proposes
a 16 : 9 aspect ratio (42 : 32 ).
Aspherical lens. A lens that has an aspherical (not spherical) surface. It is harder and more
expensive to manufacture, but it offers certain advantages over a normal spherical lens such
as a lower F number.
Astigmatism. The uneven foreground and background blur or horizontal / vertical blur that is in
an image. It is caused by distortions in the lens surface.
Asynchronous. Lacking synchronization. In video, a signal is asynchronous when its timing
differs from that of the system reference signal. A foreign video signal is asynchronous before a
local frame synchronizer treats it.
ATM. Asynchronous Transfer Mode. A transporting and switching method in which
information does not occur periodically with respect to some reference such as a frame pattern.
Attenuator. A circuit that provides reduction of the amplitude of an electrical signal without
introducing appreciable phase or frequency distortion.
ATSC. Advanced Television System Committee (the NTSC equivalent of HDTV). An
American committee involved in creating the high definition television standards.
Attenuation. The decrease in magnitude of a wave, or a signal, as it travels through a medium
or an electric system. It is usually measured in decibels (dB).
Auto iris (AI). An automatic method of varying the size of a lens aperture in response to
changes in scene illumination.
AWG. American Wire Gauge. A wire diameter specification based on the American standard.
The smaller the AWG number, the larger the wire diameter. Alternatively :- the measure of the diameter of a conductor. AWG is the U.S. standard measuring gauge for certain conductors, including copper. The higher the AWG number the thinner the wire. This measure stems from the fact that the original measurement represented the number of times the wire was
run through a wire machine which thus reduced the diameter of the wire. Thus a 24-guage wire was thinner than an 18-guage wire because it was run through a wire machine 6 more times than the 18-guage wire, reducing the overall diameter.