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AC-3
A digital surround-sound system introduced by Dolby™ laboratories. The system is usually comprised of 5.1 channels (6.1 nowadays and counting)- five/six discrete channels - left, right, center/s, back left and back right. In addition one Subwoofer covers the bass (low frequency signals, representing a limited spectrum of audio, hence the .1 designation) that belongs to all channels. As the ear is not sensitive to the direction of the very low frequencies, the "low-bass" area, one Subwoofer is sufficient. In order to retrieve and decode AC-3 sound channels, a special amplifier/receiver is needed. The AC-3 system is also available on some digital video equipment for recording and playback, such as a DVD*. Similar digital 3D sound schemes are also available such as THX®, DTS® and others, as well as Dolby surround*, which is an analog scheme.
ACC
Automatic Chroma Correction. A system built into some VCRs and TV sets for automatic adjustment of color saturation levels. Most ACC systems measure the Color Burst* amplitude and use it as reference. As the system is automatic, erroneous color levels can appear in the video scene - an original grayish scene can become over colored, or a rich, saturated-color scene can become dull.ACTIVE VIDEO
The part of a video signal visible on the screen.
ADAPTIVE VIDEO
A Comb Filter* that uses adaptation technology - changing its operation parameters by dynamically following changes in the picture.
ADC
Analog to Digital Converter. A device which converts an Analog* signal to a digital form. The process may be done with different levels of accuracy. The conversion fidelity is dependent on two factors - sampling rate and number of bits. The higher the rate and/or bits used during conversion, the more accurately the analog signal is reproduced. Until recently, the most common video sampling speed was four times the color subcarrier frequency (4xFsc - 17.7 Msamples/sec for PAL 14.3 Msamples/sec for NTSC) Today, the standard (industrial and broadcast) tends to be 13.5 Msamples/sec for luminance signals. For color difference signals it is usually half for both PAL and NTSC. Industrial video signals are digitized at 8-bit accuracy (256 levels) while broadcast signals are digitized at 10-bit accuracy (1024 levels) or even at 12 bit. Hi-Fi audio signals are usually sampled at double the highest frequency audible to the human ear, i.e., 20 kHz (or more - sampling at 44 kHz or even higher frequencies) with an accuracy of 16 bits or more.
ADTV
Advance definition Television. The “HDTV” standard which was used in Japan. “True” HDTV is now replacing this format worldwide
AES/EBU
A digital audio standard established by the Audio Engineering Society and the European Broadcasting Union. The signal is serial digital, comprising two channels. The basic sampling resolution is from 16 to 24 bit, sampled at frequencies between 32 kHz and 192 kHz (44.1 kHz is the most common). Two sets of 4 bits of information are included for other data transmission (one set can be added to the 20 bit of audio, for a 24 bit system). Additional bits are transmitted as well, and the transmitted frames of the interwoven audio channels are grouped to frame blocks.
AGC
Automatic Gain Control. Electronic circuitry that assures a fixed predefined output level, automatically compensating for varying input levels. Used in audio, video and RF equipment to ensure that output signals are maintained at constant levels in the face of wide variations in the signal-input levels. Low level signals are boosted and high level signals are attenuated to an average level. Automatic gain control has a tendency to introduce audio noise* and hiss* into the audio channel of a videotape. In sophisticated equipment, automatic gain control is often superfluous, and the user can select the option of manual control.
ALARM CAMERA SCANNING
An electronic device, mainly used in security installations, where several video cameras positioned in different locations on the premises, scan automatically, and are viewed one after the other on one monitor. When an intrusion occurs in the field-of-view of one of the cameras, a special alarm signal is sent to the scanner instantly activating the particular camera in question. It usually triggers an additional alarm device drawing attention to the event. Sophisticated scanners have internal microprocessor control, allowing them to skip or analyze every scanned source, as well as activating, when necessary, a special device, which relays a suspicious camera image to a remote location for monitoring via a telephone line.
ALIASING NOISE
An artifact created when an ADC* converts an analog signal to digital form and the sampling rate of the converter is less than double the highest frequency component within the analog signal.
ALPHA CHANNEL
The Alpha channel is a separate channel of data, transmitted alongside the original color or video information - whether it is video or computer based. It is used to specify an Alpha value for each color pixel in order to control pixel based image blending and mixing. Values of the Alpha channel range between 0 and 1. (In SDI applications the Alpha channel uses 256 different levels from 0 to 1)
ALPHA MIX
Image blending and mixing based and controlled by the Alpha Channel data stream.
AMORPHEOUSLCD
An obsolete LCD* display system, suffering from many technical flaws, replaced by polysilicone-type and other active displays.
AMPLITUD MODULATION
Amplitude Modulation, AM, is used when a signal, (usually an RF* signal) is transmitted carrying low frequency information. The low frequency signal modulates -(changes) the amplitude of the RF signal, and is recovered (detected) at the receiver's end. Video information that is transmitted on the air as TV transmission employs this system. Some radio stations use it also (AM Broadcast), although the superior FM* system dominates radio transmissions.
ANALOG MONITOR
A video monitor which accepts analog level signals. Several types of inputs are accepted by analog monitors making them very flexible: composite* video, RGBS*, YC*, YUV* and any combination of these standards. The signals transmitted to an analog monitor are usually between 0 and 1 Volt and use 75-ohm coaxial cables.
ANALOG VIDEO SIGNAL
Signal in which the output varies as a continuous function of the input, while the values of the transmitted information are within defined limits. Any variation in an analog video signal may represent a specific video parameter, e.g., when the luminance* signal is high (0.7 V above black level) the picture is very bright. When the signal is low (0.1 V), the picture is very dark; at 0 V the picture is totally black. TTL* digital signals, in contrast, are predefined as only 0 or 5 V or other fixed logic levels, and do not vary.
ANAMORPHICSIGNAL/DVD
The anamorphic horizontal compression system used in the cinema industry, optically compresses the wide-screen movie frames by a special lens, to fit on a standard film (35mm or similar). When displayed on the screen, the opposite effect is used – stretching the frames to the original wide-screen format. In the DVD format, a similar process takes place, where the frames are digitally compressed horizontally on the disc. When played on a standard, 4:3 aspect TV, a letterbox image is shown, but when set to 16:9, a full wide-screen image is shown on a Plasma screen or video projector which is capable of 16:9 image reproduction.
ANCILLARY DATA
Data added to a digital video data stream including information such as embedded digital audio, control signals, etc.
ANIMATION
Originally, creation of the appearance of movement, such as in a cartoon by flipping a series of gradually varying drawings in rapid sequence. Today, creating animation and cartoons is done more effectively using computers with appropriate graphics software and genlocking* hardware. The final product of computerized animation can be integrated into videotapes or a video production using encoding* equipment.
ANSI
American National Standards Institute
ANTIALISING
Antialiasing is a procedure which, by smoothing and filtering, eliminates or reduces, aliasing noise*. The procedure usually involves low pass filtering of the processed signal prior to digitizing in order to eliminate signals, having frequencies close to and greater than half of the sampling frequency.
AP-ACCESPOINT
Search meaning.
APERTURE
An adjustable opening that effects the amount of light entering a camera. The aperture (sometimes referred to as the Iris) is measured in F-stops. Smaller F-stop numbers mean that more light reaches the optical sensing device of the camera.
APERTUREGRILL
The screen structure element of Sony Trinitron-™ monitors.
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