A/B Roll: Editing
Video editing arrangement where scenes are edited from two source VCRs ("A" and "B") to a third (recording) VCR. Typically a switcher or mixer is used to provide transition effects between sources. Control over the machines and process can be done manually or automatically using an edit controller.

Active Pixel Region
On a computer display, the area of the screen used for actual display of pixel information.

ADPCM
Adaptive Differential Pulse Code Modulation. An encoding format for storing audio information in a digital format.

Adaptive Compression
Data compression software that continually analyzes and compensates its algorithm, depending on the type and content of the data and the storage medium.

Additive Color
Color produced by "adding" primary colors, usually the combination of red, green and blue. Additive systems are generally associated with light emitting devices (CRT's).

AGC (Automatic Gain Control)
Circuitry used to ensure that output signals are maintained at constant levels in the face of widely varying input signal levels. AGC is typically used to maintain a constant video luminance level by boosting weak (low light) picture signals electronically. Some equipment include gain controls which are switchable between automatic and manual control.

ALC (Automatic Level Control)
Circuitry used to automatically adjust the audio recording level to compensate for variations in input volume. Some equipment includes level controls which are switchable between automatic and manual control.

Algorithm
In compression software, refers to a specific formula used to compress or decompress video.

Aliasing
A form of image distortion caused by sampling frequencies being too low to faithfully reproduce image detail. (See Anti-aliasing.) Examples include:
- Temporal aliasing - e.g., rotating wagon wheel spokes apparently reversing direction
- Raster scan aliasing - e.g., twinkling or strobing effects on sharp horizontal lines
- Stair-stepping - Stepped or jagged edges of angled lines, e.g., at the slanted edges of letters.

AM (Amplitude Modulation)
Amplitude modulation is a process used for some radio (AM broadcast) and television video transmission. A low frequency (program) signal modulates (changes) the amplitude of a high frequency RF carrier signal (causing it to deviate from its nominal base amplitude). The original program signal is recovered (demodulated) at the receiver. This system is extensively used in broadcast radio transmission because it is less prone to signal interference and retains most of the original signal quality. In video, FM is used in order to record high quality signals on videotape.

Analog
A device or method which makes use of non-discrete variations in frequency, amplitude, location, etc., to symbolize or carry sounds, signals, mathematical data or other information. The signals vary continuously instead of in steps. Analog technology "mimics" information, so that, e.g., a voice is represented as an electrical signal with frequency and amplitude proportional to the pitch and volume of the voice. Traditional telephone and video technology are both analog (non-digital) in nature. Analog devices are characterized by dials and sliding mechanisms. (See Digital.)

Analog-to-Digital Converter (ADC, A/D, A-to-D)
A circuit that uses digital sampling to convert an analog signal into a digital representation of that signal.

Analog Video
A video signal that represents an infinite number of smooth gradations between given video levels. Analog video whether transmitted over cables, read from videotapes or broadcast, is subject to degradation due to noise, distortion and other electronic phenomena. Normal signal levels should be within 0.7-1 volt. By contrast, a digital video signal assigns a finite set of levels. See also Digital Video.

Analog: Monitor
A video monitor which accepts analog signals. Several types of inputs are accepted by analog monitors: composite video, RGB & Sync, Y/C, YUV and any combination of these formats. The signals transmitted to an analog monitor are usually between 0 and 1 V and use 75 ohm coaxial cables.

Anamorphic
Unequally scaled in vertical and horizontal dimensions.

Anti-aliasing
A form of interpolation used when combining images; pixels along the transitions between images are averaged to provide a smooth transition.

ANSI
American National Standards Institute. A standards-setting, non-government organization which develops and publishes standards for voluntary use in the United States.

Aperture
An adjustable opening in a lens which, like the iris in the human eye, controls the amount of light entering a camera. The size of the aperture is controlled by the iris adjustment and is measured in f-stops. A smaller f-stop number corresponds to a larger opening which passes more light.

API
Application Programmers Interface. Loosely used to describe the point at which software modules or layers meet and interconnect.

Artifact
An unintended, unwanted visual aberration in a video image (usually blurred or "blocky" portions of degraded image quality). Most common are cross color and cross luminance.

ASCII
American Standard Code for Information Interchange. The most popular coding method used by small computers for converting letters, numbers, punctuation, and control codes into digital form.

Aspect Ratio
The relationship of width and height. When an image is displayed on different screens, the aspect ratio must be kept the same to avoid "stretching" in either the vertical or horizontal direction. For standard TV or monitor, the aspect ratio is 4: 3 yielding 160X120, 320X240 and 640X480 sizes. The HDTV video format has an aspect ratio of 16 to 9 (16: 9).

Assemble Edit (Assemble Mode)
An editing mode that replaces all signals on the record tape (video, audio, control and time code tracks) with new signals.

Asymmetrical Compression
A system which requires more processing capability to compress an image than to decompress an image. It is typically used for the mass distribution of programs on media such as CD-ROM, where significant expense can be incurred for the production and compression of the program but the playback system must be low in cost.

ATM
Asynchronous Transfer Mode. A high-speed packet switching technology with the potential of creating a common "platform" for various types of transmission needs. Information is organized into standard 53 byte "cells." "Asynchronous" means that data is sent character by character with a start and stop pulse for each one. These factors allow very efficient and extremely high throughput levels. Additionally, this high-speed information transfer allows dynamic allocation of bandwidth to meet the changing requirements of the user's applications automatically.

Audio
The "other half" of any video production consisting of frequencies corresponding to a normally audible sound wave (20 Hz to 20,000 Hz), the "soundtrack" of a videotape.

Audio Bandwidth
The range of audio frequencies which directly influence the fidelity of a sound. The higher the audio bandwidth, the better the sound fidelity. The highest practical frequency which the human ear can normally hear is 20 kHz. An audio amplifier which processes all frequencies equally (flat response to 20 kHz) and a reasonably high signal-to-noise ratio, will faithfully reproduce the audio soundtracks of a video recording.

Audio Dub
VCR feature allowing replacement of the audio signals on a previously recorded tape without disturbing the video signal. When dubbing is not available via the video recorder, audio dubbing can be performed while recording using an audio mixer.

Audio Editing
Similar to video editing. Various portions of audio material are combined and recorded onto the videotape in one continuous form. For example, when a sound track is added to a videotape, various sounds such as background music, sound effects and voice narration, may be introduced in order to highlight particular movie scenes.

Audio-Follow-Video (AFV)
During video recording, the video signal is usually accompanied by an audio signal. Sometimes, during video editing, it is often necessary to separate the audio from the video signal. Audio-follow-video mixers allow accompanying audio to "follow" the video when switching video sources or not.

Audio Levels
Proper audio levels are crucial. If the audio level is too high when recording, overload of the input electronics will cause audio distortion. If audio levels are too low, the signal-to-noise ratio deteriorates. Audio levels are typically indicated either by mechanical VU-meters or electronic LED bar graph meters.

Audio Mixing
The blending of two or more audio signals to generate a combined signal which is often used for audio dub. During video processing, audio mixing may be used to insert narration or background music.

Authoring System
Software which helps developers design interactive courseware easily, without the painstaking detail of computer programming.

Auto Assembly
In video editing, the editing system automatically records all edits listed in the edit decision list (EDL). This allows the operator to complete an edit decision list (EDL) and then let the editing system perform all of the recording automatically.

AVI
Audio Video Interleaved. File format for digital video and audio under Windows wherein audio and video data are stored in alternate blocks. File format is cross-platform compatible, allowing *.AVI video files to be played under other operating systems.

AVK
Audio Video Kernel. DVI system software designed to play motion video and audio across hardware and operating system environments.

AVSS
Audio-Video Support System. DVI system software for DOS. It plays motion video and audio.