AAC
Advanced audio coder. An audio-encoding standard for MPEG-2 that is not backward-compatible with MPEG-1 audio.

AC
Alternating current. An electric current that regularly reverses direction. Adopted as a video term for a signal of non-zero frequency. Compare to DC.

AC-3
The former name of the Dolby Digital audio-coding system, which is still technically referred to as AC-3 in standards documents. AC-3 is the successor to Dolby's AC-1 and AC-2 audio coding techniques.

access time
The time it takes for a drive to access a data track and begin transferring data. In an optical jukebox, the time it takes to locate a specific disk, insert it in an optical drive, and begin transferring data to the host system.

ActiveMovie
The former name for Microsoft's DirectShow technology.

ADPCM
Adaptive differential pulse code modulation. A compression technique which encodes the difference between one sample and the next. Variations are lossy and lossless.

AES
Audio Engineering Society.

AES/EBU
A digital audio signal transmission standard for professional use, defined by the Audio Engineering Society and the European Broadcasting Union. S/P DIF is the consumer adaptation of this standard.

AGC
Automatic gain control. A circuit designed to boost the amplitude of a signal to provide adequate levels for recording. Also see Macrovision.

aliasing
A distortion (artifact) in the reproduction of digital audio or video that results when the signal frequency is more than twice the sampling frequency. The resolution is insufficient to distinguish between alternate reconstructions of the waveform, thus admitting additional noise that was not present in the original signal.

Alpha Channel
Alpha Channel, in the case of computer-based systems, is the function known as key channel or key level in conventional video technology. Masks can be superimposed on a picture signal, thus enabling different keying tricks such as inserting image signals or executing of Chroma keying. The alpha channel includes transparency information of a picture file by means of which the transparency between front and background of a picture can be controlled.

AMGM_VOBS
Video Object Set for Audio Manager Menu.

analog
A signal of (theoretically) infinitely variable levels. Compare to digital.

Anchor Point
One of a specified set of logical sector numbers at which descriptor, that identify an extent of a Volume Descriptor Sequence, may be recorded.

Angle Menu
Menu used to select the Angle number.

angle
An angle is a scene recorded from different viewpoints. Each angle is equal in time length and an Angle Block may contain up to nine (9) angles.

ANSI
American National Standards Institute (see http://www.ansi.org).

Antialiasing
Smoothing or reducing disturbing picture effects. By means of calculation of intermediate values along the sharp edges of types and graphics, these edges can be smoothed out, thus generating a smoother picture. The pixel structure along tilted or bent edges is mixed with the surrounding colors. When creating DVD Menu text, antialiasing must not be used.

AOTT_AOBS
Audio Object Set for Audio Only Title.

application format
A specification for storing information in a particular way to enable a particular use.

artifact
An unnatural effect not present in the original video or audio, produced by an external agent or action. Artifacts can be caused by many factors, including digital compression, film-to-video transfer, transmission errors, data readout errors, electrical interference, analog signal noise, and analog signal crosstalk. Most artifacts attributed to the digital compression of DVD are in fact from other sources. Digital compression artifacts will always occur in the same place and in the same way. Possible MPEG artifacts are mosquitoes, blocking, and video noise.

aspect ratio
The width-to-height ratio of an image. A 4:3 aspect ratio means the horizontal size is a third again wider than the vertical size. Standard television ratio is 4:3 (or 1.33:1). Widescreen DVD and HTDV aspect ratio is 16:9 (or 1.78:1). Common film aspect ratios are 1.85:1 and 2.35:1. Aspect ratios normalized to a height of 1 are often abbreviated by leaving off the :1.

Assemble
One of the two editing modes that are possible with video tapes. All tracks on the tape are added free of disturbances at the cutting point, but all tracks are newly written. The other editing method is known as Insert Edit.

ASV
(Audio Still Video) A still picture on a DVD-Audio disc.

ASVOBS
Audio Still Video Object Set.

ATAPI
Advanced Technology Attachment (ATA) Packet Interface. An interface between a computer and its internal peripherals such as DVD-ROM drives. ATAPI provides the command set for controlling devices connected via an IDE interface. ATAPI is part of the Enhanced IDE (E-IDE) interface, also known as ATA-2. ATAPI was extended for use in DVD-ROM drives by the SFF 8090 specification.

ATM
Asynchronous Transfer Mode. A network protocol which enables the operation of networks that allow high-speed data transfer. ATM is capable of transmitting digital high quality video or extremely large files.

ATSC
Advanced Television Systems Committee. In 1978, the Federal Communications Commission (FCC) empaneled the Advisory Committee on Advanced Television Service (ACATS) as an investigatory and advisory committee to develop information that would assist the FCC in establishing an advanced broadcast television (ATV) standard for the United States. This committee created a subcommittee, the ATSC, to explore the need for and to coordinate development of the documentation of Advanced Television Systems. In 1993, the ATSC recommended that efforts be limited to a digital television system (DTV), and in September 1995 issued its recommendation for a Digital Television System standard, which was approved with the exclusion of compression format constraints (picture resolution, frame rate, and frame sequence).

ATV
Advanced television. TV with significantly better video and audio than standard TV. Sometimes used interchangeably with HDTV, but more accurately encompasses any improved television system, including those beyond HDTV. Also sometimes used interchangeably with the final recommended standard of the ATSC, which is more correctly called DTV.

AUDIO_TS
UDF file name used for audio directory on disc volume.

Audio Channel Number
These are consecutive numbers assigned to the Audio channel of the audio stream. They range from `0' to `7' in the description are of the video title set manager area. ACH0 and ACH1 are assigned to Left channel and Right channel respectively for 2 channel stereo audio signals.

Audio Menu
Menu used to select the Audio stream

Audio Stream Number
These are consecutive numbers assigned to the Audio streams for a Title in a VTS. These range from `0' to `7' in the order described in the video title set manager area. For menus the number of audio streams is limited to 0 or 1.

authoring
For DVD-Video, authoring refers to the process of designing, creating, collecting, formatting, and encoding material. For DVD-ROM, authoring usually refers to using a specialized program to produce multimedia software.

autoplay (or automatic playback)
A feature of DVD players which automatically begins playback of a disc if so encoded.

AVI
Audio Video Interleave. A data format developed by Microsoft for digital video. Compressed picture and sound data are interleaved in such a way that they proceed synchronous to one another.