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Glossary of Terms – CD/DVD

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.

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

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.

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.

B frame - One of three picture types used in MPEG video. B pictures are bidirectionally predicted, based on both previous and following pictures. B pictures usually use the least number of bits. B pictures do not propagate coding errors since they are not used as a reference by other pictures.

BLER - Block Error Rate. A measure of the average number of raw channel errors when reading or writing a disc.

Buffer - Temporary storage space in the memory of a device. Helps smooth data flow.

Byte - A unit of data or data storage space consisting of eight bits, commonly representing a single character. Digital data storage is usually measured in bytes, kilobytes, or megabytes.

CBR - Constant Bit Rate. Data compressed into a stream with a fixed data rate. The amount of compression (such as quantization) is varied to match the allocated data rate, but as a result quality may suffer during high compression periods. In other words, data rate is held constant while quality is allowed to vary.

CD-R - Compact Disc Recordable. An extension of the CD format allowing data to be recorded once on a disc by using dye-sublimation technology. Defined by the Orange Book standard.

CD-ROM XA - CD-ROM extended architecture. A hybrid version of CD allowing interleaved audio and video.

CD-ROM - Compact Disc Read-Only Memory. An extension of the Compact disc digital audio (CD-DA) format that allows computer data to be stored in digital format. Defined by the Yellow Book standard.

Chapter - In DVD-Video, a division of a title. Technically called a Part of Title PTT).

CIRC - Cross-interleaved Reed Solomon Code. An error-correction coding method, which overlaps small frames of data.

Clamping Area - The area near the inner hole of a disc where the drive grips the disc in order to spin it.

Codec - Coder/Decoder. Circuitry or computer software that encodes and decodes a signal.

Combo Drive - A DVD-ROM drive capable of reading and writing CD-R and CD-RW media. May also refer to a DVD-R or DVD-RW or DVD+RW drive with the same capability.

Compression - The process of removing redundancies in digital data to reduce the amount that must be stored or transmitted. Lossless compression removes only enough redundancy so that the original data can be recreated exactly as it was. Lossy compression sacrifices additional data to achieve greater compression.

CSS - Content Scrambling System. An encryption scheme designed to protect copyrighted material that resides on a disc by periodically scrambling the data using encryption keys.

Contrast - The range of brightness between the darkest and lightest elements of an image.

DAT - Digital Audio Tape. A magnetic audio tape format that uses PCM [Pulse Code Modulation] to store digitized audio or digital data.

dB - Decibel. A unit of measurement expressing ratios using logarithmic scales related to human aural or visual perception.

Decode - To reverse the transformation process of an encoding method.

Decoder - A circuit that decodes compressed audio or video, taking an encoded input stream and producing output such as audio or video. DVD players use the decoders to recreate information that was compressed by systems such as MPEG-2 and Dolby Digital

DVD - Digital Versatile Disc. Generic name for a family of related disc formats encompassing Video, Audio, and computer file storage on an optical disc format. They share common physical format and logical/file structures. They differ only in content.

Digitize - To convert analog information to digital information by sampling.

DLT - Digital Linear Tape. A digital archive standard using half-inch tapes, commonly used for submitting a premastered DVD disc image to a replication service.

Dolby Digital - A perceptual coding system for audio developed by Dolby Laboratories and accepted as an international standard.

DVD-R - DVD Recordable. DVD-R offers a write-once, read-many storage format akin to CD-R and is used to master DVD-Video and DVD-ROM discs, as well as for data archival and storage applications.

DVD-ROM - The base format of DVD. ROM stands for read-only memory, referring to the fact that standard DVD-ROM and DVD-Video discs can't be recorded on. A DVD-ROM can store essentially any form of digital data.

DVD-Video - A standard for storing and reproducing audio and video on DVD-ROM discs, based on MPEG video, Dolby Digital and MPEG audio, and other proprietary data formats.

Encode - To transform data for storage or transmission, usually in such a way that redundancies are eliminated or complexity is reduced. Most compression is based on one or more encoding methods. Data such as audio or video is encoded for efficient storage or transmission and is decoded for access or display.

Enhanced CD - A general term for various techniques that add computer software to a music CD, producing a disc that can be played in a music player or read by a computer.

File System - A defined way of storing files, directories, and information about them on a data storage device.

FireWire - A standard for transmission of digital data between external peripherals, including consumer audio and video devices. The official name is IEEE 1394, based on the original FireWire design by Apple Computer.

Frame - The piece of a video signal containing the spatial detail of one complete image; the entire set of scan lines. In an interlaced system, a frame contains two fields.

Full Motion Video - Video that plays at thirty frames per second [NTSC] or 25 frames per second [PAL].

GOP - Group of Pictures. In MPEG video, one or more I pictures followed by P and B pictures. A GOP is the atomic unit of MPEG video access.

HDCD - High Definition Compatible Digital. A proprietary method of enhancing audio on CDs.

HDTV - High-definition television. A video format with a resolution approximately twice that of conventional television in both the horizontal and vertical dimensions, and a picture aspect ratio of 16:9.

I Frame - In MPEG video, an intra picture that is encoded independent from other pictures. I pictures provide a reference point for dependent P pictures and B pictures and allow random access into the compressed video stream.

Interlace - A video scanning system in which alternating lines are transmitted, so that half a picture is displayed each time the scanning beam moves down the screen. An interlaced frame is made of two fields.

Interleave - To arrange data in alternating chunks so that selected parts can be extracted while other parts are skipped over, or so that each chunk carries a piece of a different data stream. In DVD, used for seamless multi-angle and Director's cut features, in which multiplexed streams are subsequently interleaved to allow seamless playback of alternate program material.

ISO 9660 - The international standard for the file system used by CD-ROM. Allows file names of only 8 characters plus a 3-character extension.

ISRC - International Standard Recording Code.

Jitter - Temporal variation in a signal from an ideal reference clock. There are many kinds of jitter, including sample jitter, channel jitter, and interface jitter.

JPEG - Joint Photographic Experts Group. The international committee that created the standard for compressing still images.

Land - The raised area of an optical disc.

Layer - The plane of a DVD disc on which information is recorded in a pattern of microscopic pits. Each substrate of a disc can contain one or two layers.

Layer 0 - In a dual-layer disc, this is the layer closest to the optical pickup beam and surface of the disc, and the first to be read when scanning from the beginning of the disc's data.

Layer 1 - In a dual-layer disc, this is the deeper of the two layers, and the second one to be read when scanning from the beginning of the disc's data.

Letterbox - The process or form of video where black horizontal mattes are added to the top and bottom of the display area in order to create a frame in which to display video using an aspect ratio different than that of the display. The letterbox method preserves the entire video picture, as opposed to pan & scan.

Linear PCM - A coded representation of digital data that is not compressed. Linear PCM spreads values evenly across the range from highest to lowest, as opposed to nonlinear PCM, which allocates more values to more important frequency ranges.

Lossless Compression - Compression techniques that allow the original data to be recreated without loss.

Lossy Compression - Compression techniques that achieve very high compression ratios by permanently removing data while preserving as much significant information as possible. Lossy compression includes perceptual coding techniques that attempt to limit the data loss to that which is least likely to be noticed by human perception.

Macrovision - An anti-taping process that modifies a signal so that it appears unchanged on most televisions but is distorted and unviewable when played back from a videotape recording. Macrovision takes advantage of characteristics of AGC circuits and burst decoder circuits in VCRs to interfere with the recording process.

MP3 - MPEG-1 Layer III audio. A perceptual audio coding algorithm. Not supported in DVD-Video or DVD-Audio formats.

MPEG audio - Audio compressed according to the MPEG perceptual encoding system. MPEG-1 audio provides two channels, which can be in Dolby Surround format. MPEG-2 audio adds data to provide discrete multichannel audio.

MPEG video - Video compressed according to the MPEG encoding system. MPEG-1 is typically used for low data rate video such as on a Video CD. MPEG-2 is used for higher-quality video, especially interlaced video, such as on DVD or HDTV.

MPEG - Moving Pictures Expert Group. An international committee that developed the MPEG family of audio and video compression systems.

MultiRead - A standard developed by the Yokohama group, a consortium of companies attempting to ensure that new CD and DVD hardware can read all CD formats.

Multisession - A technique in write-once recording technology that allows additional data to be appended after data written in an earlier session.

Noise Floor - The level of background noise in a signal or the level of noise introduced by equipment or storage media below which the signal can't be isolated from the noise.

Noise - Irrelevant, meaningless, or erroneous information added to a signal by the recording or transmission medium or by an encoding/decoding process. An advantage of digital formats over analog formats is that noise can be completely eliminated.

NTSC - National Television Systems Committee.

Operating System - The primary software in a computer, containing general instructions for managing applications, communications, input/output, memory and other low-level tasks. DOS, Windows, Mac OS, and UNIX are examples of operating systems.

P Frame - In MPEG video, a "predicted" picture based on difference from previous pictures. P pictures (along with I pictures) provide a reference for subsequent P pictures or B pictures.

PAL - Phase Alternate Line. A video standard used in Europe and other parts of the world for composite color encoding.

Pan & scan - The technique of reframing a picture to conform to a different aspect ratio by cropping parts of the picture.

PCM - Pulse Code Modulation. An uncompressed, digitally coded representation of an analog signal. The waveform is sampled at regular intervals and a series of pulses in coded form are generated to represent the amplitude.

Perceptual Coding - Lossy compression techniques based on the study of human perception. Perceptual coding systems identify and remove information that is least likely to be missed by the average human observer.

Phase Change - A technology for rewritable optical discs using a physical effect in which a laser beam heats a recording material to reversibly change an area from an amorphous state to a crystalline state, or vice versa. Continuous heat just above the melting point creates the crystalline state [an erasure] while high heat followed by rapid cooling creates the amorphous state [a mark].

Physical Format - The low-level characteristics of the DVD-ROM and DVD-Video standards, including pits on the disc, location of data, and organization of data according to physical position.

Pit - The area of an optical disc where a physical depression exists.

Pixel - The smallest picture element of an image [one sample of each color component] A single dot of the array of dots that makes up a picture.

PGC - Program Chain. In DVD-Video, a collection of programs, or groups of cells, linked together to create a sequential presentation.

Quantize - To convert a value or range of values into a smaller value or smaller range by integer division. Quantization is a primary technique of lossless encoding.

Raster - The pattern of parallel horizontal scan lines that makes up a video picture.

Red Book - The document first published in 1982 that specifies the original compact disc digital audio format developed by Philips and Sony.

Regional Code - A code identifying one of the world regions for restricting DVD-Video playback.

Resolution - A measurement of relative detail of a digital display, typically given in pixels of width and height.

ROM - Read-only memory.

Sampling - Converting analog information into a digital representation by measuring the value of the analog signal at regular intervals, called samples, and encoding these numerical values in digital form.

Signal-to-Noise Ratio - The ratio of pure signal to extraneous noise, such as tape hiss or video interference. Signal-to-noise ratio is measured in decibels (dB). Analog recordings almost always have noise. Digital recordings, when properly pre-filtered and not compressed, have no noise.

Stamping - The process of replicating optical discs by injecting liquid plastic into a mold containing a stamper.

Subpicture - Graphic bitmap overlays used in DVD-Video to create subtitles, captions, karaoke lyrics and menu highlighting effects.

Substrate - The clear polycarbonate disc onto which data layers are stamped.

Time Code - Information recorded with audio or video to indicate a position in time. Usually consists of values for hours, minutes, seconds, and frames. Also called SMPTE time code.

Track pitch - The distance, in the radial direction, between the centers of two adjacent tracks on a disc.

Transfer Rate - The speed at which a certain volume of data is transferred from a device such as a DVD-ROM drive to a host such as a personal computer. Usually measured in bits per second or bytes per second.

UDF - Universal Disc Format. Enables file interchange among different operating systems.

User Data - The data recorded on a disc independent of formatting and error-correction overhead. Each DVD sector contains 2048 bytes of user data.

VBR - Variable Bit Rate. Data that can be read and processed at a volume that varies over time. A data compression technique that produces a data stream between a fixed minimum and maximum rate. A constant level of compression is generally maintained, with the required bandwidth increasing or decreasing depending on the complexity of the data being encoded.

VCD - Video Compact Disc. Near-VHS-quality MPEG-1 video on CD. Used primarily in Asia.

VIDEO_TS - UDF file name used for video directory on disc volume. Files under this directory name contain pointers to the sectors on the disc, which hold the program streams.

VOB - Video Object. A single, complete file composed of multiplexed Video, Audio, Sub-picture, PCI and DSI elementary streams.

Yellow Book - The document produced in 1985 by Sony and Philips that extended the Red Book compact disc format to include digital data for use by a computer. Commonly called CD-ROM.

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