compact disc

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compact disc

(CD), a small plastic disc used for the storage of digital data. As originally developed for audio systems, the sound signal is sampled at a rate of 44,100 times a second, then each sample is measured and digitally encoded on the 4 3-4 in (12 cm) disc as a series of microscopic pits on an otherwise polished surface. The disc is covered with a transparent coating so that it can be read by a laser beam. Since nothing touches the encoded portion, the CD is not worn out by the playing process. Introduced in 1982, the CD offered other advantages over the phonograph record and recording tape—smaller size, greater dynamic range, extremely low distortion—and met with rapid consumer acceptance; the CD became the music carrier of choice by 1991, when sales exceeded those of audiocassettes.

Other CD formats include CD-ROM [Compact Disc–Read Only Memory], a form of CD that is read (but not written to) by computer using a CD-ROM drive and that can contain computer programs and digitized text, sound, photographs, and video; CD-R [Compact Disc–Recordable] and CD-RW [Compact Disc–ReWritable], which can be written to one time and multiple times, respectively. Interactive CDs (CD-I, CDTV, and other formats) can store video, audio, and data. Photo CD is a format that holds digitized photographs and sound. There are also CD-ROMs that require special players with built-in microcomputers.

Other optical diskoptical disk,
any of a variety of information storage disks that are played or read using a laser. Optical disks include compact discs (CDs and CD-ROMs), laser discs (see videodisc), and digital versatile discs (or digital video discs; DVDs and DVD-ROMs).
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 formats include digital versatile (or video) discs and videodiscs. A digital versatile diskdigital versatile disc
or digital video disc
(DVD), a small plastic disc used for the storage of digital data. The successor media to the compact disc (CD), a DVD can have more than 100 times the storage capacity of a CD.
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 (DVD) holds far more information than a CD. DVD players are backward compatible to existing technologies, so they can also play a CD (or CD-ROM), but a CD player cannot be used with a DVD (or DVD-ROM). The videodiscvideodisc
or videodisk,
disk used with a special player and television to reproduce both pictures and sound. A videodisc player cannot record television programs off the air for later playback, unlike a videocassette recorder (VCR) or recordable DVD (see digital
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, or laser disk system, uses 12-in. (30-cm) disks for video recording. Its technology, unlike that of the CD, is an analog system that uses a laser to read a variable-width track, much like a conventional phonograph record.

compact disc

a small digital audio disc on which sound is recorded as a series of metallic pits enclosed in PVC; the disc is spun by the compact disc player and read by an optical laser system

Compact Disc

(storage)
(CD) (Not "disk", this spelling is part of the standard).

A 4.72 inch disc developed by Sony and Philips that can store, on the same disc, still and/or moving images in monochrome and/or color; stereo or two separate sound tracks integrated with and/or separate from the images; and digital program and information files.

The same fabrication process is used to make both audio CDs and CD-ROMs for storing computer data, the only difference is in the device used to read the CD (the player or drive).

CD Information Center.

CD

(1) See carrier detect, candela and continuous delivery.

(2) (Change Directory) A command in DOS/Windows that changes the current command line directory (see Chdir). CD also does the same thing in Unix/Linux (see Unix commands).

(3) (Compact Disc) An optical digital audio disc that contains up to 74 minutes of hi-fi stereo sound. Introduced in the U.S. in 1983, the disc is a plastic platter (120mm/4.75" diameter) recorded on one side, with individual tracks playable in any sequence. Its storage capacity is from 650MB to 700MB. Other forms of CDs, such as CD-ROM, CD-I and Video CD, all stem from the original Compact Disc-Digital Audio (CD-DA) format. CDs can be played in CD, CD-ROM, CD-R, CD-RW and most DVD drives. For more on how CDs are made, see CD-ROM.

Sound is converted into digital code by sampling the sound waves 44,056 times per second and converting each sample into a 16-bit number. CDs use 1.411 million bits for each second of stereo sound, although this bandwidth requirement is reduced considerably when music compression formats are used (see MP3 and AAC). The tracks are recorded as microscopic pits in a groove that starts at the center of the disc and spirals outward to the edge.

A Note on Terminology
In the early 1990s when CD-ROMs first became popular, "CD" meant music, and "CD-ROM" meant data. Today, "CD" refers to both audio CDs and data CD-ROMs, which also include CD-R and CD-RW media. See CD-ROM and mini CD.

The Books
Documentation for various CD formats are found in books commonly known by the color of their covers.

 Red Book    - CD-DA (Audio)

 Yellow Book - CD-ROM (Data)

 Orange Book - CD-R, CD-RW,
                 Photo CD (Recordable)

 White Book  - VCD (Video)

 Blue Book   - CD Extra (Audio and data)

 Green Book  - CD-I (Interactive)


What Happened to the Phonograph?


The audio CD was introduced in the U.S. in 1983, and within five years, CDs and CD players exceeded the sales of LPs and turntables.

From Carved Sound to Pits
Unlike phonograph records, in which the platter is literally carved with sound waves, CDs are recorded as microscopic pits covered by a clear, protective plastic layer. Instead of a needle vibrating in a groove, a laser shines onto the pits, and the reflections are decoded. Audio CDs, as well as all variations of the CD (CD-ROM, CD-R, etc.) use a spiral recording track like a phonograph record, but start at the center, not the edge. See analog audio.

Better Dynamic Range
Digital sound is cleaner than phonograph records because the numbers are turned into sound electronically. There are no needle pops and clicks and no tape hiss if the original recording was digital. In addition, the CD can handle a wider range of volume. A soft whisper can be interrupted by a loud cannon blast. If a phonograph record were recorded with that much "dynamic range," the needle would literally jump out of the groove.

Too Harsh for Critical Ears?
Pops and clicks aside, from the onset of audio CDs, many critics claimed digital sound was harsh and not as musical as the vinyl platter. DVD-Audio and SACD, two advanced digital formats with superior sound quality, came out in 1999, but neither one became popular (see DVD-Audio and SACD). See high-resolution audio.

In the meantime, turntables and vinyl records are still manufactured, although in smaller quantities, and this legacy industry is expected to persist. See turntable, laser turntable and USB turntable.
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