Cd

Cd, chemical symbol

Cd, symbol for the element cadmium.

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CD, abbreviation for compact disc

CD: see compact disc.

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(1) See also carrier detect and candela.

(2) (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. It requires approximately 1.5 million bits of storage for each second of stereo sound. The audio 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.

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.

Carved Sound
Unlike phonograph records, in which the platter is literally carved with sound waves, CDs are recorded in binary digits 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.

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 has become very popular (see DVD-Audio and SACD).

In the meantime, turntables and vinyl records are still being manufactured in small quantities, and this legacy industry is expected to persist for a while. See turntable, laser turntable and USB turntable.

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1.		CD - Compact Disc
2.	(operating system)	cd - change directory.
3.	(networking)	cd - The country code for the Democratic Republic of the Congo (formerly Zaire, zr).