Definition: CD

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

(2) (Change Directory) A command in DOS, Windows, Unix and Linux that switches the current directory (folder) to another. See Chdir and 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 CD was a revolution in convenience compared to vinyl platters and turntables. However, high-speed Internet access and streaming severely impacted CDs, and by 2022, CD sales dropped considerably (see end of this definition).

A Plastic Disc
A CD disc is a plastic platter (120mm/4.75" diameter) recorded on one side with any song accessible by track number. The binary data in the tracks are recorded as microscopic pits in a groove that start at the center of the disc and spiral outward to the edge. Storage capacity is 650MB to 700MB. Other CD formats, such as CD-ROM and CD-I, all stem from the original Compact Disc (CD-DA). CDs can also be played in most CD-ROM and DVD drives. To learn how CDs are made, see CD-ROM. See binary.

Convert the Sound Waves
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 is reduced considerably when music compression formats are used (see MP3 and AAC). See sampling.

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 CDs (CD-ROM, CD-R, CD-RW). See CD-ROM and mini CD.

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

   Books         CD Type

   Red        CD-DA (Audio)
   Yellow     CD-ROM (Data)
   Orange     CD-R, CD-RW, Photo CD
   White      VCD (Video)
   Blue       CD Extra (Audio and data)
   Green      CD-I (Interactive)

What Happened to the Phonograph?
The music CD was introduced in the U.S. in 1983, and within five years, CDs and CD players exceeded the sales of LP records and turntables. See LP and turntable.

From Carved Sound to Pits
Unlike phonograph records with sound waves literally carved into the platter, the digital bits in CDs are microscopic pits covered by a protective plastic layer. Instead of a needle vibrating in a groove, a laser shines onto the pits, and the reflections are decoded. All variations of the CD use a spiral recording track like a phonograph record, but start at the center, not the outer 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 high-resolution audio). In the meantime, vinyl records and turntables at all quality levels are being manufactured, and this legacy industry is expected to persist (see audiophile).

Rise and Fall of CD Music
The following percentages were published on CNN in March 2023 showing the CD's share of music sales in the U.S. Today, audio streaming is the primary form of music distribution (see Spotify).

      Year   CD Share

      1983      1%
      1993     65%
      2003     95%
      2013     30%
      2022      3%