See carrier detect
and continuous delivery
irectory) 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
isc) 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
). 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 CDs (CD-ROM, CD-R, CD-RW). See CD-ROM
and mini CD
Documentation for CD formats are found in books commonly known by the color of their covers.
Books CD Type
Red Book CD-DA (Audio)
Yellow Book CD-ROM (Data)
Orange Book CD-R, CD-RW, Photo CD
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 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 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
). 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
Rise and Fall of CD Music Sales
The following percentages were published on CNN in March 2023 showing the CD's share of music sales in the U.S. Today, downloads and especially streaming are the major forms of music distribution.
Year CD Share