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A Compact Disc (CD)
is an optical disc used to store digital data, originally developed
for storing digital audio. The CD, introduced in 1982, remains the
standard physical medium for commercial audio recordings as of 2006. An
audio compact disc consists of one or more stereo tracks stored using
16-bit PCM coding at a sampling rate of 44.1 kHz. Standard compact discs
have a diameter of 120 mm or 80 mm. The 120 mm discs can hold
approximately 80 minutes of audio. The 80 mm discs, sometimes used for CD
singles, hold approximately 20 minutes of audio. Compact disc technology
was later adapted for use as a data storage device, known as a CD-ROM, and
to include record-once and re-writable media (CD-R and CD-RW). CD-ROMs and
CD-Rs remain widely used technologies in the personal-computer industry as
of 2006. The CD and its extensions have been extremely successful: in
2004, the annual worldwide sales of CD-Audio, CD-ROM, and CD-R reached
about 30 billion discs.
History of the Compact
In 1979 Philips and Sony decided to join forces,
setting up a joint task force of engineers whose mission was to design the
new digital audio disc. Prominent members of the task force were Kees
Immink and Toshitada Doi. After a year of experimentation and discussion,
the taskforce produced the 'Red Book,' the Compact Disc standard. Philips
contributed the general manufacturing process, based on the video
LaserDisc technology. Philips also contributed the Eight-to-Fourteen
Modulation, EFM, which offers both a long playing time and a high
resilience against disc handling damage such as scratches and
fingerprints; while Sony contributed the error-correction method, CIRC.
The Compact Disc Story, told by a former member of the taskforce, gives
background information on the many technical decisions made, including the
choice of the sampling frequency, playing time, and disc diameter.
According to Philips, the Compact Disc was thus 'invented collectively by
a large group of people working as a
The Compact Disc reached the market in late 1982 in Asia and
early the following year in other markets. This event is often seen as the
'Big Bang' of the digital audio revolution. The new audio disc was
enthusiastically received, especially in the early-adopting classical
music and audiophile communities and its handling quality received
particular praise. The far larger popular and rock music industries were
slower to adopt the new format, especially in the huge consumer markets in
Europe and the United States, the first notable success being Brothers in
Arms in 1985.
The design of the CD was originally conceived as an evolution of
the gramophone record, rather than primarily as a data storage medium.
Only later did the concept of an 'audio file' arise, and the generalising
of this to any data file. From its origins as a music format, Compact Disc
has grown to encompass other applications. In June 1985, the CD-ROM
(read-only memory) and, in 1990, CD-Recordable were introduced, also
developed by Sony and Philips.
Compact discs are made from a 1.2 mm thick disc of
very pure polycarbonate plastic. A thin layer of Super Purity Aluminium
(or rarely gold, used for its data longevity, such as in some
limited-edition audiophile CDs) is applied to the surface to make it
reflective, which is protected by a film of lacquer. The lacquer can be
printed with a label. Common printing methods for compact discs are
silkscreening and offset printing. CD data is stored as a series of tiny
indentations (pits), encoded in a tightly packed spiral track of pits
moulded into the top of the polycarbonate layer. The areas between pits
are known as 'lands'. Each pit is approximately 100 nm deep by 500 nm
wide, and varies from 850 nm to 3.5 µm of length. The spacing between the
tracks, the pitch, is 1.6 µm. A CD is read by focusing a 780 nm wavelength
semiconductor laser through the bottom of the polycarbonate layer. The
difference in height between pits and lands leads to a phase difference
between the light reflected from a pit and from its surrounding land. By
measuring the intensity with a photodiode, one is able to read the data
from the disc. The pits and lands themselves do not directly represent the
zeroes and ones of binary data. Instead, Non-return-to-zero, inverted
encoding is used: a change from pit to land or land to pit indicates a
one, while no change indicates a zero. This in turn is decoded by
reversing the Eight-to-Fourteen Modulation used in mastering the disc, and
then reversing the Cross-Interleaved Reed-Solomon Coding, finally
revealing the raw audio data stored on the disc.
Pits are much closer to the label side of a disc so that defects
and dirt on the clear side can be out of focus during playback. Discs
consequently suffer more damage because of defects such as scratches on
the label side, whereas clear-side scratches can be repaired by refilling
them with plastic of similar index of refraction.
The digital data
on a CD begins at the center of the disc and proceeds outwards to the
edge, which allows adaptation to the different size formats available.
Standard CDs are available in two sizes. By far the most common is
120 mm in diameter, with a 74-minute audio capacity and a 650 MB
data or an 80-minute audio capacity and a 700 MB data. 80 mm
discs are also available, Each such 'miniCD' or 'Maxi CD' can hold
21 minutes of music, or 184 MB of data (this form factor has
also been called 'CD3', since it is about three inches
Audio Format or Audio CD
technical format of an audio compact disc (Compact Disc Digital Audio --
CDDA) is laid down in a document produced in 1980 by the format's joint
creators, Sony and Philips. The document is known colloquially as the 'Red
Book' after the colour of its cover. The format is a two-channel 16-bit
PCM encoding at a 44.1 kHz sampling rate. Four-channel sound is an allowed
option within the Red Book format, but has never been
The sampling rate of 44.1 kHz is inherited from a method of
converting digital audio into an analog video signal for storage on video
tape, which was the most affordable way to get the data from the recording
studio to the CD manufacturer at the time the CD specification was being
developed. A device that turns an analog audio signal into PCM audio,
which in turn is changed into an analog video signal is called a PCM
adaptor. This technology could store six samples (three samples per each
stereo channel) in a single horizontal line. A standard NTSC video signal
has 245 usable lines per field, and 59.94 fields/s, which works out at
44,056 samples/s/stereo channel. Similarly PAL has 294 lines and 50
fields, which gives 44,100 samples/s/stereo channel. This system could
either store 14-bit samples with some error correction, or 16-bit samples
with almost no error correction.
There was a long debate over whether to use 14 or 16 bit samples
and/or 44,056 or 44,100 samples/s when the Sony/Philips task force
designed the compact disc; Philips already developed a 14 bit D/A
converter, but Sony insisted on 16 bit. In the end, 16 bits and 44.1
kilosamples per second prevailed. Philips found a way to produce 16 bit
quality using their 14-bit DAC by using four times
Storage capacity and playing
The original target storage capacity for a CD was one hour of
audio content, and a disc diameter of 115 mm was sufficient. However,
according to Philips, Sony vice-president Norio Ohga suggested extending
the capacity to 74 minutes to accommodate a complete performance of
Beethoven's 9th Symphony on a single disc, however Kees Immink of Philips
denies this. The extra playing time subsequently required the change to a
12 cm disc.
According to a Sunday Tribune interview the story is
slightly more involved. At that time (1979) Philips owned Polygram, one of
the world's largest distributors of music. Polygram had set up a large
experimental CD disc plant in Hanover, Germany, which could produce huge
amounts of CDs having, of course, a diameter of 11.5cm. Sony did not yet
have such a facility. If Sony had agreed on the 11.5cm disc, Philips would
have had a significant competitive edge in the market. Sony was aware of
that, did not like it, and something had to be done. The long-playing time
of Beethoven's Ninth imposed by Ohga was used to push Philips to accept
12cm, so that Philips' Polygram lost its edge on disc
The 74-minute playing time of a CD, being more than that of most
long-playing vinyl albums, was often used to the format's advantage during
the early years when CDs and LPs vied for commercial sales. CDs would
often be released with one or more bonus tracks, enticing consumers to buy
the CD for the extra material. However, attempts to combine double LPs
onto one CD occasionally resulted in an opposing situation in which the CD
would actually offer fewer tracks than the LP equivalent.
Main physical parameters
The main parameters of the CD (taken from the September 1983
issue of the compact disc specification) are as follows:
* Scanning velocity: 1.2–1.4 m/s (constant linear velocity) -
equivalent to approximately 500 rpm at the inside of the disc, and
approximately 200 rpm at the outside edge. (A disc played from beginning
to end slows down during playback.)
* Track pitch: 1.6 µm.
diameter 120 mm.
* Disc thickness: 1.2 mm.
* Inner radius program
area: 25 mm.
* Outer radius program area: 58 mm.
spindle hole: 15 mm
The program area is 86.05 cm², so that the length of the
recordable spiral is 86.05/1.6 = 5.38 km. With a scanning speed of 1.2
m/s, the playing time is 74 minutes, or around 650 MB of data on a CD-ROM.
If the disc diameter were 115 mm, the maximum playing time would have been
68 minutes, i.e., six minutes less. A disc with data appearing slightly
more densely is tolerated by most players (though some old ones fail).
Using a linear velocity of 1.2 m/s and a track pitch of 1.5 micrometre
leads to a playing time of 80 minutes, or a capacity of 700 MB. Even
higher capacities on non-standard discs (up to 99 minutes) are available
at least as recordables, but generally the tighter the tracks are squeezed
the worse the compatibility will be.
Data structure of a CD
The smallest entity in the CD audio format is called a frame. A
frame can accommodate six complete 16-bit stereo samples, i.e. 2×2×6 = 24
bytes. A frame comprises 33 bytes, of which 24 are audio bytes (six full
stereo samples), eight CIRC-generated error correction bytes, and one
subcode byte. The eight bits of a subcode byte are available for control
and display. Under Eight-to-Fourteen Modulation (EFM) rules, each
data/audio byte is translated into 14-bit EFM words, which alternates with
3-bit merging words. In total we have 33*(14+3) = 561 bits. A 27-bit
unique synchronization word is added, so that the number of bits in a
frame totals 588. The synchronization word cannot occur in the normal bit
stream, and can thus be used to identify the beginning of a frame. Data in
a CD-ROM are organized in both frames and sectors, where a CD-ROM sector
contains 98 frames, and holds 98×24 = 2352 (user) bytes.
Current manufacturing processes allow an audio CD to contain up
to 77-78 minutes (varies from one replication plant to another) without
requiring the content creator to sign a waiver. Thus, in current practice,
maximum CD playing time has crept higher while maintaining acceptable
standards of reliability.
Wikipedia information about
CD. This article is licensed under the GNU Free Documentation License . It uses material
from the Wikipedia article 'Cd'
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