Compact Disc
For the Public Image Ltd album called "Compact Disc" on certain editions, see Album (album). |
The Compact Disc logo was inspired by that of the previous Compact Cassette. It may only be used on discs that comply with the Red Book specifications |
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 playback format for commercial audio recordings as of mid-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.
In the early
1970s, using video
Laserdisc technology,
Philips' researchers started experiments with "audio-only" optical discs, initially with wideband frequency modulation
FM and later with digitized
PCM audio signals. The compact disc was thus developed by Philips from its own 12 inch Philips
LaserVision discs. At the end of the 1970s,
Philips,
Sony, and other companies presented prototypes of digital audio discs.
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 team.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 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 is applied (or rarely
gold, used for its data longevity, such as in some limited-edition audiophile CDs) 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 represent the zeroes and ones of
binary data. Instead 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.
|
A Mini-CD is 8 centimeters in diameter |
The
Red Book specifies many mechanical parameters including the pit depth. It specifies that the pit depth should be less than (and, thus, not equal to) 130 nm. However, the Red Book implicitly specifies the pit depth by specifying the strength of both the push-pull radial tracking signal and full aperture detection signal. For a maximum full aperture signal, the optimum pit depth is λ/4n = 130 nm (refractive index n=1.5, λ=780 nm). For a maximum push-pull radial tracking signal the best choice is λ/8n = 65 nm. Most CD manufacturers, dependent on the exact pit geometry such as the slope of the pit edges etc, choose a pit depth of around 90-100 nm, (which is around λ/6n) yielding a sound trade-off between the quality of the push-pull radial tracking and full aperture detection signal.
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.
In reverse fashion to that of a vinyl record, 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. 80mm discs are also available, a format which is mainly used for audio CD singles in some regions (e.g.
Japan), much like the old
vinyl single. 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 across).
Other non-standard shapes and smaller form factors have also been sold or given away as promotional items. All of these unique shapes must fit within the 120mm ring or the 80mm ring that is standard on tray drives. Any shape falling between the 80mm ring and the 120mm ring of a tray drive, such as a credit card-sized CD business cards, must include a method of locating the disc in the tray during load and unload. This is usually a circular ridge on their underside. Irregularly shaped, non
rotationally symmetric discs with an offset
centre of mass may cause damaging vibration if played in computer CD drives, which may operate at a much higher rotational velocity than stand-alone audio CD players. Even symmetrical rectangular discs often cause far more vibration than standard circular ones.
The format of the audio disc, known as the "
Red Book" standard, was laid out by Sony and Philips in 1981. 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 implemented.
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. Similarly
PAL has 294 lines and 50 fields, which gives 44,100 samples/s. 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; 16 bits and 44.1 kilosamples per second prevailed.
Storage capacity and playing time
The original target storage capacity for a CD was one hour of audio content, and a disc diameter of 11.5 cm 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 disk.[
1] Kees Immink of Philips refutes this.[
2] The extra playing time required changing to a 12 cm disc.
According to a Sunday Tribune interview [
3] 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 fabrication.
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: 1.2â€"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.
*Track pitch: 1.6 μm.
*Disc diameter 120 mm.
*Disc thickness: 1.2 mm.
*Inner radius program area: 25 mm.
*Outer radius program area: 58 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 with will be.
Data structure
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.
For its first few years of existence, the compact disc was purely an audio format. However, in
1985 Yellow Book CD-ROM standard was established by
Sony and
Philips, which defined a non-volatile optical data
storage medium using the same physical format as audio
compact discs, readable by a computer with a CD-ROM (CDR) drive.
Pre-pressed CDs are mass-produced by a process of stamping, where a glass master disc is created and used to make "stampers", which in turn are used to manufacture multiple copies of the final disc with the pits already present.
Recordable compact discs,
CD-Rs, are injection molded with a "blank" data spiral. A photosensitive dye is then applied, and then the discs are metallized and lacquer coated. The write laser of the
CD recorder changes the color of the dye to allow the read laser of a standard CD player to see the data as it would an injection molded compact disc. CD-R recordings are permanent. The resulting discs can be read by most CD-ROM drives and played in most audio CD players.
CD-RW is a re-recordable medium that uses a metallic alloy instead of a dye. The write laser in this case is used to heat and alter the chemical properties of the alloy and hence change its reflectivity. A CD-RW does not have as great a difference in the reflectivity of lands and bumps as a pressed CD or a CD-R, and so many CD audio players cannot read CD-RW discs, although the majority of stand-alone
DVD players can.
The
Red Book audio specification, except a simple 'anti-copy' bit in the subcode, does not include any serious
copy protection mechanism. Starting in early
2002, attempts were made by record companies to market "copy-protected" non-standard compact discs. Philips has stated that such discs are not permitted to bear the
trademarked
Compact Disc Digital Audio logo because they violate the Red Book specification. Moreover, there has been great public outcry over copy-protected discs because many see it as a threat to
fair use.
*
Kees Immink, The Compact Disc Story, Journal of the Audio Engineering Society, 46(5), pp. 458-465, May 1998.
* Kenneth C. Pohlmann (1992).
The Compact Disc Handbook. Middleton, Wisconsin: A-R Editions. ISBN 895793008.
*
Sony's official CD history*
Compact disc player*
SACD*
DVD-Audio*
CD-ROM*
CD-R*
CD-RW*
CD Text*
CD Video*
Video Single Disc*
Audio format*
Audio storage*
Rainbow Books**
Red Book (audio CD standard)**
Yellow Book (CD-ROM standards)*
CD+G*
ECD*
Video CD*
SVCD*
Jewel case*
miniCD*
Optical disc**
DVD*
SPARS Code for information on the three-letter codes AAD, ADD, and DDD
