Compact disc
From Wikipedia, the free encyclopedia.
Compact disc (or CD) is an optical disc used to store digital data, originally developed for storing digital audio. It is the standard playback format for commercial audio recordings today.
A standard compact disc, often known as an "audio CD" to differentiate it from later variants, stores audio data in a format compliant with the Red Book standard. An audio CD consists of up to 99 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, though 80-mm versions exist in circular and "business-card" forms. The 120-mm discs can hold 74 minutes of audio, and versions holding 80 or even 90 minutes have been introduced. The 80-mm discs are used as "CD-singles" or novelty "business-card CDs". They hold about 20 minutes of audio. Compact disc technology was later adapted for use as a data storage device, known as a CD-ROM.
Contents |
History
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 digitized PCM audio signals. At the end of the '70s, Philips, Sony, and other companies presented prototypes of digital audio discs.
In 1979 Philips and Sony decided to join forces, setting up a joint taskforce of engineers whose mission was to design the new digital audio disc. Prominent members of the taskforce 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 large 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."[1]
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 and its handling quality received particular praise. From its origins as a music format, Compact Disc has grown to encompass other applications. Two years later, in 1985, the CD-ROM (read-only memory) was introduced. With this it was now possible to disseminate massive amounts of computer data instead of digital sound. A user-recordable CD for data storage, CD-R, was introduced in the early 1990s, and it became the de facto standard for exchange and archiving of computer data and music. The CD and its later extensions have been extremely successful: in 2004 the annual worldwide sales of CD-Audio, CD-ROM, and CD-R reached about 30 billion discs.
Physical details
Compact discs are made from a 1.2 mm thick disc of polycarbonate plastic coated with a much thinner layer of super purity aluminium (or rarely, gold, used for its data longevity, such as in some limited-edition audiophile CDs) 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. 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 (See storage capacity; this form factor has also erroneously been called "CD5" since it is about five inches across). Such a standard disc weighs 15 grams. They are also available as 80-mm discs, 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 180 MB of data (this form factor has also been called "CD3", since it is about three inches across). Other unique shapes and smaller form factors have also been sold or given away as promotional items. Examples include Business Card CDs in the shape of a rectangular card, CDs shaped like the map of a country, etc.
There is a 15-mm hole in the centre of the disc, usually used by some form of clamp or clip device within the player to hold it in place and allow it to be rotated by a motor. The diameter of the hole is the same as that of a former Dutch 10-cent coin. Philips engineers at the time used the coin to determine the size of the hole, and the coin fits inside the hole perfectly.
The information on a standard CD is encoded as a spiral track of pits moulded into the top of the polycarbonate layer (The areas between pits are known as lands). Each pit is approximately 125 nm deep by 500 nm wide, and varies from 850 nm to 3.5 μm long. The spacing between the tracks is 1.6 μm. To grasp the scale of the pits and land of a CD, if the disc is enlarged to the size of a stadium, a pit would be approximately the size of a grain of sand. The spiral begins at the center of the disc and proceeds outwards to the edge, which allows the different size formats available.
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 is a sixth of the wavelength of the laser light, leading to a third-wavelength phase difference between the light reflected from a pit and from its surrounding land. The sixth, 125 nm, (and not a quarter) of the wavelength was chosen to have a good trade-off between the push-pull radial tracking signal and the full-aperture read-out signal. The interference reduces the intensity of the reflected light compared to when the laser is reflected by just a land. By measuring this 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, finally revealing the raw data stored on the disc.
A 74-minute (650MB) disc has roughly 333,000 sectors while the 80 minute (700MB) one has approximately 360,000 sectors allowing greater storage. 90 and 99 minute versions follow the same logical extension cramming more sectors at the cost of reliability across drives. 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. Consequently, discs are much easier to ruin by scratching their label side, whereas clear-side scratches can be repaired by refilling them with plastic of similar index of refraction.
Audio format
The format of the audio disc, known as the "Red Book"/Sony standard, was laid out by Sony and Philips in 1981. Philips is responsible for the licensing program of the intellectual property pertinent to the Compact Disc including the "Compact Disc Digital Audio" logo that appears on the disc. In broad terms the format is a two-channel (four-channel sound is an allowed option within the Red Book format, but has never been implemented) stereo 16-bit PCM encoding at a 44.1 kHz sampling rate.
Innovative error correcton
Reed-Solomon error correction allows the CD to be scratched to a certain degree and still be played back. The R-S code for the compact disc is CIRC.
Origins of 44.1 kHz sampling
The highest frequency that a human ear can detect is approximately 20,000 Hz. The compact disc was designed with the capability to contain the full audible range, and thus overcome the limitations of previous consumer-level audio carriers. The Nyquist–Shannon sampling theorem states that when quantizing a signal of a given bandwidth (20 kHz in this case, 0 - 20 kHz), in order to be able to reproduce the original waveform perfectly from the digitized information, the sampling rate used in the quantization has to be more than twice the amount of bandwidth. Thus, the sample rate needed to be over 40 kHz. As real-world components do not adhere to the ideal situation behind the theorem, and CD players designed with even lower-class analogue components might not achieve desired results (producing audible distortion), the sampling rate was raised well over the ideal minimum requirement.
The exact 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 store it 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 adapter.
PCM 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 kilo-samples/s prevailed.
- The Sony PCM-1610 and PCM-1630 are well known examples of PCM adaptors used in conjunction with the Sony U-matic VCR.
Storage capacity
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 about 500 rpm at the inside of the disc, or about 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 allowable. 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. This is the limit for most conventional audio CDs today.
Another technique to increase the capacity of a disc is store data in the lead out groove that is normally used to indicate the end of a disc, and an extra minute or two of recording is often possible. However, these discs can cause problems in playback when the end of the disc is reached.
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.
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. Data in a CD-ROM are organized in both frames and sectors. A CD-ROM sector contains 98 frames, and holds 98×24 = 2352 bytes. The CD-ROM is in essence a data disc, which cannot rely on error concealment, and it requires therefore a higher reliability of the retrieved data.
- In order to achieve improved error correction and detection, a CD-ROM has a third layer of Reed-Solomon error correction.
Note that the CIRC error correction system used in the CD audio format has two interleaved layers.
- A Mode-1 CD-ROM, which has the full third layer error correction capability, contains a net 2048 bytes of the available 2352 per sector.
- In a Mode-2 CD-ROM, which is mostly used for video files, there are 2336 user-available bytes per sector.
The net byte rate of a Mode-1 CD-ROM is 44.1k×2048/(6×98) = 153.6 kbyte/s. The playing time is 74 minutes, or 4440 seconds, so that the net capacity of a Mode-1 CD-ROM is 682 Mbyte.
Subcode
Besides digital audio, a CD contains digital data called "subcode", which is multiplexed with the digital audio.
- The data in a CD are arranged in frames. A frame comprises 33 bytes, of which 24 are audio bytes (six full stereo samples), eight error correction, CIRC-generated, bytes plus one subcode byte.
- The eight bits of a subcode byte are available for control and display.
- The eight bits are used as eight different subcoding channels, and given letters designating their usage: P, Q, …, W. Thus each channel has a bit rate of 7.35 (=44.1/6) kbit/s.
In each sector there are 2352 bytes (24×98) of audio content data and 96 bytes of subchannel data.
The 96 bytes of subchannel information in each sector contain four packets of 24 bytes apiece:
1 byte for command, 1 byte for instruction, 2 bytes for parityQ, 16 bytes for data, and 4 bytes parityP.
Each of the 96 subchannel data bytes can be thought of as being divided into eight bits. Each of these bits corresponds to a separate stream of information. These streams are called "channels", and are labeled starting with the letter P, like so:
| Channel | P | Q | R | S | T | U | V | W |
| Bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Channel P is a simple pause/music flag, which can be used for low-cost search systems. Quite a few players ignore it in favor of the Q Channel.
Channel Q is used for control purposes of more sophisticated players. It contains positioning information, the Media Catalog Number (MCN), and International Standard Recording Code (ISRC). The ISRC is used by the media industry, and contains information about the country of origin, the year of publication, owner of the rights, as well as a serial number, and some additional tags:
- Data
- This track contains Data (rather than audio). Can be used for muting in audio CD players.
- Copy Flag
- Used by the Serial Copy Management System to indicate permission to digitally copy the track.
- Four Channel Audio
- The track uses four channel audio. This is very rarely used on Compact Discs.
- Pre-Emphasis
- The audio track was recorded with pre-emphasis. Used very rarely on Compact Discs.
Channels R…W are unused by Red-Book compliant CDs, and have been used for extensions to the standard.
CD-Text
CD-Text is an extension of the Red Book standard for audio CDs. It allows for storage of additional information (e.g. album name, song name, and artist) on a standards-compliant audio CD. The information is stored in the lead-in area of the CD (there is roughly five kilobytes space there), or in the Subchannels R to W on the disc, which are not used on Red-Book compliant CDs. About 31 megabytes of information can be stored there. The text is stored in a format usable by the Interactive Text Transmission System (ITTS). ITTS is also used by Digital Audio Broadcasting or the MiniDisc.
Note that other extensions such as CD+G also use those subchannels to store graphics in.
The AAD, ADD, DDD code for audio CDs
Many CDs, especially classical music, but also many popular recordings (especially on early CDs), come with a three letter code printed on the back, where "A" stands for analog and "D" stands for digital. The first letter represents how the album was recorded, the second how it was mixed, and the third how it was transferred (inevitably a D, as the CD is a digital medium). As a result, almost all early CDs are "AAD" (analog recording and mixing, digital transfer to CD). Often this code was accompanied by a short description such as "Full Digital Recording" for DDD and "Digitally Mixed Analog Recording" for ADD.
Although experimental recordings exist from the 1960s, digital recording of classical and jazz music began to be made commercially in the early 1970s, pioneered by Japanese companies such as Denon; the first 16-bit PCM recording in the United States was made by Thomas Stockham at the Santa Fe Opera in 1976 on a Soundstream recorder. In most such cases, there was no mixing stage involved; a stereo digital recording was made and used unaltered as the master tape for subsequent commercial release. These, and other subsequent unmixed digital recordings are still described as DDD, as the technology involved is purely digital. (Likewise, unmixed analog recordings are usually described as ADD, to denote a single generation of analog recording).
The first digitally recorded (DDD) popular music album was Bop Till You Drop by Ry Cooder, recorded in late 1978; it was unmixed, being recorded straight to a two-track 3M digital recorder in the studio. Many other top recording artists, such as Stevie Wonder, were early adherents of digital recording; Wonder adopted the technology in early 1979 for Journey Through the Secret Life of Plants and all subsequent recordings. Others, such as former Beatles producer George Martin, felt that the multitrack digital recording technology of the early 1980s had not reached the sophistication of analog systems; however, he used digital mixing to eliminate the distortion and noise that an analog master tape would introduce (thus ADD). An early example of an analog recording that was digitally mixed is Tusk by Fleetwood Mac, from 1979.
By the time the compact disc was introduced worldwide, digital recording and mixing was becoming commonplace among recording artists and producers known for their interest in fidelity. Two examples from 1982 are Signals by Rush, and The Nightfly by Donald Fagen.
A few examples of DAD recordings exist, mostly of works that were originally recorded digitally but later remixed by artists who preferred to work with analog technology. A notable example is Herb Alpert's Rise album from 1979.
When it started making LPs and cassettes, the originally CD-only label Ryko extended this system to the other media, so that a digital recording on an LP would be DDA, and so forth.
CD-ROM
- Main article: CD-ROM
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 drive.
Recordability
Injection moulding is used to mass produce compact discs. A "stamper" is made from the original media (audio tape, data disc, etc.) by writing to a glass disc (referred to as a glass master) coated with a photosensitive dye with a laser. This dye is then etched, leaving the data track. It is then plated to make a positive version of the CD. Polycarbonate is liquified and injected into the mold cavity where the stamper transfers the pattern of pits and lands to the polycarbonate disc. The disc is then metallized with aluminum and lacquer coated.
Recordable compact discs 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 characteristics 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 standalone DVD players can.
Copy protection
The Red Book audio specification does not include any copy protection mechanism. Ripping is the process by which the contents of an audio disc is copied out verbatim to a duplicate disc or re-encoded into some other format, such as MP3.
Starting in early 2002, attempts were made by record companies to market "copy-protected" compact discs. Some of these deliberately introduced error patterns into audio tracks severe enough to defeat the error-correcting code (and hence defeat most CD-ROM drives attempting to copy the tracks as data), but not so disruptive as to prevent interpolation from working (hence allowing the same tracks to be played in audio mode without overly affecting fidelity).
Another copy protection method places a data track (usually containing bonus software for computer users) at the end of the disc and gives it an invalid size in the disc's table of contents. This is intended to prevent the data track from being ripped, but can be defeated by ignoring the table of contents and reading the disc sector by sector.
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. It also seems likely that Philips' new models of CD recorders will be designed to be able to record from these "protected" discs. However, there has been great public outcry over copy-protected discs because many see it as a threat to fair use.
Other systems developed are Macrovision CDS-200 and Mediamax CD-3.
In any case, even if a disc cannot be directly ripped, it can still be played in audio mode, and the audio thence captured. Any loss of sound quality caused by this method is generally considered negligible. This is commonly referred to as the analog hole.
- See also: Serial Copy Management System
Non-standard CD behaviors
Some commercially released audio discs have a "secret" bonus track. These may be an extension of the last audio track or a separate track hidden from the disc's table of contents. Either way, the hidden portion is heard when the disc is played to the end.
Other discs hide the extra material at the beginning of the disc. On most discs, the location of the first track listed in the table of contents immediately follows the table of contents itself. In this case, the hidden track is an unlisted track sandwiched between the two. To hear the hidden track, the listener must usually "rewind" the player past the beginning of the first listed track. Not all players allow this.
Name
Notwithstanding the variability of general usage between "disk" and "disc" [2], the customary spelling is "compact disc", rather than "compact disk". This may be in large degree due to its status as a Philips trademark under that spelling.
References
- Kees Immink, The Compact Disc Story, AES Journal, pp. 458-465, May 1998 [3].
- Kenneth C. Pohlmann (1992). The Compact Disc Handbook. Middleton, Wisconsin: A-R Editions. ISBN 895793008.
