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 several stereo tracks stored using 16-bit PCM coding at a sampling rate of 44.1 kHz. Standard compact discs have a diameter of 120mm, though 80mm versions exist in circular and "business-card" forms. The 120mm discs can hold 74 minutes of audio, and versions holding 80 or even 90 minutes have been introduced. The 80mm discs are used as "CD-singles" or novelty "business-card CDs". They hold about 20 minutes of audio.
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 it was to design the new digital audio disc. Prominent members of the taskforce were Kees Immink and Toshi Doi. After a year of experiment and discussion the taskforce produced the 'Red Book', the Compact Disc standard. Philips contributed the general manufacturing process, based on the (unsuccessful) 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, (http://www.exp-math.uni-essen.de/~immink/pdf/cdstory.pdf) 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." (http://www.research.philips.com/newscenter/dossier/optrec/index.html)
The Compact Disc reached the market in 1983, and 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.
Compact discs are made from a 1.2 mm thick disc of polycarbonate plastic coated with a much thinner layer of Super Purity Aluminium (originally gold, which is sometimes still used for its data longevity) layer 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). They are also available as 80 mm discs, a format which is mainly used for audio CD singles, much like the old vinyl single. The 80 mm disc can hold 21 minutes of music, or 180 MB of data. (see miniCD)
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 ¼m long. The spacing between the tracks is 1.6 ¼m. 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 one quarter of the wavelength of the laser light, leading to a half-wavelength phase difference between the light reflected from a pit and from its surrounding land. The destructive interference this reduces the intensity of the reflected light compared to when the laser is focused on 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.
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).
- Track pitch: 1.6 micrometer.
- Disc diameter 120 mm.
- Disc thickness: 1.2 mm.
- Inner radius program area: 25 mm.
- Outer radius program area: 58 mm.
We find that the program area equals 86.05 square cm, so that the length of the recordable spiral is 86.05/1.6 = 5.38 km. Given a scanning speed of 1.2 m/s, we note that the playing time is 74 minutes, or around 650 MB of data on a CD-ROM. In case the disc diameter would have been 115 mm, the maximum playing time would have been 68 minutes, i.e., 6 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 disk, 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 format of the audio disc, known as the 'Red Book' 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 'CDDA' 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) stereo 16-bit PCM encoding at a 44.1 kHz sampling rate. Reed-Solomon error correction allows the CD to be scratched to a certain degree and still be played back.
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 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 adaptor. This technology could store 6 samples (3 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 a second, which works out at 44,056 samples/second. Similarly PAL has 294 lines and 50 fields, which gives 44,100 samples/second. 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 k or 44.1 k samples/s when the Sony/Philips taskforce designed the compact disc; 16 bits and 44.1 k 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.
Originally, audio-format CDs came with a three letter code on the back, where "A" stood for analog and "D" stood for digital. The first letter represented how the album had been recorded, the second how it had been mixed, and the third how it had been transferred. As a result, almost all early CDs are "AAD" (analog recording and mixing, digital transfer to CD) quality. The rock band Rush was the first musical act to record a full digital, "DDD," album—Signals.
Besides digital audio, a CD contains digital data called 'subcode', which is multiplexed with the digital audio. The data in a CD is 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 8 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 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:
- 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 digially copy the track.
- Four Channel Audio
- The track uses four channel audio. This is very rarely used on Compact Discs.
- 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 is an extension of the Red Book standard for audio CDs. It allows to store additional information (like album name, song name, artist,..) on a standards-compilant audio CD. The information is stored in the lead-in area of the CD (there's roughly 5 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.
Please note that other extensions like CD+G also use those subchannels to store graphics in.
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.
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-I 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, we have 2336 user-available bytes per sector. The net byte rate of a Mode-1 CD-ROM is 44.1*2048/(6*98) = 153.6 byte/s. The playing time is 74 minutes, or 4440 seconds, so that the net capacity of a Mode-I CD-ROM equals 682 Mbyte.
Injection molding 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.
However, there are also CD-recordable discs which can be recorded by a laser beam using a CD-R writer (most often connected to a computer, though standalone units are also available) and can be played on most compact disc players. CD-R recordings are permanent and cannot be recorded more than once, so the process is also called "burning" a CD. (See also CD burner and overburning.)
CD-RW is a medium that allows multiple recordings on the same disc over and over again. 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, so many CD audio players cannot read CD-RW discs, although the majority of standalone DVD players can.
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 burner 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.
The Red Book audio specification does not include any copy protection mechanism and discs can be easily duplicated or the contents "ripped" to a computer. Starting in early 2002, attempts were made by record companies to market "copy-protected" compact discs. These rely on deliberate errors being introduced into the data recorded on the disc. The intent is that the error-correction in a music player will enable music to be played as normal, while computer CD-ROM drives will fail with errors. This approach is the subject of an evolutionary arms race or cat-and-mouse game — not all current drives fail, and copying software is being adapted to cope with these damaged data tracks. The recording industry then works on further approaches.
Some copy protection mechanism may mean that the disc is in truth a mixed-mode CD, which has a data track after all audio tracks. The data track contains whatever the manufacturer chose to make available to computer users. The Table of Contents of the disc however specifies an invalid size of the data track, which prevents the disc from being copied. Copying all the audio tracks usually works fine though.
Philips has stated that such discs, which do not meet the Red Book specification, are not permitted to bear the trademarked Compact Disc Digital Audio logo. 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.
In any case, even if a disc cannot be ripped directly, it can still be played on a CD player connected to a computer via analogue inputs. 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
Secret bonus tracks and other nonstandard CD behaviors
Main article: hidden track
Some commercial CDs are released with "secret" bonus tracks. On some of these discs, the bonus material follows the last track listed on the cover, often after a silence. The extras and silence may be part of the last song's track or in a separate track. Either way, the hidden portion is heard any time the disc is allowed to play to the end. A good example of this is the presence of the track "Her Majesty" in The Beatles' Abbey Road CD, which is present directly after 20 seconds of total silence after the final listed track, "The End".
Other discs hide the extra material at the beginning of the disc. The "table of contents" at the beginning of an audio disc informs the player where each track starts. The location of the first track usually follows the table at the next allowable position. The first listed track's location in the table of contents is after the end of the unlisted track. To hear the unlisted track, the listener must typically "rewind" the player past the beginning of the first listed track. Not all players allow this.
Notwithstanding the variability of general usage between "disk" and "disc"  (http://www.bartleby.com/61/16/C0521600.html), 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.