Image of a recordable compact disc (pencil included for scale)
A compact disc (or CD) is an optical disc used to store digital data, originally developed for storing digital audio.
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. Most compact discs have a diameter of 120 mm, which is designed to hold 74 minutes of audio, and in practice slightly more.
Compact disc technology was later adapted for use as a data storage device, known as a CD-ROM.
History
The Compact Disc is not an invention: it is the convergence of a series of enabling technologies, such as laser technology, mechanics, electronics, and coding technology. Therefore, nobody can claim that he or she is the inventor of the Compact Disc.
In the mid-1970s, using Laserdisc video 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 developed prototypes of digital audio discs as electronics and solid-state laser technology made it now possible to produce the required key components in mass quantities.
In 1979, Philips and Sony decided to join forces, and they set up a joint taskforce of engineers, whose mission it was to design the new digital audio disc. After a year of experiments and discussion the taskforce produced the 'Red Book '. Philips contributed the general manufacturing process, based on their earlier flopped 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 of the many technical decisions made, including the choice of the sampling frequency, playing time, and disc diameter.
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.
Physical details
Compact discs are made from a 1.2 mm thick disc of polycarbonate plastic coated with a much thinner 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.
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 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 causes 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.
Audio format
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 intelectual 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 3 samples 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 or 44.1 samples/sec when the Sony/Philips taskforce designed the compact disc; 16 bits and 44.1 samples/sec 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 compact disc specification recommends a constant linear velocity (CLV) of 1.22 m/s and a track pitch of 1.59 micrometres. This leads to a maximum audio program length of 74 minutes on a 120 mm disc, or around 650 MB of data on a CD-ROM. However, in order to allow for variations in manufacturing, a disc with data appearing slightly more densely is allowable. By deliberately making a disc with this density, we can increase capacity and remain within or near spec. Using a linear velocity of 1.1975 m/s and a track pitch of 1.497 micrometres leads to a new maximum capacity of 79 minutes and 40 seconds, or 702 MB. Although such discs allow for little variation in manufacturing, they are generally reliable and only a small number of players are known to reject them.
Some blank discs (see recordability) are available in 90 and even 99 minute configurations. Besides the increased density of their tracks, these run into two other technical problems. The first is that the maximum capacity a disc can declare itself as having is, according to the recordable CD specification, just under 80 minutes. The second is that timing markers on the disc with a value between 90 and 99 minutes are normally used to indicate to the player it is reading the beginning of the disc, not the end. These problems, as well as variable compatibility with CD recorders and software, mean discs larger than 80 minutes are generally regarded as a niche product.
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.
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 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 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.
Copy protection
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.
Other systems developed are Macrovision CDS-200 and Mediamax CD-3.
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
Name
Notwithstanding the variability of general usage between "disk" and "disc" [1], 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.
Secret bonus tracks and other nonstandard CD behaviors
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.
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.
See also
