Compact disc

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 was developed in 1979 by Philips and Sony. Philips developed the general manufacturing process, based on their earlier Laserdisc technology, while Sony contributed the error-correction method.

Early compact disc prototypes produced by Philips were 115 mm in diameter, with a 14 bit resolution and a 60 minute capacity. Sony insisted on a 16 bit resolution and 74 minute capacity, which increased the size of the disc to 120 mm. The reason for the increase in capacity is often rumored to be to hold even the slowest versions of Beethoven's 9th Symphony, though this has never been verified.

Compact discs were first mass produced in 1982, in Langenhagen near Hanover, Germany.

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 . They are also available as 8 cm discs, a format which is mainly used for audio CD singles, much like the old vinyl single. The 8 cm 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 the Dutch electronics company Philips, who own the rights to the licensing of the 'CDDA' logo that appears on the disc. In broad terms the format is a two-channel 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 unusual sampling rate of 44.1 kHz is inherited from a method of converting digital audio into a 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. This technology could store 3 samples in a single horizontal line. A standard NTSC video signal has 245 usable lines per field, and 60 fields a second, which indeed works out at 44,100 samples/second. Similarly PAL has 294 lines and 50 fields, which also 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 debate over whether to use 14- or 16-bit samples when they designed the compact disc; 16 bits prevailed. Hence, the decision to use the 16-bit, 44.1 kHz sampling rate. The Sony PCM-1630, an early CD mastering machine, was just a modified 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 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.

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.

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 have 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.

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.