CIRCULAR DISKS

 

 circular disks

A Compact Disc (or CD) is an optical disc used to store digital data, originally developed for storing digital audio. The CD, available on the market since late 1982, remains the standard playback medium for commercial audio recordings to the present day.

Standard CDs have a diameter of 120 mm and can hold up to 80 minutes of audio. There is also the Mini CD, with diameters ranging from 60 to 80 mm; they are sometimes used for CD singles, storing up to 24 minutes of audio.

The technology was later adapted and expanded to include data storage (CD-ROM), write-once audio and data storage (CD-R), rewritable media (CD-RW), SACD, VCD, SVCD, PhotoCD, PictureCD, CD-i, and Enhanced CD. CD-ROMs and CD-Rs remain widely used technologies in the computer industry. The CD and its extensions have been extremely successful: in 2004, worldwide sales of CD audio, CD-ROM, and CD-R reached about 30 billion discs.[1] By 2007, 200 billion CDs had been sold worldwide.

History

The compact disc is a successful spin-off of the much less successful Laserdisc technology. In 1979, Sony and Philips Consumer Electronics set up a joint task force of engineers to design a new digital audio disc. The task force, led by prominent members [[Kees Immink] and Toshitada Doi (土井利忠), progressed the research into laser technology and optical discs that had been started by Philips in 1977.[2] 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 video LaserDisc technology. Philips also contributed Eight-to-Fourteen Modulation (EFM), which offers both a long playing time and a high resilience against disc defects such as scratches and fingerprints, while Sony contributed the error-correction method, CIRC. The Compact Disc Story,[3] 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."[4]

The first Compact Disc for commercial release rolled off the assembly line on August 17, 1982, at a Philips factory in Langenhagen, near Hanover, Germany. The first title released was ABBA's The Visitors (1981).[5] CDs and Sony's CD player CDP-101 reached the market on October 1, 1982 in Japan, and early the following year in the United States and 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. As the price of players sank rapidly, the CD began to gain popularity in the larger popular and rock music markets. The first artist to sell a million copies on CD was Dire Straits, with their 1985 album Brothers in Arms.[6]

The CD was originally thought of 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.

A Compact Disc is made from a 1.2 mm thick disc of almost pure polycarbonate plastic and weighs approximately 16 grams. A thin layer of aluminium or, more rarely, gold is applied to the surface to make it reflective, and is protected by a film of lacquer. The lacquer is normally spin coated directly on top of the reflective layer. On top of that surface, the label print is applied. Common printing methods for CDs are screen-printing and offset printing.

CD data is stored as a series of tiny indentations (pits), encoded in a tightly packed spiral track molded 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 in length.

The spacing between the tracks, the pitch, is 1.6 μm. A CD is read by focusing a 780 nm wavelength (near infrared) semiconductor laser through the bottom of the polycarbonate layer. The change in height between pits and lands results in a difference in intensity in the light reflected. By measuring the intensity change with a photodiode, the data can be read from the disc.

The pits and lands themselves do not directly represent the zeros and ones of binary data. Instead, Non-return-to-zero, inverted (NRZI) encoding is used: 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 data stored on the disc.

While CDs are significantly more durable than earlier audio formats, they are susceptible to damage from daily usage and environmental factors. 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, or by careful polishing. Early music CDs were known to suffer from "CD rot" or "laser rot" where the internal reflective layer itself degrades. When this occurs the CD may become unplayable.

Disc shapes and diameters
A Mini-CD is 8 centimeters in diameter.

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 or 80-minute audio capacity and a 650 or 700 MB data capacity. This diameter has also been adopted by later formats, including Super Audio CD, DVD, HD DVD, and Blu-ray. 80 mm discs ("Mini CDs") were originally designed for CD singles and can hold up to 21 minutes of music or 184 MB of data but never really became popular. Today nearly all singles are released on 120 mm CDs, which is called a Maxi single.