From ECDL.web

A Compact Disc (also known as a CD) is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage CD-ROM, write-once audio and data storage CD-R, rewritable media CD-RW, Video Compact Discs (VCD), Super Video Compact Discs (SVCD), PhotoCD, PictureCD, CD-i, and Enhanced CD. Sony first publicly demonstrated an optical digital audio disc in September 1976. In 1979 Philips publicly demonstrated a prototype of an optical digital audio disc. Later in 1979, Sony and Philips set up a joint task force of engineers to design a new digital audio disc and presented the Compact Disc standard a year later. Audio CDs and audio CD players have been commercially available since October 1982. From its origins as a musical format, CDs have grown to encompass other applications. In June 1985, the computer readable CD-ROM (read-only memory) and in 1990, CD-Recordable were introduced. CD-ROMs and CD-Rs remain widely used technologies in the computer industry. The CD and its extensions are successful: in 2004, worldwide sales of CD audio, CD-ROM, and CD-R reached about 30 billion discs. By 2007, 200 billion CDs had been sold worldwide.

[edit] Physical details

Standard CDs have a diameter of 120 millimetres (4.7 in) and can hold up to 80 minutes of uncompressed audio (700 MB of data). The Mini CD has various diameters ranging from 60 to 80 millimetres (2.4 to 3.1 in); they are sometimes used for CD singles, storing up to 24 minutes of audio or delivering device drivers.

A CD is made from 1.2 millimetres (0.047 in) thick, polycarbonate plastic and weighs 15–20 grams. From the center outward, components are: the center spindle hole (15 mm), the first-transition area (clamping ring), the clamping area (stacking ring), the second-transition area (mirror band), the program (data) area, and the rim. The inner program area occupies a radius from 25 to 58 mm.

A thin layer of aluminium or, more rarely, gold is applied to the surface making it reflective. The metal is protected by a film of lacquer normally spin coated directly on the reflective layer. The label is printed on the lacquer layer.

CD data are stored as a series of tiny indentations known as "pits", encoded in a spiral track moulded 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 distance between the tracks, the pitch, is ~1.6 µm.

[edit] Disc shapes and diameters

The digital data on a CD begins at the center of the disc and proceeds toward 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 millimetres (4.7 in) in diameter, with a 74- or 80-minute audio capacity and a 650 or 700 MB data capacity. This diameter has been adopted by subsequent formats, including Super Audio CD, DVD, HD DVD, and Blu-ray disc.
• 80 mm discs ("Mini CDs") were originally designed for CD singles and can hold up to 24 minutes of music or 210 MB of data but never became popular.

[edit] Logical formats

• Audio CD: the logical format of an audio CD (officially Compact Disc Digital Audio or CD-DA) is described in a document produced by the format's joint creators, Sony and Philips in 1980. The 74-minute playing time of a CD, which was longer than the 20 minutes per side typical of long-playing (LP) vinyl albums, was often used to the CD's advantage during the early years when CDs and LPs vied for commercial sales.
• Super Audio CD (SACD) is a high-resolution read-only optical audio disc format that provides much higher fidelity digital audio reproduction than the CD-DA. Introduced in 1999, it was developed by Sony and Philips, and competed with DVD-Audio, but neither has replaced audio CDs.
• CD-ROM: for the first few years of its existence, the Compact Disc was a medium used purely for audio. However, in 1985 the CD-ROM standard was established by Sony and Philips, which defined a non-volatile optical data computer data storage medium using the same physical format as audio Compact Discs, readable by a computer with a CD-ROM drive. A standard 120 mm, 700 MB CD-ROM can actually hold about 737 MB (703 MiB) of data with error correction (or 847 MB total). In comparison, a single-layer DVD-ROM can hold 4.7 GB of error-protected data, more than 6 CD-ROMs.
• Video CD (VCD, View CD, and Compact Disc digital video) and Super Video CD (SVCD) are standard digital formats for storing video media on a CD. VCDs are playable in dedicated VCD players, most modern DVD-Video players, personal computers, and some video game consoles. The VCD standard was created in 1993 by Sony, Philips, Matsushita, and JVC.
• Photo CD is a system designed by Kodak for digitizing and storing photos on a CD. Launched in 1992, the discs were designed to hold nearly 100 high quality images, scanned prints and slides using special proprietary encoding. They are intended to play on CD-i players, Photo CD players and any computer with the suitable software irrespective of the operating system.

[edit] Recordable CD

Recordable Compact Discs, CD-Rs, are injection molded with a "blank" data spiral. A photosensitive dye is then applied, after which the discs are metalized and lacquer-coated. The write laser of the CD recorder changes the color of the dye to allow the read laser of a standard CD player to see the data, just as it would with a standard stamped disc. The resulting discs can be read by most CD-ROM drives and played in most audio CD players.

CD-R recordings are designed to be permanent. Over time the dye's physical characteristics may change, however, causing read errors and data loss until the reading device cannot recover with error correction methods. The design life is from 20 to 100 years, depending on the quality of the discs, the quality of the writing drive, and storage conditions. However, testing has demonstrated such degradation of some discs in as little as 18 months under normal storage conditions.

[edit] ReWritable CD

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 properties (amorphous vs. crystalline) of the alloy, and hence change its reflectivity. A CD-RW does not have as great a difference in reflectivity as a pressed CD or a CD-R, and so many earlier CD audio players cannot read CD-RW discs, although most later CD audio players and stand-alone DVD players can.