Compact Disc

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 in late 1982, remains the standard physical medium for commercial audio recordings as of 2007. An audio CD consists of one or more stereo tracks stored using 16-bit PCM coding at a sampling rate of 44.1 kHz. Standard CDs have a diameter of 120 mm and can hold approximately 80 minutes of audio. There are also 80 mm discs, sometimes used for CD singles, which hold approximately 20 minutes of audio. Compact Disc technology was later adapted for use as a data storage device, known as a CD-ROM, and to include record-once and re-writable media (CD-R and CD-RW). CD-ROMs and CD-Rs remain widely used technologies in the personal-computer industry as of 2007. The CD and its extensions have been extremely successful: in 2004, the annual worldwide sales of CD-Audio, CD-ROM, and CD-R reached about 30 billion discs.

History

In 1979 Philips and Sony set up a joint task force of engineers to design the new digital audio disc. Prominent members of the task force were Joop Sinjou, Kees Immink and Toshi tada Doi. 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 the Eight-to-Fourteen Modulation, EFM, which offers both a long 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[1], 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[2]."

The Compact Disc reached the market in late 1982 in Asia and early the following year in other markets; for example, it was released in the United States in March, the first CDs available being 16 Japanese-made titles from CBS/Sony. 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 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 generalizing 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

Physical details

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 Super Purity Aluminium (or rarely gold, used for its data longevity, such as in some limited-edition audiophile CDs) is applied to the surface to make it reflective, and is protected by a film of lacquer. The lacquer is normally printed directly and not with an adhesive label. Common printing methods for compact discs are screen-printing and offset printing. CD data is stored as a series of tiny indentations (pits), encoded in a tightly packed 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 spacing between the tracks, the pitch, is 1.6 μm. 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 leads to a phase difference between the light reflected from a pit and that from its surrounding land. By measuring the intensity with a photodiode, it is possible to read the data 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 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.

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.

CD-ROM

When Sony and Philips invented the Compact Disc (CD) in the early 1980s, even they couldn't ever have imagined what a versatile carrier of information it would become. Launched in 1982, the audio CD's durability, random access features and audio quality made it incredibly successful, capturing the majority of the market within a few years. CD-ROM followed in 1984, but it took a few years longer to gain the widespread acceptance enjoyed by the audio CD. This consumer reluctance was mainly due to a lack of compelling content during the first few years that the technology was available. However, there are now countless games, software applications, encyclopaedias, presentations and other multimedia programs available on CD-ROM and what was originally designed to carry 74 minutes of high-quality digital audio can now hold up to 650MB of computer data, 100 publishable photographic scans, or even 74 minutes of VHS-quality full-motion video and audio. Many discs offer a combination of all three, along with other information besides.
Today's mass produced CD-ROM drives are faster and cheaper than they've ever been. Consequently, not only is a vast range of software now routinely delivered on CD-ROM, but many programs (databases, multimedia titles, games and movies, for example) are also run directly from CD-ROM - often over a network. The CD-ROM market now embraces internal, external and portable drives, caddy- and tray-loading mechanisms, single-disc and multichanger units, SCSI and EIDE interfaces, and a plethora of standards.
In order to understand what discs do what and which machine will read what, it is necessary to identify clearly the different formats. The information describing a CD standard is written on pages bound between the coloured covers of a book. A given standard is known by the colour of its cover. All CD-ROM drives are Yellow Book- and Red Book-compatible, along with boasting built-in digital-to-analogue converters (DACs) which enable you to listen to Red Book audio discs directly through headphone or line audio sockets.

Red Book (audio CD standard)

From Wikipedia, the free encyclopedia
(Redirected from CD Audio)

Red Book is the standard for audio CDs (Compact Disc Digital Audio system, or CDDA). It is named after one of a set of colour-bound books that contain the technical specifications for all CD and CD-ROM formats.
The first edition of the Red Book was released in June 1980 by Philips and Sony; it was adopted by the Digital Audio Disc Committee and ratified as IEC 908. The standard is not freely available and must be licensed from Philips. At the time of writing, the cost as per the relevant Philips order form (document no. 28/10/04-3122 783 0027 2) is US$5000. As of 2006, the IEC 908 document is also available as a PDF download for $210

Technical details

The Red Book specifies the physical parameters and properties of the CD, the optical "stylus" parameters, deviations and error rate, modulation system and error correction, and subcode channels and graphics.
It also specifies the form of digital audio encoding (2-channel signed 16-bit PCM sampled at 44100 Hz).
Bit rate = 44100 samples/s × 16 bit/sample × 2 channels = 1411.2 kbit/s (more than 10 MB per minute)
Sample values range from -32768 to +32767.
On the disc, the data is stored in sectors of 2352 bytes each, read at 75 sectors/s. Onto this is added the overhead of EFM, CIRC, L2 ECC, and so on, but these are not typically exposed to the application reading the disc.
By comparison, the bit rate of a "1x" data CD is defined as 2048 bytes/sector × 75 sectors/s = exactly 150 KiB/s = about 8.8 MB per minute.

Copy-protection (C)

Recently, some major recording publishers have begun to sell CDs that violate the Red Book standard for the purposes of copy prevention, using systems like Copy Control, or extra features such as DualDisc, which features a CD-layer and a DVD-layer. The CD-layer is much thinner, 0.9mm, than required by the Red Book, which stipulates 1.2mm. Philips and many other companies have warned them that including the Compact Disc Digital Audio logo on such non-conforming discs may constitute trademark infringement; either in anticipation or in response, the long-familiar logo is no longer to be seen on recent CDs, as well as stickers and warnings that the CD is not standard and may not play in all CD players.

Obsolescence

With the rise of MP3 players and high-capacity Secure Digital cards the usage of CD's has declined as Digital Audio Players are smaller in terms of physical size, can store more tracks in binary format and don't require the user to carry their CD collection around with them. Online music sites such as iTunes have also contributed to the demise of the CD . Purchasing music on a CD still remains a viable choice for buying new music although due to the decline of portable CD players and the rise of MP3 players users commonly rip the tracks from the CD then archive it, but most often with a loss of sound quality, thus turning sometimes excellent recordings into mediocre sound files.