Compact Disc



A Compact Disc (CD) is an optical disc used to store digital data, originally developed for storing digital audio. The CD, introduced in 1983, remains the standard playback format for commercial audio recordings as of mid-2006. An audio compact disc consists of one or more stereo tracks stored using 16-bit PCM coding at a sampling rate of 44.1 kHz. Standard compact discs have a diameter of 120 mm or 80 mm. The 120 mm discs can hold approximately 80 minutes of audio. The 80 mm discs, sometimes used for CD singles, 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 2006. 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.

The digital compact disc, now commonplace in stereos and computers, was invented in the late 1960s by James T. Russell. Russell was born in Bremerton, Washington in 1931. At age six, he invented a remote-control battleship, with a storage chamber for his lunch. Russell went on to earn a BA in physics from Reed College in Portland in 1953. Afterward, he went to work as a physicist in General Electric's nearby labs in Richland, Washington.

At GE, Russell initiated many experimental instrumentation projects. He was among the first to use a color TV screen and keyboard as the sole interface between computer and operator; and he designed and built the first electron beam welder. In 1965, when Columbus, Ohio - based Battelle Memorial Institute opened its Pacific Northwest Laboratory in Richland, Washington, Russell joined the effort as Senior Scientist. He already knew what avenue of research he wanted to pursue.

Russell was an avid music listener. Like many audiophiles of the time, he was continually frustrated by the wear and tear suffered by his vinyl phonograph records. He was also unsatisfied with their sound quality: his experimental improvements included using a cactus needle as a stylus. Alone at home on a Saturday afternoon, Russell began to sketch out a better music recording system --- and was inspired with a truly revolutionary idea.

Russell envisioned a system that would record and replay sounds without physical contact between its parts; and he saw that the best way to achieve such a system was to use light. Russell was familiar with digital data recording, in punch card or magnetic tape form. He saw that if he could represent the the binary 0 and 1 with dark and light, a device could read sounds or indeed any information at all without ever wearing out. If he could make the binary code compact enough, Russell saw that he could store not only symphonies, but entire encyclopedias on a small piece of film.

Battelle let Russell pursue the project, and after years of work, Russell succeeded in inventing the first digital-to-optical recording and playback system (patented in 1970). He had found a way to record onto a photosensitive platter in tiny "bits" of light and dark, each one micron in diameter; a laser read the binary patterns, and a computer converted the data into an electronic signal --- which it was then comparatively simple to convert into an audible or visible transmission.

This was the first compact disc. Although Russell had once envisioned 3x5-inch stereo records that would fit in a shirt pocket and a video record that would be about the size of a punch card, the final product imitated the phonographic disc which had been its inspiration. Through the 1970s, Russell continued to refine the CD-ROM, adapting it to any form of data. Like many ideas far ahead of their time, the CD-ROM found few interested investors at first; but eventually, Sony and other audio companies realized the implications and purchased licenses.

By 1985, Russell had earned 26 patents for CD-ROM technology. He then founded his own consulting firm, where he has continued to create and patent improvements in optical storage systems, along with bar code scanners, liquid crystal shutters, and other industrial optical instruments. His most revolutionary recent invention is a high-speed optical data recorder / player that has no moving parts. Russell earned another 11 patents for this "Optical Random Access Memory" device, which is currently being refined for the market.

James T. Russell has many interests beyond optical data devices. In fact, he has claimed, "I've got hundreds of ideas stacked up --- many of them worth more than the compact disc. But I haven't been able to work on them." Digital engineers and consumers alike will be lucky if he does find the time.

Compact discs are made from a 1.2 mm thick disc of very pure polycarbonate plastic. A thin layer of Super Purity Aluminium is applied (or rarely gold, used for its data longevity, such as in some limited-edition audiophile CDs) to the surface to make it reflective, 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. CD data is stored as a series of tiny indentations (pits), encoded in a tightly packed spiral track of pits 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 of 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 from its surrounding land. By measuring the intensity with a photodiode, one is able to read the data from the disc. The pits and lands themselves do not directly represent the zeroes 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 audio 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.

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-minute audio capacity and a 650 MB data or an 80-minute audio capacity and a 700 MB data. 80 mm discs are also available, Each such "miniCD" or "Maxi CD" can hold 21 minutes of music, or 184 MB of data (this form factor has also been called "CD3", since it is about three inches across).

The technical format of a audio compact disc (Compact Disc Digital Audio -- CDDA) is laid down in a document produced in 1980 by the format's joint creators, Sony and Philips. The document is known colloquially as the "Red Book" after the colour of its cover. The format is a two-channel 16-bit PCM encoding at a 44.1 kHz sampling rate. Four-channel sound is an allowed option within the Red Book format, but has never been implemented.

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 get the data from the recording studio to the CD manufacturer 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 six samples (three samples per each stereo channel) in a single horizontal line. A standard NTSC video signal has 245 usable lines per field, and 59.94 fields/s, which works out at 44,056 samples/s. Similarly PAL has 294 lines and 50 fields, which gives 44,100 samples/s. 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 44,100 samples/s when the Sony/Philips task force designed the compact disc; Philips thought that 14 bits were 'good enough' at the time, but Sony insisted on 16 bit, arguing that the format had to last at least 20 years and therefor be future-proof. In the end, 16 bits and 44.1 kilosamples per second prevailed.

The original target storage capacity for a CD was one hour of audio content, and a disc diameter of 115 mm was sufficient. However, according to Philips, Sony vice-president Norio Ohga suggested extending the capacity to 74 minutes to accommodate a complete performance of Beethoven's 9th Symphony on a single disk. Kees Immink of Philips refutes this. The extra playing time required changing to a 12 cm disc.

According to a Sunday Tribune interview the story is slightly more involved. At that time (1979) Philips owned Polygram, one of the world's largest distributors of music. Polygram had set up a large experimental CD disc plant in Hanover, Germany, which could produce huge amounts of CDs having, of course, a diameter of 11.5cm. Sony did not yet have such a facility. If Sony had agreed on the 11.5cm disc, Philips would have had a significant competitive edge in the market. Sony was aware of that, did not like it, and something had to be done. The long-playing time of Beethoven's Ninth imposed by Ohga was used to push Philips to accept 12cm, so that Philips' Polygram lost its edge on disc fabrication.

The 74-minute playing time of a CD, being more than that of most long-playing vinyl albums, was often used to the format's advantage during the early years when CDs and LPs vied for commercial sales. CDs would often be released with one or more bonus tracks, enticing consumers to buy the CD for the extra material. However, attempts to combine double LPs onto one CD occasionally resulted in an opposing situation in which the CD would actually offer fewer tracks than the LP equivalent.

The main parameters of the CD (taken from the September 1983 issue of the compact disc specification) are as follows:

* Scanning velocity: 1.2–1.4 m/s (constant linear velocity) - equivalent to approximately 500 rpm at the inside of the disc, and approximately 200 rpm at the outside edge.
* Track pitch: 1.6 μm.
* Disc diameter 120 mm.
* Disc thickness: 1.2 mm.
* Inner radius program area: 25 mm.
* Outer radius program area: 58 mm.

The program area is 86.05 cm², so that the length of the recordable spiral is 86.05/1.6 = 5.38 km. With a scanning speed of 1.2 m/s, the playing time is 74 minutes, or around 650 MB of data on a CD-ROM. If the disc diameter were 115 mm, the maximum playing time would have been 68 minutes, i.e., six minutes less. A disc with data appearing slightly more densely is tolerated by most players (though some old ones fail). Using a linear velocity of 1.2 m/s and a track pitch of 1.5 micrometre leads to a playing time of 80 minutes, or a capacity of 700 MB. Even higher capacities on non-standard discs (up to 99 minutes) are available at least as recordables, but generally the tighter the tracks are squeezed the worse the compatibility will be.

The smallest entity in the CD audio format is called a frame. A frame can accommodate six complete 16-bit stereo samples, i.e. 2×2×6 = 24 bytes. A frame comprises 33 bytes, of which 24 are audio bytes (six full stereo samples), eight CIRC-generated error correction bytes, and one subcode byte. The eight bits of a subcode byte are available for control and display. Under Eight-to-Fourteen Modulation (EFM) rules, each data/audio byte is translated into 14-bit EFM words, which alternates with 3-bit merging words. In total we have 33*(14+3) = 561 bits. A 27-bit unique synchronization word is added, so that the number of bits in a frame totals 588. The synchronization word cannot occur in the normal bit stream, and can thus be used to identify the beginning of a frame. Data in a CD-ROM are organized in both frames and sectors, where a CD-ROM sector contains 98 frames, and holds 98×24 = 2352 (user) bytes.

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 (CDR) drive.

Replicated CDs are mass-produced using a hydraulic injection mold, where a glass master disc is created and used to make "stampers." Small granules of raw plastic are fed into the barrel while under heat and increasing amount of pressure melt the plastic and force the liquified material into the mold cavity. Equipped with a metal stamper, with the pattern of pits (for pre-recorded discs) the mold closes, allowing the plastic to cool and harden. Once opened, the disc substrate is removed from the mold by a robotic arm, and a 15 mm diameter center hole (called a stacking ring) is removed.

Recordable compact discs, CD-Rs, 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 recorder changes the color 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. CD-R recordings are permanent. The resulting discs can be read by most CD-ROM drives and played in most audio CD players. Over time however (estimated to be about 5 years) the dye will fade causing read errors and data loss until the reading device cannot recover with error correction methods.

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 chemical properties of the alloy and hence change its reflectivity. 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, and so many CD audio players cannot read CD-RW discs, although the majority of stand-alone DVD players can.

The Red Book audio specification, except a simple 'anti-copy' bit in the subcode, does not include any serious copy protection mechanism. Starting in early 2002, attempts were made by record companies to market "copy-protected" non-standard compact discs. Philips has stated that such discs are not permitted to bear the trademarked Compact Disc Digital Audio logo because they violate the Red Book specification. Moreover, there has been great public outcry over copy-protected discs because many see it as a threat to fair use.Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts.
Virtual Magic is a human knowledge database blog. Text Based On Information From Wikipedia, Under The GNU Free Documentation License. Copyright (c) 2007 Virtual Magic. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".

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