Information AboutCompact Disc |
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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 Recording s as of 2007 . An audio CD consists of one or more Stereo Track s 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. The 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 respectively). CD-ROMs and CD-Rs remain widely used technologies in the computer industry as of 2007. The CD and its extensions have been extremely successful: in 2004, the worldwide sales of CD audio, CD-ROM, and CD-R reached about 30 billion discs. In 2007, on CDs 25th birthday, 200 billion legitimate CD-audio discs have been sold worldwide since the introduction. Compact disc hits 25th birthday HISTORY 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).2 CDs reached the market in late 1982 in Asia , 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 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-R ecordable were introduced, also developed by Sony and Philips . While CDs are significantly more durable than earlier audio formats, they are susceptible to damage from daily usage and environmental factors. Libraries and archives should enact Optical Media Preservation procedures to ensure continued usability. 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 Aluminium (or, more rarely, Gold , used for its 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 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 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 , 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 , or by polishing. Disc shapes and diameters 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. 80 mm discs (" Mini CD s") 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 . "Shaped CD" Novelty Shaped CD s are also available in a number of shapes and sizes, and are mostly used for marketing. The most common variant is a "business card" CD, a CD-single with portions removed at the top and bottom to more closely resemble the form-factor of a Business Card . AUDIO FORMAT The technical format of an audio compact disc (officially compact disc digital audio or CD-DA) 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 color 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/stereo channel. Similarly, PAL has 294 lines and 50 fields, which gives 44,100 samples/s/stereo channel. 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 44,056 or 44,100 samples/s, when the Sony/Philips task force designed the compact disc; Philips had already developed a 14 bit D/A Converter , but Sony insisted on 16 bit. In the end, 16 bits and 44.1 kilosamples per second prevailed. Philips found a way to produce 16-bit quality using their 14-bit DAC by using four times Oversampling . Storage capacity and playing time The original target storage capacity for a CD was an hour of audio content, and a disc diameter of 115 mm was sufficient for this. 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 ; 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 plant in Hanover , Germany , which could produce huge amounts of CDs having, of course, a diameter of 115 mm. Sony did not yet have such a facility. If Sony had agreed on the 115 mm disc, Philips would have had a significant competitive edge in the market. Sony decided that something had to be done. The long playing time of Beethoven's Ninth imposed by Ohga was used to push Philips to accept 120 mm, 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 CD’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. An example is the 1987 album '' reissue of Fleetwood Mac 's '' Tusk '' album, which substituted the long album version of "Sara" with the shorter single version. Enough complaints were lodged to eventually convince Warner Bros. to remaster the album in the mid-1990s with the original contents intact. Main physical parameters The main parameters of the CD (taken from the September 1983 issue of the Compact Disc Specification ) are as follows:
The program area is 86.05 cm&2 and the length of the recordable spiral is 86.05 cm&2 / 1.6 μm = 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 only 115 mm, the maximum playing time would have been 68 minutes, i.e., six minutes less. A disc with data packed 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 μm 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. Data structure The smallest entity in a CD is called a ''frame''. A frame consists of 33 bytes and contains six complete 16-bit stereo samples (2 bytes × 2 channels × six samples equals 24 bytes). The other nine bytes consist of eight Cross-Interleaved Reed-Solomon Coding error correction bytes and one Subcode byte, used for control and display. Each byte is translated into a 14-bit word using Eight-to-Fourteen Modulation , 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 (of which only 192 bits are music). These 588-bit frames are in turn grouped into sectors. Each sector contains 98 frames, totalling 98 × 24 = 2352 bytes of music. The CD is played at a speed of 75 sectors per second, which results in 176,400 bytes per second. Divided by 2 channels and 2 bytes per sample, this results in a sample rate of 44,100 samples per second. For CD-ROM data discs, the physical frame and sector sizes are the same. Since error concealment cannot be applied to non-audio data in case the CIRC error correction fails to recover the user data, a third layer of error correction is defined, reducing the payload to 2048 bytes per sector for the Mode-1 CD-ROM format. To increase the data-rate for Video CD , Mode-2 CD-ROM, the third layer has been omitted, increasing the payload to 2336 user-available bytes per sector, only 16 bytes (for synchronisation and header data) less than available in Red-Book audio. "Frame" For the Red Book stereo audio CD, the time format is commonly measured in minutes, seconds and frames (mm:ss:ff), where one frame corresponds to one sector, or 1/75th of a second of stereo sound. Note that in this context, the term ''frame'' is erroneously applied in editing applications and does not denote the physical frame described above. In editing and extracting, the frame is the smallest addressable time interval for an audio CD, meaning that track start and end positions can only be defined in 1/75 second steps. Logical structure The largest entity on a CD is called a track. A CD can contain up to 99 tracks (including a data track for Mixed Mode discs). Each track can in turn have up to 100 indexes, though players which handle this feature are rarely found outside of Pro Audio , particularly Radio Broadcasting . The vast majority of songs are recorded under index 1, with the Pre-gap being index 0. Sometimes Hidden Track s are placed at the end of the last track of the disc, often using index 2 or 3. This is also the case with some discs offering "101 Sound Effect s", with 100 and 101 being index 2 and 3 on track 99. The index, if used, is occasionally put on the track listing as a Decimal part of the track number, such as 99.2 or 99.3. ( Information Society 's '' Hack '' was one of very few CD releases to do this, following a release with an equally-obscure CD+G feature.) The track and index structure of the CD carried forward to the DVD as title and chapter, respectively. Manufacturing tolerances Current manufacturing processes allow an audio CD to contain up to 80 minutes (variable from one replication plant to another) without requiring the content creator to sign a Waiver . Thus, in current practice, maximum CD playing time has crept higher by reducing minimum Engineering Tolerance s, while still maintaining acceptable standards of Reliability . CD-ROM See Also: CD-ROM For its first few years of existence, the compact disc was purely an audio format. However, in 1985 the Yellow Book 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. MANUFACTURE See Also: CD manufacturing Replicated CDs are mass-produced initially using a hydraulic press. Small granules of raw polycarbonate plastic are fed into the press while under heat. A screw forces the liquified plastic into the mold cavity. The mold closes with a metal stamper in contact with the disc surface. The plastic is allowed 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. The cycle time, the time it takes to "stamp" one CD, is usually 2–3 seconds. This method produces the clear plastic blank part of the disc. After the metallic layer is applied to the clear blank substrate, the disc goes under a UV light for drying and it is ready to go to press. To press the CD, first a glass master is cut using a high-power laser on a device similar to a CD writer. This glass master is a positive master. After testing, it is used to make a die by pressing it against a metal disc. The die then becomes a negative image: a number of them can be made depending on the number of pressing mills that are to be running off copies of the final CD. The die then goes into the press and the image is pressed onto the blank CD, leaving a final positive image on the disc. A small circle of lacquer is then applied as a ring around the center of the disc, and a fast spin spreads it evenly over the surface. The disc can then be printed and packed. Manufactured CDs that are sold in stores are wrapped/sealed via a process called " Polywrapping " or are Shrink Wrap ped. RECORDABLE CD See Also: CD-R CD-R]] 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 CD audio players cannot read CD-RW discs, although '''most''' stand-alone DVD players can. CD-Rs follow the Orange Book standard. COPY PROTECTION The Red Book audio specification, except for 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, which cannot be Ripped (copied) to hard drives or easily converted to MP3 s. One major drawback to these copy-protected discs is that most will not play on computer CD-ROM drives, as well as some standalone CD players that use CD-ROM mechanisms. Philips has stated that such discs are not permitted to bear the Trademark ed ''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 . Numerous copy-protection systems have been countered by readily-available, often free, software. Also, any CD that can play on a standard audio CD player can be extracted via the standard S/PDIF digital output, rendering any copy protection ineffective. LOGOS The various Compact Disc logos/trademarks, from Philips (copyright holder) graphic files, for identification purposes: SEE ALSO
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