   #copyright

Compact Disc

2007 Schools Wikipedia Selection. Related subjects: Computing hardware and
infrastructure

   CAPTION: Compact Disc

   Media type: optical disc
    Capacity:  up to 700 MiB
     Usage:    audio and data storage
                Optical disc authoring
     * Optical disc
     * Optical disc image
     * Recorder hardware
     * Authoring software
     * Recording technologies
          + Recording modes
          + Packet writing

                 Optical media types
     * Laserdisc
     * Compact Disc/ CD-ROM: CD-R, CD-RW
     * DVD: DVD-R, DVD-R DL, DVD+R, DVD+R DL,
       DVD-RW, DVD+RW, DVD+RW DL, DVD-RAM
     * Blu-ray Disc: BD-R, BD-RE
     * HD DVD: HD DVD-R

                      Standards
     * Rainbow Books
     * File systems
          + ISO 9660
               o Joliet
               o Rock Ridge
                    # Amiga extensions to Rock Ridge
               o El Torito
               o Apple ISO9660 Extensions
          + Universal Disk Format
               o Mount Rainier

   A Compact Disc (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 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.

History

   The optical lens of a CD drive.
   Enlarge
   The optical lens of a CD drive.

   In 1979 Philips and Sony decided to join forces, setting up a joint
   task force of engineers whose mission was to design the new digital
   audio disc. Prominent members of the task force were Kees Immink and
   Toshitada 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 the 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, 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."

   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, with 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 design of 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

   Compact Discs are made from a 1.2 mm thick disc of very pure
   polycarbonate plastic. 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, 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 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 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.
   A Mini-CD is 8 centimeters in diameter
   Enlarge
   A Mini-CD is 8 centimeters in diameter

Disc shapes and diameters

   The digital data on a CD begins at the centre 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 CDs")
   were originally designed for CD singles and can hold up to 21 minutes
   of music or 184 MB of data. Today many singles are released on 120mm
   CDs, called a Maxi single.
   Physical size marketed "650 MB" marketed "700 MB"
   12 cm         682 MB (650 MiB)  737 MB (703 MiB)

Audio format

   The technical format of an 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/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/or
   44,056 or 44,100 samples/s when the Sony/Philips task force designed
   the Compact Disc; Philips 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 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 disc , however Kees
   Immink of Philips denies this. The extra playing time subsequently
   required the change 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. An example is the 1987 album Kiss Me, Kiss Me, Kiss Me by
   The Cure, which says the following on the back of the CD edition: “The
   track Hey You!!! which appears on the double album and cassette has
   been omitted so as to facilitate a single compact disk.”

Main physical parameters

   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. (A disc played from
       beginning to end slows down during playback.)
     * 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.
     * Centre spindle hole diameter: 15 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.

Data structure

   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.

   Current manufacturing processes allow an audio CD to contain up to
   77-78 minutes (varies 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 while maintaining
   acceptable standards of reliability.

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 storage medium using the same physical format as audio compact
   discs, readable by a computer with a CD-ROM (CDR) drive.

Manufacture

   Replicated CDs are mass-produced initially using a hydraulic press.
   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
   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 centre hole (called a stacking ring) is removed. This
   method produces the clear plastic blank part of the disc. After the
   foil layer is applied to the clear blank substrate the disc is ready to
   go to press. To press the CD first a Glass Master is cut using a high
   power laser on a device not dissimilar to a CD writer, the glass master
   being around 12 inches (30 cm) diameter and up to one inch (25 mm)
   thick as it needs to be strong for pressing. This glass master is a
   positive master. After testing it will then be 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 will be running off copies of the final CD. The die will
   then go into the press and press the image onto the foil layer of the
   blank CD leaving a final positive image on the disc. A small circle of
   varnish is then applied as a ring around the centre of the disc and a
   fast spin will evenly spread it over the surface. The disc will then be
   printed and packed. The method used to press an LP record is very
   similar except the molding of the plastic disc is a separate process
   with CDs.

Recordable CD

   A typical 700-megabyte CD-R
   Enlarge
   A typical 700- megabyte CD-R

   Recordable compact discs, CD-Rs, are injection molded with a "blank"
   data spiral. A photosensitive dye is then applied, and then the discs
   are metalized and lacquer coated. The write laser of the CD recorder
   changes the colour 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.

   CD-Rs follow the Orange Book standard.

Copy protection

   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.
   Retrieved from " http://en.wikipedia.org/wiki/Compact_Disc"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
