Editing Compact disc (section)

== Physical details ==
{{More citations needed section|date=May 2016|talk=Numbers in the Physical Details section}}
{{see also|Shaped compact disc}}
[[File:CD layers.svg|thumb|Diagram of CD layers {{ordered list |list_style_type=upper-alpha
 |1=<!--A-->A polycarbonate disc layer has the data encoded by using bumps.
 |2=<!--B-->A shiny layer reflects the laser.
 |3=<!--C-->A layer of lacquer protects the shiny layer.
 |4=<!--D-->Artwork is [[Screen printing|screen printed]] on the top of the disc.
 |5=<!--E-->A laser beam is reflected off the CD to a sensor, which converts it into electronic data.
}}|alt=]]

A CD is made from {{convert|1.2|mm|adj=on}} thick, [[polycarbonate]] plastic, and weighs 14–33 grams.<ref>{{Cite book|last=Pohlmann|first=Ken C.|url=https://books.google.com/books?id=GkIaGZ0HWcMC&q=compact+disc+weight&pg=PA303|title=The Compact Disc: A Handbook of Theory and Use|date=1989|publisher=A-R Editions, Inc.|isbn=978-0-89579-228-0|language=en}}</ref> From the center outward, components are: the center spindle hole (15&nbsp;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&nbsp;mm.

A thin layer of [[aluminum]] or, more rarely, [[Gold compact disc|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, usually by [[screen printing]] or [[offset printing]].

[[File:Compactdiscar.jpg|thumb|left|''Pits'' and ''lands'' of a compact disc under a [[microscope]] ]]CD data is represented as tiny indentations known as ''pits'', encoded in a spiral track molded into the top of the polycarbonate layer. The areas between pits are known as ''lands''. Each pit is approximately 100&nbsp;[[Nanometre|nm]] deep by 500&nbsp;nm wide, and varies from 850&nbsp;nm to 3.5&nbsp;[[μm]] in length.<ref>{{cite web |url=http://www.laesieworks.com/digicom/Storage_CD.html |title=Compact Disc|access-date=6 May 2016 |url-status=live |archive-url=https://web.archive.org/web/20160512100137/http://www.laesieworks.com/digicom/Storage_CD.html |archive-date=12 May 2016}}</ref> The distance between the windings (the ''pitch'') is 1.6&nbsp;μm (measured center-to-center, not between the edges).<ref>{{cite web |url=http://www.multimediadirector.com/help/technology/downloads/tech_docs/cdintroduction.pdf |title=Introduction to CD and CD-ROM |last=Sharpless |first=Graham |date=July 2003 |publisher=Deluxe Global Media Services Ltd|access-date=3 May 2016 |url-status=live |archive-url=https://web.archive.org/web/20160309135812/http://www.multimediadirector.com/help/technology/downloads/tech_docs/cdintroduction.pdf |archive-date=9 March 2016}}</ref><ref name="IEC908">{{cite web |url=https://webstore.iec.ch/publication/3885 |title=IEC 60908 Audio recording - Compact disc digital audio system |url-status=live |archive-url=https://web.archive.org/web/20160506194944/https://webstore.iec.ch/publication/3885 |archive-date=6 May 2016}}</ref><ref name="IEC10149">{{cite web |url=https://webstore.iec.ch/publication/9362 |title=ISO/IEC 10149 Information technology -- Data interchange on read-only 120 mm optical data disks (CD-ROM) |url-status=live |archive-url=https://web.archive.org/web/20160506194949/https://webstore.iec.ch/publication/9362 |archive-date=6 May 2016}}</ref>

When playing an audio CD, a motor within the CD player spins the disc to a scanning velocity of 1.2–1.4&nbsp;m/s ([[constant linear velocity]], CLV)—equivalent to approximately 500 RPM at the inside of the disc, and approximately 200 RPM at the outside edge.<ref name=":0">{{Cite web |date=2024-03-07 |title=Compact disc (CD) {{!}} Definition & Facts {{!}} Britannica |url=https://www.britannica.com/technology/compact-disc |access-date=2024-03-24 |website=www.britannica.com |language=en}}</ref> The track on the CD begins at the inside and spirals outward so a disc played from beginning to end slows its rotation rate during playback.

[[File:Comparison CD DVD HDDVD BD.svg|thumb|upright=1.35|left|Comparison of various optical storage media<!-- parameters: track pitch (p), pit width (w) and minimum length (l), and laser spot size (⌀) and wavelength (λ). -->]]
The program area is 86.05&nbsp;cm<sup>2</sup> and the length of the recordable spiral is {{no wrap|86.05&nbsp;cm<sup>2</sup> / 1.6 μm {{=}}}} {{no wrap|5.38&nbsp;km.}} With a scanning speed of 1.2&nbsp;m/s, the playing time is 74 minutes or 650&nbsp;MiB of data on a CD-ROM. 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&nbsp;m/s and a narrower track pitch of 1.5&nbsp;μm increases the playing time to 80 minutes, and data capacity to 700&nbsp;MiB. Even denser tracks are possible, with semi-standard 90 minute/800&nbsp;MiB discs having 1.33&nbsp;μm, and 99 minute/870&nbsp;MiB having 1.26&nbsp;μm,<ref>{{cite conference |title=Optical measurements upon compact discs in education in optoelectronics |book-title=Electronics Technology (ISSE), 2010 33rd International Spring Seminar on Electronics Technology |last1=Pencheva |first1=Tamara |last2=Gyoch |first2=Berkant |last3=Mashkov |first3=Petko |date=2010-05-01 |pages=531–535 |isbn=978-1-4244-7849-1 |url=https://www.researchgate.net/figure/for-CD-R-74-min-are-obtained-as-follows_tbl1_261263539 }}</ref> but compatibility suffers as density increases.

[[File: CD Pits at 6.25x Magnification.jpg|alt=This is a photomicrograph of the pits at the inner edge of a CD-ROM; 2-second exposure under visible fluorescent light.|thumb|The pits in a CD are 500 [[nanometre|nm]] wide, between 830 nm and 3,000 nm long and 150 nm deep.]]

A CD is read by focusing a 780&nbsp;nm [[wavelength]] ([[near infrared]]) [[semiconductor laser]] (early players used [[Helium–neon laser|He{{nbndash}}Ne laser]]<ref>{{Cite book | url=https://books.google.com/books?id=Ad5G1HWtDRgC&q=cd+player+hene&pg=PA824 |title = Springer Handbook of Lasers and Optics|isbn = 9783642194092|last1 = Träger|first1 = Frank|date = 5 May 2012| publisher=Springer }}</ref>) through the bottom of the polycarbonate layer. The change in height between pits and lands results in a difference in the way the light is reflected. Because the pits are indented into the top layer of the disc and are read through the transparent polycarbonate base, the pits form bumps when read.<ref>An Introduction to Digital Audio, John Watkinson, 1994</ref> The laser hits the disc, casting a circle of light wider than the modulated spiral track reflecting partially from the lands and partially from the top of any bumps where they are present. As the laser passes over a pit (bump), its height means that the round trip path of the light reflected from its peak is 1/2 wavelength out of phase with the light reflected from the land around it. This is because the height of a bump is around 1/4 of the wavelength of the light used, so the light falls 1/4 out of phase before reflection and another 1/4 wavelength out of phase after reflection. This causes partial [[Wave interference|cancellation]] of the laser's reflection from the surface. By measuring the reflected intensity change with a [[photodiode]], a modulated signal is read back from the disc.<ref name=":0" />

To accommodate the spiral pattern of data, the laser is placed on a mobile mechanism within the disc tray of any CD player. This mechanism typically takes the form of a sled that moves along a rail. The sled can be driven by a [[worm gear]] or [[linear motor]]. Where a worm gear is used, a second shorter-throw linear motor, in the form of a coil and magnet, makes fine position adjustments to track eccentricities in the disk at high speed. Some CD drives (particularly those manufactured by Philips during the 1980s and early 1990s) use a swing arm similar to that seen on a gramophone.

[[File:CDM210 cd laufwerk.jpg|thumb|Philips CDM210 CD Drive]]

The pits and lands do ''not'' directly represent the 0s and 1s of [[binary data]]. Instead, [[non-return-to-zero, inverted]] encoding is used: a change from either pit to land or land to pit indicates a 1, while no change indicates a series of 0s. There must be at least two, and no more than ten 0s between each 1, which is defined by the length of the pit. 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. These encoding techniques (defined in the ''[[Compact Disc Digital Audio#Data encoding|Red Book]]'') were originally designed for [[CD Digital Audio]], but they later became a standard for almost all CD formats (such as [[CD-ROM]]).

=== Integrity ===
CDs are susceptible to damage during handling and from environmental exposure. Pits are much closer to the label side of a disc, enabling defects and contaminants on the clear side to be out of focus during playback. Consequently, CDs are more likely to suffer damage on the label side of the disc. Scratches on the clear side can be repaired by refilling them with similar refractive plastic or by careful polishing. The edges of CDs are sometimes incompletely sealed, allowing gases and liquids to enter the CD and corrode the metal reflective layer and/or interfere with the focus of the laser on the pits, a condition known as [[disc rot]].<ref name="clir">Council on Library and Information Resources: [https://www.clir.org/pubs/reports/pub121/sec5.html Conditions that Affect CDs and DVDs] {{webarchive|url=https://web.archive.org/web/20160915012213/https://www.clir.org/pubs/reports/pub121/sec5.html |date=15 September 2016 }}</ref> The fungus ''[[Geotrichum candidum]]'' has been found—under conditions of high heat and humidity—to consume the polycarbonate plastic and aluminium found in CDs.<ref>{{Cite journal|url=http://www.nature.com/news/1998/010628/full/news010628-11.html|title=Fungus eats CD|year=2001|journal=Nature|doi=10.1038/news010628-11|url-status=live|archive-url=https://web.archive.org/web/20131212044729/http://www.nature.com/news/1998/010628/full/news010628-11.html|archive-date=12 December 2013|last1=Bosch|first1=Xavier |issn=0028-0836|url-access=subscription}}</ref><ref>{{cite news|url=http://news.bbc.co.uk/2/hi/science/nature/1402533.stm|title=Fungus 'eats' CDs|date=June 2001|publisher=BBC|url-status=live|archive-url=https://web.archive.org/web/20131212220948/http://news.bbc.co.uk/2/hi/science/nature/1402533.stm|archive-date=12 December 2013}}</ref>

The [[data integrity]] of compact discs can be measured using [[Optical Disc#Surface error scanning|surface error scanning]], which can measure the rates of different types of data errors, known as ''C1'', ''[[C2 error|C2]]'', ''CU'' and extended (finer-grain) error measurements known as ''E11'', ''E12'', ''E21'', ''E22'', ''E31'' and ''E32'', of which higher rates indicate a possibly damaged or unclean data surface, low media quality, [[disc rot|deteriorating media]] and [[CD-R|recordable media]] written to by a malfunctioning [[CD writer]].

Error scanning can reliably predict data losses caused by media deterioration. Support of error scanning differs between vendors and models of [[Optical disc drive#Appliances|optical disc drive]]s<!-- TSSTcorp and LiteON support it according to forum screenshots, but we need a better source. -->, and ''extended'' error scanning (known as ''"advanced error scanning"'' in [[Nero DiscSpeed]]) which reports the six aforementioned E-type errors has only been available on [[Plextor#Optical drives|Plextor]] and some [[BenQ]] optical drives so far, as of 2020.<ref name=gleitz>{{cite web|language=de|url=https://gleitz.info/forum/index.php?thread/32049-philips-dvd-r-8x-infodiscr20/&postID=308650#post308650|date=2006-11-18|title=Philips DVD-R 8x (InfodiscR20) - Philips - Gleitz}}</ref><ref name=qpx-g>{{cite web |title=QPxTool glossary |url=https://qpxtool.sourceforge.io/glossar.html |website=qpxtool.sourceforge.io |publisher=QPxTool |access-date=22 July 2020 |date=2008-08-01 |ref=QPx-Glossary}}</ref>

=== Disc shapes and diameters ===
[[File:Comparison disk storage.svg|thumb|upright=1.35|Comparison of several forms of disk storage showing tracks (not to scale); green denotes start and red denotes end.<br/><nowiki>*</nowiki> Some CD-R(W) and DVD-R(W)/DVD+R(W) recorders operate in ZCLV, CAA or CAV modes.]]

The digital data on a CD begins at the center of the disc and proceeds toward the edge, which allows adaptation to the different sizes available. Standard CDs are available in two sizes. By far, the most common is {{convert|120|mm}} in diameter, with a 74-, 80, 90, or 99-minute audio capacity and a 650, 700, 800, or 870&nbsp;MiB (737,280,000-byte) data capacity. Discs are {{convert|1.2|mm}} thick, with a {{convert|15|mm}} center hole. The size of the hole was chosen by Joop Sinjou and based on a Dutch 10-cent coin: a [[dubbeltje]].<ref>{{Cite web|title=Perfecting the Compact Disc System - The six Philips/Sony meetings - 1979-1980 |url=https://dutchaudioclassics.nl/The-six-meetings-Philips-Sony-1979-1980-The-Start-of-Digital-Audio/|access-date=2022-01-26|website=DutchAudioClassics.nl }}</ref> Philips/Sony patented the physical dimensions.<ref>{{Cite web |date=2015-06-25 |title=Response To Koninklijke Philips Electronics, N.V.'s, Sony Corporation of Japan's And Pioneer Electronic Corporation of Japan's Request For Business Review Letter |url=https://www.justice.gov/atr/response-koninklijke-philips-electronics-nvs-sony-corporation-japans-and-pioneer-electronic |access-date=2022-05-14 |website=justice.gov |language=en}}</ref>

The official Philips history says the capacity was specified by Sony executive [[Norio Ohga]] to be able to contain the entirety of [[Beethoven's Ninth Symphony]] on one disc.<ref name="Ohgaobituary"/> 
This is a myth<ref name="Imminknature">{{Cite journal|journal=Nature Electronics|volume=1|date=2018|title=How we made the compact disc
|author=K.A. Schouhamer Immink|author-link=Kees Schouhamer Immink
|url=https://www.researchgate.net/publication/324571504|access-date=2018-04-16
}}</ref> according to [[Kees Immink]], as the [[Eight-to-fourteen modulation|EFM]] code format had not yet been decided in December 1979, when the 120&nbsp;mm size was adopted. The adoption of EFM in June 1980 allowed 30 percent more playing time that would have resulted in 97 minutes for 120&nbsp;mm diameter or 74 minutes for a disc as small as {{convert|100|mm}}. Instead, the information density was lowered by 30 percent to keep the playing time at 74 minutes.<ref name="Immink2"/><ref>Tim Buthe and Walter Mattli, The New Global Rulers: The Privatization of Regulation in the World Economy, Princeton University Press, Feb. 2011.</ref> The 120&nbsp;mm diameter has been adopted by subsequent formats, including [[Super Audio CD]], [[DVD]], [[HD DVD]], and [[Blu-ray]] Disc. The {{convert|80|mm|adj=on}} diameter discs ("[[Mini CD]]s") can hold up to 24 minutes of music or 210&nbsp;MiB.

{| class="wikitable"
|-
! Physical size
! Audio capacity
! CD-ROM data capacity
! Definition
|-
| 120&nbsp;mm || 74–80 min || 650–700&nbsp;[[Megabyte|MB]] || Standard size
|-
| 80&nbsp;mm || 21–24 min || 185–210&nbsp;MB || Mini-CD size
|-
| 80×54&nbsp;mm&nbsp;– 80×64&nbsp;mm || ~6 min || 10–65&nbsp;MB || [[Business card]] size
|}

===SHM-CD===
[[File:SHM-CD_Subline_logo.svg|thumb|Logo used for SHM-CDs]]
'''SHM-CD''' (short for ''Super High Material Compact Disc'') is a variant of the Compact Disc, which replaces the [[polycarbonate]] base with a proprietary material. This material was created during joint research by [[Universal Music Japan]] and [[JVC]] into manufacturing high-clarity [[Liquid-crystal display|liquid-crystal displays]].

SHM-CDs are fully compatible with all CD players since the difference in light refraction is not detected as an error. JVC claims that the greater fluidity and clarity of the material used for SHM-CDs results in a higher reading accuracy and improved sound quality.<ref>{{Cite web |last=CDJapan |title=All About SHM-CD Format |url=https://www.cdjapan.co.jp/feature/shmcd_allabout |access-date=2024-02-22 |website=CDJapan |language=en}}</ref> However, since the CD-Audio format contains inherent [[Compact Disc Digital Audio#Data encoding|error correction]], it is unclear whether a reduction in read errors would be great enough to produce an improved output.