View source for Constant angular velocity

{{Short description|Qualifier for the rated speed of an optical disc drive}}
{{refimprove|date=April 2012}}
{{missing information|P-CAV (partial constant angular velocity)|date=July 2020}}
[[File:Comparison disk storage.svg|thumb|300px|Comparison of several forms of disk storage showing tracks (not-to-scale); green denotes start and red denotes end. Note Apple 3.5–inch floppy disk drives used five speeds, increasing capacity by a fourth relative to simpler CAV drives.<br /><nowiki>*</nowiki> Some CD-R(W) and DVD-R(W)/DVD+R(W) recorders operate in ZCLV, CAA or CAV modes.]]
In [[optical storage]], '''constant angular velocity''' ('''CAV''') is a qualifier for the rated speed of any [[disk storage|disc]] containing information, and may also be applied to the writing speed of recordable [[optical disc|discs]]. A drive or disc operating in CAV mode maintains a constant [[angular velocity]], contrasted with a [[constant linear velocity]] (CLV).<ref> {{cite journal |last1=Christodoulakis | first1=S. |last2=Ford | first2=D. A. | title=File organizations and access methods for CLV disks |date=1989 |journal=SIGIR Forum |pages =152-159|volume=23 |doi=10.1145/75335.75350}}
</ref>

A typical [[CD-ROM]] drive operates in CLV mode, in contrast to a floppy or hard disk drive, or gramophone, which operates in CAV mode. <ref>{{cite journal
 | first = Larry
 | last= Press
 | title = Optical storage
 | journal = Encyclopedia of Computer Science
 | year = 2003
 | pages = 1336–1339
 | isbn = 0470864125
 | publisher = John Wiley and Sons Ltd.
}}</ref>  In CAV mode, the spindle motor turns at a constant speed, which makes the medium pass by the read/write head faster when the head is positioned at the outside of the disk.  In contrast, in CLV mode, the spindle motor speed varies so that the medium passes by the head at the same speed regardless of where on the disk the head is positioned.  

If the disk is recorded at the same areal density throughout, then when read or written in CAV mode, the data rate is higher for the outer tracks than for the inner tracks (because for the same rotation on the inner track, more distance is travelled on the outer track), whereas in CLV mode, the data rate is the same everywhere.

An advantage of CAV mode over CLV is that the drive mechanism is easier to engineer (and less expensive to build).  Another advantage is that a device can switch from reading one part of a disk to reading another part more quickly, because in CLV mode, when the device moves the head in or out, it must change the speed of the disc.

== Dimensions ==
[[File:Basic Illustration and mesures of a CD.svg|thumb|Dimensions of a standard-size (12&nbsp;cm diameter) [[optical disc]]]]
In case of a 12 cm standard diameter disc, data at the outermost edge (6 cm from center) is accessed at 2.4 times the speed of data at the inner edge of the data-containing program area (2.5 cm from center).<ref name=OD-Diameter>{{Cite web |url=https://docplayer.es/74808946-Fundamentos-de-hardware.html |title=''FUNDAMENTOS DE HARDWARE.'' – Page 37 of 45 (graphic) |access-date=2020-07-13 |archive-date=2022-02-23 |archive-url=https://web.archive.org/web/20220223102343/https://docplayer.es/74808946-fundamentos-de-hardware.html |url-status=dead }}</ref>

For a [[miniCD|miniature disc]] with a diameter of 8 cm (radius of 4 cm), the speed ratio of outer to inner data edge is 1.6<!--‾-->.

This means that, for example, if a disc is accessed at a constant angular velocity of ×24, the equivalent [[linear velocity]] is ×24 while accessed at the outermost edge of the data area while being ×10 at the innermost data area.

==Uses==
[[File:Laserdisc CAV.jpg|thumb|Constant-angular-velocity (CAV) [[LaserDisc]], showing the NTSC field setup and individual scan lines]]
[[Gramophone record]]s have always used CAV, including [[Capacitance Electronic Disc|CED]]s which provide video signals.

Some high-speed CD and DVD drives can use CAV.{{specify|date=April 2012}} It allows for shorter [[access time]]s because the rotation speed (angular velocity) does not need to be changed when the laser seeks across the disc, similarly to the [[Disk read-and-write head|magnetic head]] of a [[hard disk drive]]. With a given angular (rotation) speed, the linear speed at the outermost track is nearly 2.4 times as fast as that of the innermost track.

CAV was used in the [[LaserDisc]] format for interactive titles; it was also used with [[special edition]]s of certain films.<ref>{{cite web| url= http://www.voyagerco.com/criterion/catalogpage.cgi?ghostbusters| archiveurl= https://web.archive.org/web/19970213145931/http://www.voyagerco.com/criterion/catalogpage.cgi?ghostbusters | archivedate=1997-02-13| title= The Criterion Collection: Ghostbusters| publisher= Voyager Media Store | work=voyagerco.com| accessdate=2012-04-30}}</ref> CAV allowed for perfect still frames, as well as random access to any given frame on a disc. Playing time, however, was 30 minutes on each side (these discs are also known as "standard play"), as opposed to 60 minutes on each side for CLV-based discs ("extended play").

CAV is used with the [[Nintendo optical disc|Nintendo GameCube Game disc and Wii Optical disc]], the [[Xbox (console)|original Xbox]] discs, and Sega's [[GD-ROM]].{{citation needed|date=April 2012}}

=== Partial constant angular velocity (P-CAV) ===
If the used media (e.g. [[DVD-RAM]] variants) has a linear (writing) speed limitation that can not be attained with physically supported angular (rotation) speeds at the inner edge of the recordable area, the disc writing can start off with a constant angular velocity, until the increasing linear velocity has reached the writing speed limitation of the disc.

At that point, the drive transitions into writing with a constant linear velocity.<ref name=P-CAV>[https://qpxtool.sourceforge.io/glossar.html#p_cav QPxTool glossary: P-CAV (partial constant angular velocity)]</ref>
<ref name="cdrlabs-SE208">{{cite web |title=CDRLabs.com - Samsung SE-208GB Portable 8x DVD Writer - Reviews |url=http://www.cdrlabs.com/reviews/samsung-se-208gb-portable-8x-dvd-writer/print.html |website=CDRLabs |accessdate=9 August 2020 |language=en-gb |date=2014-12-03}}</ref>

==See also==
* [[Constant linear velocity]] (CLV)
* [[Zone bit recording]] (ZCAV)
* {{section link|Optical_storage_media_writing_and_reading_speed#Various_optical_disc_formats_writing/reading_speeds}}

==References==
{{reflist}}

==External links==
* http://www.pcguide.com/ref/hdd/geom/tracksZBR-c.html
[[Category:Rotating disc computer storage media]]
[[Category:Audio storage]]
[[Category:Video storage]]
[[Category:Optical computer storage]]