Editing Constant angular velocity (section)

{{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.