Hardware Reference
In-Depth Information
ated is a maximum that is seen only when reading data near the outside (end) of a disc.
The speed near the beginning of the disc might be as little as half that, and of course aver-
age speeds are somewhere in the middle.
With today's optical drives supporting multiple disc formats, multiple read and write spe-
cifications are given for each form of media a drive supports.
CD Drive Speed
Because CDs originally were designed to record audio, the speed at which the drive reads
the data had to be constant. To maintain this constant flow, CD data is recorded using a
technique called
constant linear velocity
(CLV).
In the quest for greater performance, drive manufacturers began increasing the speeds of
their drives by making them spin more quickly. A drive that spins twice as fast was called
a 2x drive, one that spins four times faster was called 4x, and so on. This was fine until
about the 12x point, where drives were spinning discs at rates from 2,568 rpm to 5,959
rpmtomaintainaconstantdatarate.Athigherspeedsthanthis,itbecamedifficulttobuild
motorsthatcouldchangespeeds(spinupordown)asquicklyasnecessarywhendatawas
read from different parts of the disc. Because of this, most drives rated faster than 12x
spin the disc at a fixed rotational, rather than linear speed. This is termed CAV because
the angular velocity (or rotational speed) remains a constant.
CAV drives are also generally quieter than CLV drives because the motors don't have to
trytoaccelerateordecelerateasquickly.Adrive(suchasmostrewritables)thatcombines
CLVandCAVtechnologiesisreferredtoas
Partial-CAV
or
P-CAV
.Mostwritabledrives,
for example, function in CLV mode when burning the disc and in CAV mode when read-
ing.
Table 11.23
compares CLV and CAV.
Table 11.23 CLV Versus CAV Technology Quick Reference
CD-ROM drives have been available in speeds from 1x up to 52x. Most nonrewritable
drives up to 12x were CLV; most drives from 16x and up are CAV. With CAV drives, the
disc spins at a constant speed, so track data moves past the read laser at various speeds,
depending on where the data is physically located on the CD (near the inner or outer part
of the track). This also means that CAV drives read the data at the outer edge (end) of the
