Hardware Reference
In-Depth Information
data)isalwaysmovingpastthereadlaseratthesamespeed,whichoriginallywasdefined
as 3.49 meters per second (or 3.84 mps on dual-layer discs). Because the track is a spiral
that is wound more tightly near the center of the disc, the disc must spin at varying rates
to maintain the same track linear speed. In other words, to maintain a CLV, the disk must
spinmorequicklywhentheinnertrackareaisbeingreadandmoreslowlywhentheouter
trackareaisbeingread.Thespeedofrotationina1xdrive(3.49meterspersecondiscon-
sidered 1x speed) varies from 1,515 rpm when reading the start (inner part) of the track
down to 570 rpm when reading the end (outer part) of the track.
Single-speed (1x) DVD drives provide a data transfer rate of 1.385MBps, which means
the data transfer rate from a DVD at 1x speed is roughly equivalent to a 9x CD (1x CD
data transfer rate is 153.6KBps, or 0.1536MBps). This does not mean, however, that a 1x
DVD drive can read CDs at 9x rates: DVD drives actually spin at a rate that is just un-
der three times faster than a CD drive of the same speed. So, a 1x DVD drive spins at
aboutthesamerotationalspeedasa2.7xCDdrive.ManyDVDdriveslisttwospeeds,for
example, a DVD drive listed as a 16x/40x would indicate the performance when reading
DVDs/CDs, respectively.
As with CD drives, DVD drive manufacturers began increasing the speeds of their drives
bymakingthemspinmorequickly.Adrivethatspinstwiceasfastwascalleda2xdrive,a
drive that spins four times as fast was 4x, and so on. At higher speeds, it became difficult
to build motors that could change speeds (spin up or down) as quickly as needed when
data was read from different parts of the disc. Because of this, faster DVD drives spin the
disc at a fixed rotational speed rather than linear speed. This is termed CAV because the
angular velocity (or rotational speed) remains a constant.
The faster drives are useful primarily for data, not video. Having a faster drive can reduce
or eliminate the pause during layer changes when playing a DVD video disc, but having a
faster drive has no effect on video quality.
DVD drives are available in speeds up to 20x or more, but because virtually all are CAV,
they actually achieve the rated transfer speed only when reading the outer part of a disc.
Table 11.25
shows the data rates for DVD drives reading DVDs and how that rate com-
pares to a CD drive.
Table 11.25 DVD Speeds and Transfer Rates
