Chapter 1

TABLE 4-1 DVD and CD speed ratings
DVDs and Computers 121
DVD Equivalent CD reading
drive speed Data rate CD rate speed
1x 11.08 Mbps (1.32 MB/s) 9x 8x–18x
2x 22.16 Mbps (2.64 MB/s) 18x 20x–24x
4x 44.32 Mbps (5.28 MB/s) 36x 24x–32x
5x 55.40 Mbps (6.60 MB/s) 45x 24x–32x
6x 66.48 Mbps (7.93 MB/s) 54x 24x–32x
8x 88.64 Mbps (10.57 MB/s) 72x 32x–40x
10x 110.80 Mbps (13.21 MB/s) 90x 32x–40x
16x 177.28 Mbps (21.13 MB/s) 144x 32x–40x
The bigger the cache (memory buffer) in a DVD-ROM drive, the faster it
can supply data to the computer. This is useful primarily for data, not video.
It may reduce or eliminate the pause during layer changes, but has no effect
on video quality.
Rewritable DVD drives (see “What About Recordable DVD: DVD-R,
DVD-RAM, DVD-RW, DVD�RW, and DVD�R?”) write at about half their
advertised speed when the data verification feature is turned on, which
reads each block of data after it is written. Verification is usually on by
default in DVD-RAM drives. Turning it off will speed up writing. Whether this
endangers your data is a subject of debate. Verification is off in DVD-RW
and DVD�RW drives.
In order to maintain constant linear density, typical CD-ROM and DVD-
ROM drives spin the disc more slowly when reading near the outside where
there is more physical surface in each track. (This is called CLV, constant linear
velocity.) Some faster drives keep the rotational speed constant and use
a buffer to deal with the differences in data readout speed. (This is called
CAV, constant angular velocity.) In CAV drives, the data is read fastest at the
outside of the disc, which is why specifications often list “max speed.”
NOTE: When playing movies, a fast DVD drive gains you nothing
more than possibly smoother scanning and faster searching. Speeds
above 1x do not improve video quality from DVD-Video discs. Higher
speeds only make a difference when reading computer data, such as
when playing a multimedia game or when using a database.