Hardware Reference
In-Depth Information
to reading completely full discs. The real-world average could be much less than that.
Table 11.24
contains data showing CD drive speeds along with transfer rates and other interesting
data. This information also applies to DVD or BD drives when CDs are used.
Table 11.24. CD-ROM Drive Speeds and Transfer Rates
Vibration problems can cause high-speed drives to drop to lower speeds to enable reliable reading.
Your disc can become unbalanced, for example, if you apply a small paper label to its surface to
identify the disc. For this reason, many of the faster optical drives come with autobalancing or
vibration-control mechanisms to overcome these problems. The only drawback is that if they detect a
vibration, they slow down the disc, thereby reducing the transfer rate performance.
Most recent optical drives use Z-CLV (zoned CLV) or P-CAV (partial CAV) designs, which help
increase average performance while keeping rotational speeds under control.
DVD Drive Speed
As with CDs, DVDs rotate counterclockwise (as viewed from the reading laser) and typically are
recorded at a constant data rate called CLV. Therefore, the track (and thus the data) is always moving
past the read laser at the same speed, which originally was defined 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 spin more quickly when the inner track area is being read and more
slowly when the outer track area is being read. The speed of rotation in a 1x drive (3.49 meters per
second is considered 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 under three times faster than a CD drive of the
same speed. So, a 1x DVD drive spins at about the same rotational speed as a 2.7x CD drive. Many
DVD drives list two speeds; for example, a DVD drive listed as a 16x/40x would indicate the
performance when reading DVDs/CDs, respectively.
