Hardware Reference
In-Depth Information
disk more quickly than data near the center (beginning). This allows for some misleading
advertising. For example, a 12x CLV drive reads data at 1.84MBps no matter where that
data is on the disc. On the other hand, a 16x CAV drive reads data at speeds up to 16x
(2.46MBps) on the outer part of the disc, but it also reads at a much lower speed of only
6.9x (1.06MBps) when reading the inner part of the disc (that is the part they don't tell
you). On average, this would be only 11.5x, or about 1.76MBps. In fact, the average is
actually overly optimistic because discs are read from the inside (slower part) out, and an
average would relate only 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 in-
teresting 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
onlydrawbackisthatiftheydetect avibration, theyslowdownthedisc,therebyreducing
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 typ-
ically are recorded at a constant data rate called CLV. Therefore, the track (and thus the
 
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