Hardware Reference
In-Depth Information
CD Drive Speeds
CD drives can have different RPMs—that is, different speeds at which the discs spin. On
a CD drive, the RPM and the amount of data it can access per second are directly related,
so the faster the RPM, the faster the drive can deliver data. (This relationship is much more
direct with CD drives than it is with hard disk drives, where a variety of other factors
complicate the equation.)
CD speeds are defi ned in comparison to the original speed of a CD back when the
technology was fi rst invented in the 1980s. This original speed, known as 1x, is between
200 and 500 RPM. That range varies because the original CD drives used
constant linear
velocity (CLV)
. The drives read data at a consistent speed, no matter which spot on the
disk was being read at the time. Because the inner rings on a disk are much smaller than
the outer ones, the speed at which the disk spun had to change when accessing different
areas. The disk spun fastest when reading the inner rings, at 500 RPM. It spun slowest
when reading the outer rings, at 200 RPM.
Later CD drives, including all writeable CD drives (covered later in this chapter),
changed to a different technology known as
constant angular velocity (CAV)
. With CAV,
the disc always spins at the same speed, and it's the reading speed that varies.
Table 2.3 lists some speeds and compares their maximum data rates and RPMs.
TABLE 2.3
CD speeds
Speed
Maximum Data Rate
(Megabits per Second)
RPM
Technology
1x
1.23
200-500
CLV
2x
2.46
400-1,000
CLV
10x
9.83
800-2,000
CLV
20x
24.6
4,000
CAV
40x
49.2
7,200
CAV
56x
68.8
11,200
CAV
In addition to considering a drive's speed, you should also consider its access time.
Access time
is the amount of time that elapses between a PC's request for data from the CD
and the drive's delivery of the fi rst part of that data. This is mostly a measure of the drive's
mechanical ability to move the head to the correct spot. Access time isn't directly related to
the drive's rotational speed, although drives with faster speeds also tend to have superior
mechanics. A 1x drive has a typical access time of around 400 milliseconds (ms). Today's
best-performing drives have an access time of around 75 ms.
CD drives, like hard drives, have built-in caches for buffering data—the larger the cache,
the more effi cient the drive. A good CD drive should have at least a 512 KB cache.
