Hardware Reference
In-Depth Information
transfer at 153.6KBps according to the original standard. Drives that transfer twice that are 2x, 40
times that are 40x, and so on. DVD drives transfer at 1,385KBps at the base rate, whereas drives that
are 20 times faster than that are listed as 20x. Note that because almost all faster drives feature CAV,
the “x” speed usually indicated is a maximum that is seen only when reading data near the outside
(end) of a disc. The speed near the beginning of the disc might be as little as half that, and of course
average speeds are somewhere in the middle.
With today's optical drives supporting multiple disc formats, multiple read and write specifications
are given for each form of media a drive supports.
CD Drive Speed
Because CDs originally were designed to record audio, the speed at which the drive reads the data
had to be constant. To maintain this constant flow, CD data is recorded using a technique called
constant linear velocity
(CLV).
In the quest for greater performance, drive manufacturers began increasing the speeds of their drives
by making them spin more quickly. A drive that spins twice as fast was called a 2x drive, one that
spins four times faster was called 4x, and so on. This was fine until about the 12x point, where drives
were spinning discs at rates from 2,568 rpm to 5,959 rpm to maintain a constant data rate. At higher
speeds than this, it became difficult to build motors that could change speeds (spin up or down) as
quickly as necessary when data was read from different parts of the disc. Because of this, most drives
rated faster than 12x spin the disc at a fixed rotational, rather than linear speed. This is termed CAV
because the angular velocity (or rotational speed) remains a constant.
CAV drives are also generally quieter than CLV drives because the motors don't have to try to
accelerate or decelerate as quickly. A drive (such as most rewritables) that combines CLV and CAV
technologies is referred to as
Partial-CAV
or
P-CAV
. Most writable drives, for example, function in
CLV mode when burning the disc and in CAV mode when reading.
Table 11.23
compares CLV and
CAV.
Table 11.23. CLV Versus CAV Technology Quick Reference
CD-ROM drives have been available in speeds from 1x up to 52x. Most nonrewritable drives up to
12x were CLV; most drives from 16x and up are CAV. With CAV drives, the disc spins at a constant
speed, so track data moves past the read laser at various speeds, depending on where the data is
physically located on the CD (near the inner or outer part of the track). This also means that CAV
drives read the data at the outer edge (end) of the 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
