CHAPTER 13

704 Chapter 13 Optical Storage
Each of the columns in Table 13.7 contains interesting information, explained here:
■ Column 1. Indicates the advertised drive speed. This is a constant speed if the drive is CLV (most
12x and lower) or a maximum speed only if CAV.
■ Columns 2 and 3. Indicate how long it would take to read a full disc if the drive was CLV. For
CAV drives, those figures would be longer because the average read speed is less than the advertised
speed. The fourth column indicates the data transfer rate, which for CAV drives would be a
maximum figure only when reading the end of a disc.
■ Columns 3–6. Indicate the actual minimum “x” speed for CAV drives, along with the minimum
transfer speed (when reading the start of any disc) and an optimistic average speed (true only
when reading a full disc; otherwise, it would be even lower) in both “x” and byte-per-second
formats.
■ Columns 7–8. Indicate the maximum linear speeds the drive will attain, in both meters per second
and miles per hour. CLV drives maintain those speeds everywhere on the disc, whereas CAV
drives reach those speeds only on the outer part of a disc.
■ Columns 9–12. Indicate the rotational speeds of a drive. The first of those shows how fast the
disc spins when reading the start of a disc; this would apply to either CAV or CLV drives. For
CAV drives, that figure is constant no matter where on the disc it is reading. The last column
shows the maximum rotational speed if the drive were a CLV type. Because most drives over
12x are CAV, those figures are mostly theoretical for the 16x and faster drives.
Vibration problems can cause high-speed drives to drop to lower speeds to enable reliable reading of
CD-ROMs. Your CD-ROM can become unbalanced, for example, if you apply a small paper label to its
surface to identify the CD or affix its serial number or code for easy reinstallation. For this reason,
many of the faster CD and DVD 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. .
TrueX Technology
Drives that are 16x or faster are usually CAV drives. Even with CAV, at these speeds the rotational
speed of the disc is nearly 12,000rpm and the data is moving at nearly 163 miles per hour past the
laser on the outer part of the track! Rather than try to spin discs even faster for higher speeds, a company
called Zen Research has developed a technology they call TrueX, (also called Multibeam), which
uses multiple laser beams to achieve constant high transfer rates without the limitations of CAV.
Currently, this technology is licensed by several companies, although Kenwood has been the primary
promotor and manufacturer of TrueX drives and has released drives in 42x, 52x, 62x, and 72x models,
which have benchmarked as the fastest CD-ROM drives available.
TrueX drives use a diffraction grating to split a single beam into seven beams reading seven tracks
simultaneously to improve the transfer rate while maintaining a slower rotational speed that reduces
noise and vibration. Drives with TrueX technology are the fastest CD-ROM drives on the market,
capable of sustaining near-CLV performance at high speeds consistently no matter where they are
reading on the disc.
The net effect is that the reading speed is consistently higher than equivelant speed CAV drives, and
yet the TrueX drives spin at a slower speed. For example, a 52x CAV drive performs from about 22x at
the start of a disk to 52x at the end, whereas a 52x TrueX drive performs at about 45x at the start of a
disc to 52x at the end. This results in a much higher average performance that is nearly consistent
with CLV drives.