Hardware Reference
In-Depth Information
The original Orange Book Part III Volume 1 (CD-RW specification) allowed for CD-RW writing at
up to 4x speeds. In May 2000, Part III Volume 2 was published, defining CD-RW recording at speeds
from 4x to 10x. This revision of the CD-RW standard is called
High-Speed Rewritable
, and both the
discs and drives capable of CD-RW speeds higher than 4x indicate this via the logos printed on them.
Part III Volume 3 was published in September 2002 and defines Ultra-Speed drives, which are CD-
RW drives capable of recording speeds 8x-24x.
Because of the differences in High-Speed and Ultra-Speed media, High-Speed media can be used
only in High-Speed and Ultra-Speed drives; Ultra-Speed Media can be used only in Ultra-Speed
drives. Both High-Speed and Ultra-Speed drives can use standard 2x-4x media, enabling them to
interchange data with computers that have standard-speed CD-RW drives. Thus, choosing the wrong
media to interchange with another system can prevent the other system from reading the media. If you
don't know which speed of CD-RW media the target computer supports, I recommend you either use
standard 2x-4x media or create a CD-R.
Because of differences in the UDF standards used by the packet-writing software that drags and drops
files to CD-RW drives, the need to install a UDF reader on systems with CD-ROM drives, and the
incapability of older CD-ROM and first-generation DVD-ROM drives to read CD-RW media, I
recommend using CD-RW media for personal backups and data transfer between your own
computers. However, when you send CD data to another user, CD-R is universally readable, making
it a better choice.
MultiRead Specifications
The original Red and Yellow Book CD standards specified that, on a CD, the lands should have a
minimum reflectance value of about 70%, and the pits should have a maximum reflectance of about
28%. Therefore, the area of a disc that represents a land should reflect back no less than 70% of the
laser light directed at it, whereas the pits should reflect no more than 28%. In the early 1980s when
these standards were developed, the photodetector diodes used in the drives were relatively
insensitive, and these minimum and maximum reflectance requirements were deliberately designed to
create enough brightness and contrast between pits and lands to accommodate them.
On a CD-RW disc, the reflectance of a land is approximately 20% (plus or minus 5%), and the
reflectivity of a pit is only 5%—obviously well below the original requirements. Fortunately, it was
found that by the addition of a relatively simple AGC circuit, the ratio of amplification in the detector
circuitry can be changed dynamically to allow for reading the lower-reflective CD-RW discs.
Therefore, although CD-ROM drives were not initially capable of reading CD-RW discs, modifying
the existing designs to enable them to do so wasn't difficult. Where you might encounter problems
reading CD-RW discs is with CD audio drives, especially older ones. Because CD-RW first came
out in 1996 (and took a year or more to become popular), most CD-ROM drives manufactured in
1997 or earlier have problems reading CD-RW discs.
DVDs also have some compatibility problems. With DVD, the problem isn't just simple reflectivity
as it is an inherent incompatibility with the laser wavelength used for DVD versus CD. The problem
in this case stems from the dyes used in the recording layer of CD-R and RW discs, which are very
sensitive to the wavelength of light used to read them. At the proper CD laser wavelength of 780nm,
they are reflective, but at other wavelengths, the reflectivity falls off markedly. Normally, CD drives
use a 780nm (infrared) laser to read the data, whereas DVD drives use a shorter wavelength 650nm
(red) laser. Although the shorter wavelength laser works well for reading commercial CD-ROM
