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reflectance requirements were deliberately designed to create enough brightness and con-
trast 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
thelower-reflectiveCD-RWdiscs.Therefore,althoughCD-ROMdriveswerenotinitially
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 very reflective, but at other
wavelengths, the reflectivity falls off markedly. Normally, CD drives use a 780nm (in-
frared) laser to read the data, whereas DVD drives use a shorter wavelength 650nm (red)
laser.AlthoughtheshorterwavelengthlaserworkswellforreadingcommercialCD-ROM
discs because the aluminum reflective layer they use is equally reflective at the shorter
DVD laser wavelength, it doesn't work well at all for reading CD-R or RW discs.
Fortunately, a solution was introduced by Sony and then similarly by all the other DVD
drive manufacturers. This solution consists of a dual-laser pickup that incorporates both a
650nm (DVD) and 780nm (CD) laser. Some of these used two discrete pickup units with
separate optics mounted to the same assembly, but they eventually changed to dual-laser
units that use the same optics for both, making the pickup smaller and less expensive. Be-
cause most manufacturers wanted to make a variety of drives—including cheaper ones
without the dual-laser pickup—a standard needed to be created so that someone purchas-
ing a drive would know the drive's capabilities.
So how can you tell whether your CD or DVD drive is compatible with CD-R and RW
discs? In the late 1990s, the OSTA created the MultiRead specifications to guarantee spe-
cific levels of compatibility:
MultiRead —For CD drives
MultiRead2 —For DVD drives
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