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 photo-detector 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 CDRW 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 (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 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. Because 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 purchasing a drive would know the drives 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 specific levels of compatibility:
- Multiread - For CD drives
- MultiRead2 - For DVD drives
In addition, a similar MultiPlay standard exists for consumer DVD-Video and CD-DA devices.
Note that MultiRead also indicates that the drive is capable of reading discs written in Packet Writing mode because this mode is now being used more commonly with both CD-R and DVD rewritable media.
If you use only rewritable CD or DVD drives, you don’t need to worry about compatibility. However, if you still use non-rewritable drives, you should check compatibility with other types of media. Although the MultiRead and MultiRead2 logos shown in Figure 11.8 are not widely used today, you can determine a particular drive’s compatibility with a given media type by viewing its specification sheet.