CDR-RW Multi-Read Technology

The recorded tracks on a CD-RW disc are read in the same way as regular CD tracks: by detecting transitions between low and high reflectance, and measuring the length of the periods between the transitions. The only difference is that the reflectance is lower than for regular CDs. This does, however, mean that CD-RW discs cannot be read by many older CD-ROM drives or CD players.

To outline the solution to this problem, it is helpful to consider the original CD reflectance specifications: 70% minimum for lands, 28% maximum for pits, that were introduced to allow the relatively insensitive photodiodes of the early 1980s to read the signal pattern reliably. But with today’s photodiodes able to detect much smaller reflectance differences, these stringent specifications are no longer necessary.

The CD-RW disc has a reflectance of 15-25% for lands. The CD-Rewritable system, therefore, works at reflectances about one-third of those of the original CD specification. However, with modern photodiodes this presents no problem. All that is needed to reliably read the recorded pattern is extra amplification. The MultiRead specification drawn up by Philips and Hewlett Packard and approved by the Optical Storage Technology Association (OSTA) provides for the necessary adjustments, thus solving any compatibility issues.

Moreover, the maximum and minimum reflectances of a CD-RW disc meet the CD specification requirements for a minimum modulation of 60%. Looking to the future, the CD-RW phase-change technology is significantly independent of the recording/read-out laser wavelength. CD-RW discs can be read out by the 650 nm lasers used in DVD systems as well as by the present 780 nm lasers used in other CD drives. Clearly, this creates additional options for drive manufacturers.