Theory of CD-R and CD-RW
to that used in making photographs, the cyanine and phthalocyanine
dyes used in CD-R (CD-Recordable)
discs are photosensitive organic compounds. When a CD-R is recorded
the dye is heated by the writing laser and becomes opaque (or
absorbtive) through a chemical reaction to the heat. Because
of this chemical reaction it is not likely that any process
will be developed to reverse the process and make previously
burned CD-Rs reusable.
uses a different kind of data-bearing layer from that in ordinary
CD-R. CD-RW uses a phase change process to alter its state from
a reflective state to a light absorbing state. This phase change
can be reversed to make the area erasable and reusable. The
phase change technology used in CD-RW alters the state of the
recording layer from crystaline to non-crystaline and vise versa.
The cystaline portion allows the metalized layer to reflect
the laser better while the non-crystaline portion absorbs the
laser beam making it is not being reflected.
As in pressed
CDs and CD-Recordables, these alternating binary states represent
the data-bearing signal. In order to achieve these effects in
the recording layer, the CD-Rewritable recorder use three different
"Write Power" (the highest laser power) creates a non-crystaline
(absorbtive) state on the recording layer.
"Erase Power" (the middle power) melts the recording layer
and converts it to a reflective crystaline state.
- The "Read
Power" (the lowest power) does not alter the state of the
recording layer and it is used for reading the data.
are not usually readable in most CD-ROM or CD players unless
they are "multi-read" compliant. The reason for this is that
CD-RW technology emerged a lot later than CD-ROM or CD-R and
consequently many older or non-"multi-read" systems do not recognise
CD-RW media. CD-R media on the other hand, when finalised, is
readable in most recent CD-ROM drives and CD player.
- How CD-Recordable
and CD-Rewritable Dyes Work, 1996, by Kaz Chigita and Katherine
Cochrane, the CD Info Company Inc.
Book part III