Greg Iverson asked:
What is the difference in how CDs purchased in a store, CDRs, CDR/Ws and
DVDs are written? I understand the part about DVD using a different
kind of laser using a smaller wave length so it takes up less space. Is
the plastic different so that over time the molecules may flow?
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The data bits of ordinary CD's consist of mechanically pressed pits.
They are manufactured similar to the way vinyl, analog records were. A
plastic blank is pressed against a metal master which leaves an
impression. The pits are as permanent as the plastic they're pressed
into.
CDR's are "burned" with a laser. The laser causes local heating and
warping of the underlying metalized layer. The warping is permanent in
that re-exposing it to the laser and re-heating it would not be able to
invert the state of a bit by unwarping the metallic layer.
CD/RW's are also "burned" with a laser. However, instead of warping a
metallic layer with heat the laser causes a phase change in the
underlying layer and changes the angle of reflection of the beam. This
phase change is reversible WHEN EXPOSED TO THE PROPER INTENSITY AND
WAVELENGTH OF LIGHT. That's why they're re-writable.
So, CD's and CDR's should be considered pretty safe from data loss
unless exposed to really hostile conditions. I don't know what
environmental conditions might alter the data on a CD-RW but it's not
hard to imagine that there MIGHT be some. I think the advice to stick
with CDR for permanent records is probably very good advice. It's also
cheaper.
DVD's are more of the same, greater density.
Chuck Norcutt
Woburn, Massachusetts, USA
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