7 Dec
2000
7 Dec
'00
2:05 p.m.
See below...
On 7 Dec 2000, Eric Smith wrote:
Chuck wrote:
Scratches on the bottom of the disk cause the servo system to lose
focus and/or tracking. This is why you will occasionally get a CD
to repeat a short section of music, similiar to skipping on an LP.
No, different elements. One uses aluminum foil
and the other gold
foil. Aluminum has the unfortunate tendency to oxidize once oxygen
permeates(sp?) the plastic or the top seal. This oxidation shows up as
"black crud" growing from a crack or scratch.
John wrote:
Hmm. And how much plastic has to be scratched
off before the foil is
exposed, and how wide of a scratch on either Al or Au will blow
away data, given the ECC? If a CD gets scratched, oxidation
won't happen for a while. Will I notice the scratch before
it's too late? Am I mistreating archival CDs in the first place?
On a standard CD or CD-R, the data surface is actually on the *top*
side of the disk, covered by a thin layer of lacquer and the
silkscreened label.
Scratches to the bottom of the disc are unlikely to cause problems,
because the laser is focused through the disk onto the reflective
layer near the top.
Scratches on the top of the disk are much more likely
to cause
problems. The ECC can almost always handle small radial scratches.
Big scratches, or scratches at an angle further from radial (so that
it damages along a track) will cause data loss.
Oxygen seeps through the lacquer and will eventually oxidize the
aluminimum. This won't happen with gold CDRs.
Scratches in the top can help oxygen seep in more quickly.