World's Crappiest Drives (was Re: A&J Microdrive)

From: Clint Wolff <vaxman_at_uswest.net>
Date: Thu Dec 7 14:05:14 2000

See below...

On 7 Dec 2000, Eric Smith wrote:

> Chuck wrote:
> > 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 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.

> 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.
>
>
Received on Thu Dec 07 2000 - 14:05:14 GMT