Media Longitevity/Care

Guy Sotomayor ggs at shiresoft.com
Wed Mar 16 19:59:52 CST 2005


On Wed, 2005-03-16 at 17:43 -0800, Eric Smith wrote:
> Guy wrote:
> > It doesn't
> > require *any* lab environment (just the right software and suitably
> > compliant hardware...ie a CDwriter that allows "raw" writes).
> 
> I was assuming a "normal" CD-writer, which can't be assumed to
> support raw writes.  Many CD-writers don't, and some that do claim
> to support raw writes do so incorrectly.

The one's I've used have done a reasonably good job of it.  In many
cases it's the software that gets it wrong.

> 
> If you can do raw writes, you can guarantee the alignment of the
> "data" to the subcode framing.  Otherwise (with a non-raw-write-capable
> drive, you cannot, and thus you can't predict the complete details
> of the actual EFM sequence that will be written to disc.  Though to
> be fair, that only will affect 14 channel code bits out of every 1372.
> 

Yep.

> But even after you've done all that, you're not going to find a way to
> generate any channel data pattern that is significantly more succeptible
> to externally induced error (e.g., oxidation) than what you will get
> from random data.  That was my original point.
> 
> > I've done this work.
> 
> Why?
> 
> I hacked my own software to read a CD based on raw preamp output, in
> order to investigate copy-protected discs, but what would the point
> be in developing anything that could generated a desired EFM channel
> pattern from application-level data, unless you were developing or
> validating a CD writer?

It was a while ago (1997).  A certain company (who shall remain
nameless) said that certain things w.r.t. CDROMs could only be done with
their media and their writers.  I took that as a challenge and was able
to reproduce what they were doing with generic media and writers that
were available.  It required that I delve deeply into how the CDROM
format worked and specifically the Reed-Soloman codes (since I was
writing "raw", the writer wouldn't generate them, so I had to).  I was
able to write whatever I wanted (well there were limits) onto the CDROM.
Even after I had done this and showed that my results were identical to
theirs, I had their engineers telling me that it was impossible.

Which only goes to prove: "The possible we do immediately, the
impossible takes a little longer".
-- 

TTFN - Guy




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