16 Mar
2005
16 Mar
'05
5:59 p.m.
On Wed, 2005-03-16 at 17:43 -0800, Eric Smith wrote:
Guy wrote:
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.
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.
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.
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
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?