Idea: The ultimate CD/DVD auditing tool

Peter Fairbrother zenadsl6186 at
Tue Jul 8 10:40:08 PDT 2003

Tyler Durden wrote:

> As a basic idea it seems relatively workable. However, there's one detail
> that perhaps you might want to know about:
> "We can push the idea a step further, making a stripped-down CD/DVD drive
> that would be able basically just to follow the spiral track with its head
> in constant linear velocity"
> Unlike a vinyl record, the CD grooves don't form a spiral...they are
> concentric circles. Also, the beginning of the CD is towards the center, the
> end towards the edge.

Eh? It's a spiral.

The constant linear velocity applies to the scan velocity (1.3 m/s at 1x),
not the head velocity, which might cause problems.

Also the spiral track/ holes in the centre aren't accurate enough to follow
the track without real-time correction, done by some complex optical tricks
and feedback loops.

However, it should be quite easy to get a signal from somewhere in the CD
player (especially from early ones which split the functions between lots of
chips), probably best would be from the EFM (eight to fourteen modulation)
or frame output. This will include all the interesting subcodes etc., plus
sync and C1/C2 parity bytes.

That's a fairly clean (it's digital, but with errors) signal at about 2.1
Mb/s at 1x speed, so it shouldn't be hard to capture and analyse in real
time  :). However, you will still have to do the CIRC decoding.

If you are feeling adventurous you could just take a signal from the laser
head, and do the timing, EFM, circ etc yourself. That will give you the
pits, plus the errors, plus a lot (!) of work. Not recommended really,
unlees you need it for some (anti?) copy-protection purpose.

As for getting the player to actually follow the track on a protected disc,
again the solution is probably to go for an older player and hack about. I
used to repair them (a long time ago, when it was worth doing), it should be
quite easy (though I'm no expert on CD copy protection). There was a mod
involving just putting a few volts on one chip lead on an early Sony model,
but I can't remember enough details to find a ref.

A curiosity, only tenuously related - I just came across a Feb 1994 copy of
Elector magazine, with plans for a S/PDIF copybit eliminator (for SCMS).
Seems people have been defeating copy protection for a while..

Peter Fairbrother

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