[vorbis] Copy protected CDs (off-topic)

[Gregory Maxwell]

On Wed, Dec 20, 2000 at 03:25:03PM -0800, Dan Hollis wrote:
> Have you tried this with a test CD pressed from glass master also?
> 
> I think you will see the pulse dance around also. This is because even
> on great CD players, the motor speed varies. So you've proven that gee, we
> can see motor speed varying on a CD player! Wow!

I haven't but other people have done similar test and gotten much better
results.

Here you illustrate it perfectly, the motor speed should *not* be determining
the output! But in many players, IT DOES.

The DAC should be clocked against a high quality oscillator, with
1-2 orders of magnitude more stability then needed to keep the output 'bit
accurate'. The dac should run independent of the motor via a FIFO. The motor
should be driven will PLL with the input of the FIFO, perhaps with an 'extra
kick' at the low/high watermarks. 

The motor should be stable enough and the buffer should be big enough that
the motor speed should never effect the clocking of the output. The motor
should be electrically isolated enough never to effect the output.

The output on my scope test will stay at relatively the same position,
depending on the quality of the internal clock and without respect to the
media type.
 
> Now, attach the oscilloscope to the DAC clock. That's a better test.

Sure, then we will be measuring the output against it's own incorrect view
of the world. It will look fine. But that doesn't mean it will sound fine.
With this measurement, I could replace the oscillators in the CD player and
halve it's speed. That test will look fine. :)

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