Raw laser readout from compact disc?

Archived from groups: rec.audio.tech (More info?)

Hi all,

I am trying to gain an in-depth understanding of the digital encoding
used in compact discs, out of hobby interests. I think I got a grasp of
all the algorithms involved (CIRC encoding, EFM, and so on) but I would
like to verify my understanding by looking at the actual raw (analog)
signal as seen by the laser sensor in the CD's optical system.

For this purpose, it would be quite ideal to have access to a couple of
seconds worth of audio data as seen by the laser reflection pickup
sensor; something like 20 Msamples/sec at 8 bits/sample would be perfect
for me.

Would anybody know how I could get hold of this kind of data? I assume
that it is routinely produced by producers of CD hardware, but they will
probably not be keen to share :-)

Any pointers would be greatly appreciated.

Best regards,

Sidney Cadot
The Netherlands

Archived from groups: rec.audio.tech (More info?)

Richard Tomkins wrote:

> Get a good microscope. All you see it pits, that's all the Laser sees.

Are they really visible using an optical microscope? Given that the pit
depth is the quarter of the wavelength of the laser (780 nm) I would
find it surprising if these could be seen using anything other than an
electron microscope.

> A pit disperses the laser light so in that bit time there is no signal and
> no pit reflects the light there is a signal.

As I understand it, the psysical effect that attenuates the reflected
signal is not dispersion but rather destructive interference (given the
pit depth of 1/4*lambda, the extra pathlength is 1/2*lambda, so this
interferes destructively).

> The signal in question is a 1 or a 0.

The transition land/pit and pit/land denote ones, no transition denotes
zeros. All this is then fed into the EFM decoder.

Unfortunately, all this doesn't really help to find what I'm looking
for: some real-life data recorded directly from the laser sensor.

My problem isn't so much that I need a feel for the physical layout
(interesting as that may be); I would like to verify my understanding of
the algorithms, which are largely mathematical in nature. There is some
funky mathematics going on in the error correction - I like to think I
understand it, but it would be nice to check.

For that I really need at least a couple of thousands samples of raw
information from the laser as it follows the track (preferably still
analog).

Best regards,

Sidney

Archived from groups: rec.audio.tech (More info?)

Sidney Cadot <sidney@jigsaw.nl> wrote in message news:<ca2h48$asb$1@news.tudelft.nl>...
> Hi all,
>
> I am trying to gain an in-depth understanding of the digital encoding
> used in compact discs, out of hobby interests. I think I got a grasp of
> all the algorithms involved (CIRC encoding, EFM, and so on) but I would
> like to verify my understanding by looking at the actual raw (analog)
> signal as seen by the laser sensor in the CD's optical system.
>
> For this purpose, it would be quite ideal to have access to a couple of
> seconds worth of audio data as seen by the laser reflection pickup
> sensor; something like 20 Msamples/sec at 8 bits/sample would be perfect
> for me.
>
> Would anybody know how I could get hold of this kind of data? I assume
> that it is routinely produced by producers of CD hardware, but they will
> probably not be keen to share :-)
>
> Any pointers would be greatly appreciated.
>
> Best regards,
>
> Sidney Cadot
> The Netherlands

This link shows you the signal from the laser known as the "eye pattern"


jeff