Thank you Bertus.

 

I have a problem in it writing in spirals though, since a stepping motor is involved. I don't know the subject, but the idea of it being like a vinyl record disturbs me a little. Oh well.

 

Hello,

There are a couple of servo mechanisms to keep track:

Two main servomechanisms are used, the first one to maintain a correct distance between lens and disc, and ensure the laser beam is focused on a small laser spot on the disc. The second servo moves a head along the disc's radius, keeping the beam on a groove, a continuous spiral data path.

This quote comes from this wiki page:
http://en.wikipedia.org/wiki/Optical_disc_drive

Bertus

 

Thanks so much for the feedback, everyone, and thanks, Bertus, for finding what appears to be the answer to my initial question (by what looks like consensus).

I'd like to make sure that I understand what you mean, though, in your post that has the picture of the spirals:

"When you scan that with a XEROX (wich has a straight scanline),
you will see dark parts where the pits are inline with the XEROX scanner.
When the pits are NOT inline with the XEROX scanner, there will be a light pattern shown."

What I'm picturing is a plane wave, perpendicular to our screens, traveling horizontally across our screens from left to right across the spirals in the picture you posted. If that is what's happening, I'm not quite able to see why that creates the xerox pattern we see. So when you say "where the pits are inline with the XEROX scanner", I'm having a hard time picture what that looks like. Could you please help me picture/understand this better?

Also, since this scenario occurs with every single CD, is it safe to say that all CDs are fabricated with pits in the same places which is why we get the same patterns each time?

Thanks

 

My understanding is they blank CD has a spiral engraved into it, and the pits burned by the laser is put between them. A conventional CD (prerecorded) is much like a vinyl record, and molded with the pits as part of the mold. The rewritables are interesting too, the laser moves die around and doesn't burn anything.

 

I have been doing some more investigation which has led to further support for the birefringance theory.

Here are three scans.

The first is using a clear plastic disk of CD size, backed by a reflective sheet (cooking foil).

The second is the same plastic disk backed by the non recordable side of a CD, where I believe there are no grooves.

The third is of the Cd without the intervening clear disk.

The maltese cross is clearly discernable in all three. The mirror backing would be conclusive, if I had a suitable mirror.

go well

 

Very interesting!

I guess my plans to pirate CD's by photocopying them will have to be abandoned. I thought that because the whole CD thing works by reading the information with light, you could just run a CD through a photocopier and glue the paper copy to a blank disk. Now I'm dissappointed to find my nefarious pot spoiled by reality.