Thank you for your answers, and excuse me for my bad english.

This is what I have:

-LD player (Pioneer CLD-D925), with Toslink optical output.

-Sound card Terratec Aureon 5.1 Sky, with Toslink optical input.

And this is what I want to do:

I would like to transfer the digital sound track of some movies on
LD to my Hard Disk, with any loss of quality, this mean perfect
transaction bit to bit.

The sound on LD is digital PCM (the same system on CD audio).

I'm sure that I'm transfering the digital track because the digital
output on the LD player only can out the digital track. The movie
also have a analog audio track, but this track can not be out to the
digital output, only to the analog one.

I'm sure that the digital transaction is far to be perfect because I
do two consecutive transfers of the same sound, later I compare the
two samples and the waveform is diferent.

Of course, I strip the zero samples and start comparing from the
first real audiosample.

Take a look here and you will see what I mean:

http://personales.ya.com/deharo/digitalsoundtest.JPG

I have try to use a portable computer with digital output instead
the LD player, and I have the same results.

Michel, you say you have done perfect transactions, how have you
verified that? What hardware have you used? Please, tell me what
source (CD player, LD, minidisc), what sound card (manufacturer and
model) and what kind of conections, optical or coaxial. Your
information is very important for me.

Thank you