After trying the builtin ALSA driver with kernel 2.6.24-rc4 and alsa-utils-1.0.14 from portage with no success to get the S/PDIF output from my Intel Corporation 82801G sound card, I was about to give up, then I thought may be the lastest ALSA driver would do. I grabbed alsa-driver-1.0.16-rc2, compiled, installed... then again no S/PDIF output. I thought may be I'd wait for a few months but I would give a last shot, I downloaded the snapshot alsa-driver-hg20080207.tar.bz2, compiled, installed and voila, my Logitech Z5500 was able to detect the optical input signal. Now I can bypass the crappy DAC of the sound card on my Fujitsu Amilo Si1520.
You'll need to disable ALSA and OSS in the kernel, but you must let Kernel sound support on.
To config alsa driver:
[code]./configure --with-cards=hda-intel --with-card-options=hda-codec-conexant[/code]
And then make, make install as the INSTALL doc says.
To test S/PDIF
[code]speaker-test -Diec958 -c2[/code]
If things go well, you might be able to see the blinking red light in the S/PDIF output from the sound card.
You need alsa-utils to do speaker-test.
My sound card is:
Audio device: Intel Corporation 82801G (ICH7 Family) High Definition Audio Controller (rev 02)
Which is intel-hda in ALSA
Toslink to 3.5 mini (Optical)