[MPlayer-users] hwac3 with 5.1 on old nforce 220D s/pdif gives stereo only

[Michael Holland]

Folks,
I'm using an old Asus A7N266-VM motherboard for a HTPC, and cannot get 
surround sound from the digital audio out. This has the nVidia 220D
nforce-1 chip with dolby digital.
   I don't know if the dolby-encoding is supposed to work under Linux,
but figure I should be able to simply pipe the AC3 from a DVD or digital
TV straight out the s/pdif for decoding in the amp?

I'm using Ubuntu, which auto-configured the sound. I think snd_intel8x0
is the driver.
By fiddling with alsamixer, I enabled analog 5.1 out, and the
s/pdif out.

$ mplayer  -ac hwac3 -vo null sample,dvd,ac3-5.1.avi
Forced audio codec: hwac3
Opening audio decoder: [hwac3] AC3/DTS pass-through S/PDIF
No accelerated IMDCT transform found
hwac3: switched to AC3, 448000 bps, 48000 Hz
AUDIO: 48000 Hz, 2 ch, ac3, 448.0 kbit/29.17% (ratio: 56000->192000)
Selected audio codec: [hwac3] afm: hwac3 (AC3 through S/PDIF)

Why is it cutting to 2 channels?
If I leave out the "-ac hwac3", I get correct decoding to the analogue.

Should I be using the 'nvsound' binary driver? That seems to be 
OSS-only, not ALSA.
Please - point me in the right direction. Not sure what I need to learn 
here. Is this an mplayer problem, or my ALSA setup, or shouldn't I be 
using ALSA at all?

Mike.