[mythtv-users] ac3 passthrough problems

Richard Krehbiel richard.krehbiel at gmail.com
Sat Sep 30 20:57:41 UTC 2006

I'm building a mythtv system now (about as slowly as anyone possibly can),
and I'm stumped on the S/PDIF problem.

I have an MSI K9NGM2-FID - Socket AM2, nVidia GeForce 6150, and I'm using
on-board sound with Intel HDA drivers (snd-intel-8x0).  I installed Fedora
Core 5, current updates.  I've got a coax from my S/PDIF out to my Onkyo
304, and Onkyo set to consume coax.  (It works from an actual DVD player.)

I haven't done any alsa configuration.  I looked into it, but frankly alsa
is impenetrable  (is there a good doc?  I haven't seen one) - plus I've read
comments saying it should work out of the box.

I've got a DVD in right now.  I'll play it:

mplayer -v -ao alsa -afm hwac3 -monitoraspect 16:9 -dvd-device /dev/cdrom

I get a nice picture, but no sound.

The log from mplayer says:

Trying to force audio codec driver family hwac3...
Opening audio decoder: [hwac3] AC3/DTS pass-through S/PDIF
dec_audio: Allocating 8192 bytes for input buffer.
dec_audio: Allocating 16384 + 65536 = 81920 bytes for output buffer.
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)
Building audio filter chain for 48000Hz/2ch/ac3 -> 0Hz/0ch/??...
[libaf] Adding filter dummy
[dummy] Was reinitialized: 48000Hz/2ch/ac3
[dummy] Was reinitialized: 48000Hz/2ch/ac3
alsa-init: requested format: 48000 Hz, 2 channels, 100
alsa-init: using ALSA 1.0.11rc2
alsa-spdif-init: playing AC3, 2 channels
alsa-init: using device iec958:{CARD 0 AES0 0x02 AES1 0x82 AES2 0x00 AES3
alsa-lib: pcm_hw.c:1217:(snd_pcm_hw_open) open /dev/snd/pcmC0D1p failed: No
such file or directory
alsa-init: playback open error: No such file or directory
Could not open/initialize audio device -> no sound.

Okay, my /dev/snd directory contains pcmC0D0c, pcmC0D0p, pcmC0D1c, and
pcmC0D2p.  Whic is a clue - it seems I need to change a 1 to a 2 somewhere.

Please, help.  ALSA's gettin' me down.
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