[mythtv-users] AC3 / DTS passthrough over nVidia HDMI.

[Martin Long (myth)]

Hi, 

I'm having some trouble getting AC3 and DTS passthrough working on myth.
First let me clarify, I have Linear PCM working with hdmi on myth using:

	alsa:plughw:0,3

This works perfectly, and I get pro-logic surround etc. It also works using
just hw:0,3 but then, of course my mp3s won't play. 

Using alsa:hw:0,3 as the pass through device ALMOST works. If I switch off
passthrough, I get perfect picture AND sound (but only LPCM). If I switch
passthrough on, then the picture starts to jerk slightly, and the sound
jumps in and out (and the DD light on my receiver is going on and off).

Now... I'm certain my alsa setup is spot on, as it works perfectly through
mplayer. I get smooth picture, and perfect streamed Dolby Digital AC3 (see
the command line below, and text output from mplayer).

My question... why is this working from mplayer, and not from myth, when I'm
effectively using the same settings? This is the same playing a DVD, or
playing an .mkv file for example. 

It seems like I'm so close, I'm not getting anything wrong as such, just a
buffer needs tweaking, or something is being misinterpreted in the code
somewhere.  


Some details: 

card 0: NVidia [HDA NVidia], device 0: ALC1200 Analog [ALC1200 Analog]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: NVidia [HDA NVidia], device 1: ALC1200 Digital [ALC1200 Digital]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: NVidia [HDA NVidia], device 3: NVIDIA HDMI [NVIDIA HDMI]
  Subdevices: 0/1
  Subdevice #0: subdevice #0

NVidia drivers: latest

ALSA driver: 1.0.19

Distro: Debian something (but I've fiddled with Kernel, drivers etc... might
as well be hand rolled)

Output from mplayer:

# mplayer -ao alsa:device=hw=0.3 -ac hwac3
Dollhouse.S01E08.720p.HDTV.x264-CTU.mkv -y 1080 -x 1920
MPlayer SVN-r29078-4.1.2 (C) 2000-2009 MPlayer Team

Playing Dollhouse.S01E08.720p.HDTV.x264-CTU.mkv.
[mkv] Track ID 1: audio (A_AC3), -aid 0, -alang und
[mkv] Track ID 2: video (V_MPEG4/ISO/AVC), -vid 0
[mkv] Will play video track 2.
Matroska file format detected.
VIDEO:  [avc1]  1280x720  24bpp  23.976 fps    0.0 kbps ( 0.0 kbyte/s)
==========================================================================
Opening video decoder: [ffmpeg] FFmpeg's libavcodec codec family
Selected video codec: [ffh264] vfm: ffmpeg (FFmpeg H.264)
==========================================================================
==========================================================================
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)
==========================================================================
[AO_ALSA] alsa-lib: conf.c:3843:(parse_args) Unknown parameter AES0
[AO_ALSA] alsa-lib: conf.c:3969:(snd_config_expand) Parse arguments error:
No such file or directory
[AO_ALSA] alsa-lib: pcm.c:2144:(snd_pcm_open_noupdate) Unknown PCM
hw:0,3,AES0=6
AO: [alsa] 48000Hz 2ch ac3 (1 bytes per sample)
Starting playback...


If you need anything else, let me know. 

I'm fairly up on how asound.conf works, and I've tried using direct, plug,
hw aliases etc, as well as not asound.conf at all. All of them give the same
result, when I plumb them up correctly. 

I hope someone is able to help. 

Thanks

Martin