[mythtv] Air2PC / DVB audio playback fails in Mythtv - AC3 handling problem?

[Mark Anderson]

On Tuesday 15 February 2005 05:22, Blammo [doh] wrote:
> That's odd.
>
> ahhh.. wait.. I think I got it... can't test this yet, but I have a theory:
>
> MythTV is set up for output to /dev/adsp, which is the ANALOG outputs
> of my sound card, IE directly to the TV. MythVideo is setup up for
> /dev/dsp, which is the DIGITAL outputs of my sound card. In the past,
> this has made complete sense, since no TV was digital, and all movies
> were.
>
This will be part of your problem.

> However, in this case, you can't send AC-3 out an analog port. so that
> may be why we're hearing the chiff-chiff.
>
> Which now brings up several question:
>
> If I turn off AC-3 passthrough, will it play back correctly, IE convert it?
> Is there any way to choose a different output device, only for AC-3?
> IE, use /dev/adsp for normal output, use /dev/dsp for AC-3.
>
Yes

> It's somewhat important, at least in my case, to keep the MythTV audio
> going out the analogs, in that it gives volume control to Myth. As
> soon as I sent the audio out the digital, I lose volume control. I've
> played with all the mixer settings, and nothing seems to affect that.
> Other people use the frontend way more than I do, they have a hard
> time working multiple remotes.
>
> Thoughts?
>
> Thanks Isaac for your help with this. I hope it turns out being this easy..
>
> On Mon, 14 Feb 2005 12:45:51 -0500, Isaac Richards <ijr at case.edu> wrote:
> > On Monday 14 February 2005 11:38 am, Blammo [doh] wrote:
> > > Small is somewhat of a misnomer when it comes to DVB files, but here
> > > is a short clip. I was on live, hit record, waited a few seconds, and
> > > stopped the recording:
> > >
> > > http://216.27.229.222/tmp/air2pc.sample.nuv.gz
> > >
> > > Apologies, it is 30M, even GZ'd.
> >
> > Plays fine here in myth.
> >
> > Isaac
It does not play fine for me. With SPDIF passthrough enabled I get a small 
burst of audio then nothing (it is AC3 2/0), except the video runs slow and 
the logs show a video/audio divergence. My log is different to your's in that 
it does detect the audio as AC3 and initialise appropriately. 

With SPDIF passsthrough off I get stuttering in the audio.

Take all of this with a grain of salt since the sample is HD and I am not sure 
my processor can actually keep up with it, so that may be what was causing my 
stuttering.

BTW where are you located?

If changing to /dev/dsp doesn't work then try TS recording, it does not 
process the AC3 as much as PS does.


Log from my startup:

2005-02-15 07:59:08.198 AVFD
2005-02-15 07:59:08.198 AVFD: Opening Stream #0: codec id 2
2005-02-15 07:59:08.200 Using libmpeg2 for video decoding
2005-02-15 07:59:08.200 detectInterlace(Detect Scan, Detect Scan, 59.9401, 
720) ->Progressive Scan
2005-02-15 07:59:08.200 Interlaced: Progressive Scan  video_height: 720  fps: 
59.9401
2005-02-15 07:59:08.200 AVFD: Looking for decoder for 2
2005-02-15 07:59:08.200 AVFD
2005-02-15 07:59:08.200 AVFD: Opening Stream #1: codec id 86020
2005-02-15 07:59:08.200 AVFD: Looking for decoder for 86020
2005-02-15 07:59:08.201 Stream #1 (audio track #1) is an audio stream with 2 
channels.
2005-02-15 07:59:08.201 Auto-selecting AC3 audio track (stream #1).
2005-02-15 07:59:08.201 Initializing audio parms from stream #0.
2005-02-15 07:59:08.202 Estimated bitrate = 45384
2005-02-15 07:59:08.224 Position map filled from DB to: 417
2005-02-15 07:59:08.225 SyncPositionMap prerecorded, from DB: 35 entries
2005-02-15 07:59:08.225 detectInterlace(Ignore Scan, Progressive Scan, 
59.9401, 720) ->Progressive Scan
2005-02-15 07:59:08.225 Interlaced: Progressive Scan  video_height: 720  fps: 
-1
Input #0, mpeg, from '/media/1032_20050215075400_20050215075400.nuv':
  Stream #0.0: Video: mpeg2video, 1280x720, 59.94 fps, 45000 kb/s
  Stream #0.1: Audio: ac3, 48000 Hz, stereo, 384 kb/s
2005-02-15 07:59:08.225 Position map found
2005-02-15 07:59:08.226 Commercial Detection initialized: width = 1280, height 
= 720, fps = 59.9401, method = 3
2005-02-15 07:59:08.227 Using Sample Spacing of 8 horizontal & 8 vertical 
pixels.
2005-02-15 07:59:08.228 CommDetect::ClearAllMaps()
2005-02-15 07:59:08.229 Killing AudioOutputDSP
2005-02-15 07:59:08.230 Opening audio device '/dev/dsp'.
2005-02-15 07:59:08.230 Opening OSS audio device '/dev/dsp'.
2005-02-15 07:59:08.231 Audio fragment size: 4096