[mythtv-users] MythTV 0.24 HDMI audio problem

Jean-Yves Avenard jyavenard at gmail.com
Thu Dec 9 06:18:06 UTC 2010


On 9 December 2010 16:56, John P Poet <jppoet at gmail.com> wrote:
> When I kick on timestretch, the pre-amp switched from showing Dolby
> Digital 5.1, to PCM 2.0 and only the two primary speakers are used --
> the others are quiet.  This make me think myth is trying to use LPCM,
> but it is not making it to my pre-amp.

If you haven't checked the "Stereo PCM" flag in the advanced option
and you are using an hdmi output (or an audio device reporting more
than 2 channels), then decoded AC3 won't be re-encoded as AC3, but
instead passed as multichannels LPCM ; no point re-encoding as it only
adds extra audio losses and is slower.

Mind you, depending on your amplfier, not all accepts multichannels
LPCM. I know that the yamaha one I use for my main frontend, when I
pass it 5.1 PCM, all I can hear is the left and right front channel
(which is the reason I added that "Stereo PCM" flag, I needed it)

> When I run alsamixer it lists ALL the outputs from the nvidia GT220
> card as being S/PDIF.  Is that correct?  I would half expect HDMI to
> be different from S/PDIF.

that's how it shows mine too. doesn't bother me too much. It does
bother me that it shows 4 spdif device there and I have no clue what
they are there for

> I have not tried your latest updates yet.  Is multi-channel ( > 2 )
> LPCM working for you?

well, that's the embarassing thing: my last fix to enable LPCM to be
used, revealed another bug.
If you use the upmixer, if 2 channels audio is upmixed as 6 channels
audio, and you have LPCM ; then it's all screwed up.

I believe the issue is related to the timestamp calculation, and it
doesn't handle properly the fact that the audio stream is suddenly 3
times bigger ; in turn I get "Waiting for video ..."

When LPCM wasn't in used, and instead audio was re-encoded to AC3, it
wasn't a problem because AC3 is also embedded in a stereo PCM stream,
so the amount of data going to the audio card is the same as before it
was upmixed...

That's a guess anyway. I spent the whole day yesterday on this but I
can't find why it's behaving that way...
I'll have more time tonight, but the wife said that me staying up to
3AM to work on mythtv isn't acceptable anymore (and as such, can't
wake up at 6.30 to manage the kids)


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