[mythtv-users] how to tell if 5.1 audio channels mapped correctly?

[Dan Christensen]

Jean-Yves Avenard <jyavenard at gmail.com> writes:

> A great deal of time was spent getting the channel ordering right in
> mythtv.

That's good.  I've only found a few test files that also had video, but
they all worked right for me in myth except for one, and I think that
one was made incorrectly.

> Now, the difference you may hear may come on how you are transmitting
> the audio ; are you using digital passthrough or not ?

Ah, good point.  With mythtv, I'm using digital passthrough, but I 
guess I wasn't with the other players.  I take it that with digital
passthrough, the question of channel mapping is completely out of the
software's control?  And I can probably trust that my amp is designed
correctly.

> If you do, there could be a difference between the AAC and DTS track ;
> as AAC would have to be first decoded then sent as LPCM with channel
> reordering occurring in the process.

> They probably do not re-order the channels from SMPTE back to ALSA order.
> If you were to play the DTS sample using passthrough instead ; how do
> you hear the channels with mplayer ?
> Does it sound like in mythtv ?

Yes.  I've now tried mplayer with digital passthrough for the DTS track,
and it plays correctly too.

> If that's the case, at least it shows that myth is actually doing the
> right thing, it's the others that are wrong.

I'm pretty sure that's correct.  All of this worry started because
alsa's speaker-test program got it wrong too!

I should say that I'm on Debian lenny, which is almost two years old,
so it would be interesting to see if newer versions of speaker-test,
mplayer, xine and vlc handle these things better.  If someone can
test one of the clips linked to earlier using mplayer, I'd be really
curious whether they play correctly without digital passthrough.

In any case, I'm glad to hear that mythtv has it right!

> It does, but there you will play LPCM audio ; if you want passthrough,
> you'll need something like:

I used "-ac hwdts" on the mplayer command line.  Is there a way to set
up alsa so that by default most software will use digital passthrough?
Or does that need to be configured for each piece of software?

> Mine does lock up from time to time, usually it's the nvidia drivers ;

Hmm, that's too bad.  Mine is very solid, but it replaced a board
that had occasional lock-ups.

> I am using hdmi 

Does your sound cut-out when your screen blanks (e.g. "xset dpms force
off")?  Sometimes I want to play music, so I've got to disable the
screensaver for this to work.

Also, for testing, have you found a good command line tool for playing
the audio files you have?  How do you play them from within myth?
(I haven't installed mythmusic myself.)

Dan