[mythtv-users] 7+1 audio with alsa, xine and 5+1 DVDs

Michael T. Dean mtdean at thirdcontact.com
Sun Jun 5 18:20:53 UTC 2005

Franco wrote:

>I have a 7+1 soundcard (Audigy 2 ZS Platinum Pro)
>with 7+1 speakers connected.
I'm understanding this to mean you're going directly from the sound card 
to the speakers with no A/V receiver in between.

>Unfortunately, when I watch a 5+1 DVD using xine,
>it is NOT upmixed to 7+1 so two speakers remain silent.
Which is probably a good thing (keep reading)...

>Should I change the configuration of xine or alsa
>to get the desired effect (and, how do I do it?)
You could "easily"* do this using ALSA's .asoundrc to specify a device 
using the route and plug plugins.  Basically, with this approach, you 
would simply copy 2 channels to the silent speakers (i.e. copy left- and 
right-surround to left- and right-back, respectively).  Then, set up 
xine to use this ALSA device for 5.1 soundtracks.

Note that while doing this will keep those speakers from being silent, 
you may actually be harming the overall effect of the audio.  The 
different channels in a 5.1 audio recording are used to convey position 
information using precise and complex timing of the sounds on each of 
the channels.  If you "copy" channels to extra speakers, you will 
lose/destroy much of the intended effect (i.e. your position information 
will likely seem "washed out").

Even trying to calculate the timing and audio information to add to the 
channels would not work (i.e. if you were able to come up with some 
incredibly complex read-ahead and isolation technology that allows you 
to identify movement using front and surround channels and create the 
appropriate audio information for the back channels, you would be 
inventing new information--which is likely imprecise and would 
definitely change the effect).  Think of it like trying to print a 
640x480 digital image in 10 x 8 in format--it won't look very good 
because you're inventing way too much information.  IMHO, your best bet 
is to leave the speakers silent.

For more information why it's better not to mess with the original, take 
a look at some of the A/V and home theater guides on the 'net and/or the 
requirements for THX certification.  Both will give you an appreciation 
of the precision of the source audio.

*It's easy to do once you know how.  Figuring out how is where you'll 
spend the time.  Check out the FAQ at http://alsa.opensrc.org/ for more 
info--especially http://alsa.opensrc.org/index.php?page=FAQ028 .  Same 
ideas apply, but routing from 5.1 to 7.1 is more complex than from 
stereo to 5.1.

>Actually, I'd like to upmix to 7+1 every kind of
>audio source, but this seems to automaticaly happen
>whith e.g. mythmusic (stereo music always becomes 7+1)
>as well as mplayer when playing stereo videos...
This is easy to do with stereo audio and doesn't detract from the effect 
(i.e. stereo has only imprecise position information embedded within 
it--generally only allowing the user to distinguish whether the source 
was on the left or right), so there's typically no harm in copying 
left/right channels to left/right back and/or left/right surround 
speakers.  However, in truth, you would ideally reposition your speakers 
when changing from listening to music to listening to a 5.1 movie 
soundtrack.  More info is available on the guides.



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