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

Paul Woodward paul.woodward at gmail.com
Sun Jun 5 20:32:44 UTC 2005

 What you say is interesting and I had no idea it was possible. It's given 
me an idea - my brother's A/V receiver has a 6ch stereo mode which basically 
replicates replicates the front speakers to the rear and it sounds fantatic 
for music. Is it possible to do something similar with alsa config?
 Thanks, Paul

 On 6/5/05, Michael T. Dean <mtdean at thirdcontact.com> wrote: 
> 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.
> HTH.
> Mike
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