Re: help with multichannel sound, alsa 1.0.x

Mon Apr 26 2004, Paul Grayson wrote:
> Hello, I have just spent a few hours trying to get surround sound
> working with Ogle 0.9.2.  From the alsa test in configure script it's
> clear that nobody is using this with Alsa 1.0.x...  I happen to have
> Alsa 1.0.1, so I tweaked configure to allow ogle to compile with alsa
> support.  The default oglerc now gives me fine two-speaker sound.
> 
There where no Alsa 1.0 at the time of the last Ogle release, so we
missed the broken configure test.  This is fixed in CVS though.

> However, my motherboard (nforce/shuttle AN35N ultra) has outputs for
> rear and center speakers, and I'd love to have them working.  So I
> uncommented the "if you have a multichannel card" section, and tried
> ogle again.  When the title screen comes up, I hear a loud, annoying
> clicking sound instead of the normal audio.
> 
This should be read as here are somethings you migth want if you have
a multichannel card and/or a card with S/PDIFF output.

What you hear is S/PDIFF data beeing sent out as audio.

> Here are the logs:  http://www.deconfusion.org/tmp/ogle/ogle-debug.txt
> 
> I tried removing the AC3 section and I get a slightly different clicking
> sound.  I'd like to figure out what I'm doing wrong, but the manual is
> no help - it doesn't even mention that AC3 is a possible chtype, so
> there is no way to figure out what the difference between AC3 and the
> explicit channel listing is.  The FAQ and Google were no help here.
> 
I see that you are correct, we never seem to have written anything
regarding them in the man page.

> Can someone explain the difference between the different types of
> channel settings, and what those are actually for?

AC3, DTS, LPCM, MPEG is for S/PDIFF digital output, add if you want that
type of data sent out.

>  Do they control how the sound is read off of the DVD, 

There isn't much to control here.. it's data, read just like any other data..

> how it is sent to the sound card, 

Well, rather what is sent to the audio driver.

> how sound is processed internally by ogle, 

That two, since if the thing beeing sent to the audio driver isn't the
same as the selected audio track on the DVD then it needs to be decoded
and possibly down-/up-mixed.

> or how the sound card is configured?

That is mostly up to the audio driver, but there are different parameters
that can be set and the depend on what kind of samples we are sending to it.

>  Shouldn't all four of these be independently controlled?
>
How, or why?  I can only think of one combination that isn't covered, and
that is letting Ogle decode the data and still send it as S/PDIFF PCM data.
The audio drivers seem to support this fine though (selectable form the mixer).
 
> Someone mentioned that you might have to specify an explicit alsa device
> name - how do I find the list of names and options?
> 
We have no clue, there are interfaces in Alsa to switch these things from
within and we used them (or at least used to), they never did work though.

Since it's some Alsa configuration file where you can give any
arbitrary name to any setup you configure, there really isn't much
Ogle can do to guess the correct name.

So kind of semi standards appears to be 'surround40' and 'surround51'.

For a 4 channel setup use the first, for a full (or nearly full)
use the second.
      <alsa>
        <name>surround40</name>
      </alsa>
resp.
      <alsa>
        <name>surround51</name>
      </alsa>

The speakers section should be something like the following if you don't
use S/PDIFF digital output.

    <speakers>
      <channel_config>
        <chtype>Left</chtype>
        <chtype>Right</chtype>
      </channel_config>
      <channel_config>
        <chtype>Left</chtype>
        <chtype>Right</chtype>
        <chtype>RearLeft</chtype>
        <chtype>RearRight</chtype>
      </channel_config>
    </speakers>

or for a 5.1 setup

    <speakers>
      <channel_config>
        <chtype>Left</chtype>
        <chtype>Right</chtype>
      </channel_config>
      <channel_config>
        <chtype>Left</chtype>
        <chtype>Right</chtype>
        <chtype>RearLeft</chtype>
        <chtype>RearRight</chtype>
        <chtype>Center</chtype>
        <chtype>LFE</chtype>
      </channel_config>
    </speakers>

You can switch the channels around if you get some channel in the wrong
speaker.

Regards
-- 
Håkan Hjort