Re: help with multichannel sound, alsa 1.0.x

I have a feeling that I am getting closer.  I've modified my oglerc to
include

    <device>
      <driver>alsa</driver>
      <path>/dev/dsp</path>
      <alsa>
	<name>surround40</name>
      </alsa>
    </device>
    <speakers>
      <channel_config>
	<chtype>Left</chtype>
	<chtype>Right</chtype>
	<chtype>RearLeft</chtype>
	<chtype>RearRight</chtype>
      </channel_config>
    </speakers>

and *sometimes* the rear (or center?) channel is not mixed in to my
front speakers.  I never get anything out on the other channels with
ogle, but occasionally I can get multichannel sound with mplayer like this:

  mplayer dvd://1 -v -aid 128 -ao oss -channels 6

So I know my other channels are capable of producing sound.  But mplayer
doesn't work reliably, and of course, I'd like to use ogle, since it is
a better player :)

Do you know anything that could cause the behavior of the channels to
vary randomly?  Could running mplayer be affecting a later invocation of
ogle, or could my motherboard possibly be detecting my continual
rearrangements of the speaker plugs?

Does ogle use .asoundrc and should I be trying to do something useful
with it?  I noticed that surround40, surround51, and so on are defined
in /usr/share/alsa/alsa.conf - but I see no reason that surround51
should fail for me while surround40 plays sound.  If I try

  aplay -D plug:surround51 <some sound>

I get the error

  set_params:882: Can't use period equal to buffer size (1254 == 1254)

while plug:surround40 works.  I don't know what the "plug:" part means,
but it is necessary for aplay.  The documentation on .asoundrc is really
difficult for me to comprehend.

> >  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).
 
I still don't have a good understanding of how this all works - what
happens in my case when six channels are read from the DVD and I only
have four speakers?  Does the center sound go forward or back?  Where
are the names "RearLeft" etc. defined, and how is the correspondance to
"LeftSurround" etc. (displayed with -debug) made?  Is this just all
defined in the dolby specifications?

Sorry about all of the questions, and thanks for your help!

-Paul