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Did some testing: ALSA works great with multi channel... except our channel mapping
is incorrect.  It looks like a lot of the audio got pumped to the left speaker on my
headphones.

We can either downmix to stereo (lame but easy) or we can swizzle the channels to
ALSA's preferred format (testing = watch some 5.1 movies!!!)

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folding is the easier change, because it can channel swap for free.
The ALSA channel layout appears to be:
0 - Front Left
1 - Front Right
2 - Rear Left
3 - Rear Right
4 - Center
5 - LFE

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 Issue 23304  has been merged into this issue.

Moving this out of mstone-4 and into mstone-5 after talking to the Linux 
team.

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The following revision refers to this bug:
    http://src.chromium.org/viewvc/chrome?view=rev&revision=29299 

------------------------------------------------------------------------
r29299 | ajwong@chromium.org | 2009-10-16 11:41:56 -0700 (Fri, 16 Oct 2009) | 10 lines
Changed paths:
   M http://src.chromium.org/viewvc/chrome/trunk/src/media/audio/linux/alsa_output.cc?r1=29299&r2=29298
   M http://src.chromium.org/viewvc/chrome/trunk/src/media/audio/linux/alsa_output.h?r1=29299&r2=29298
   M http://src.chromium.org/viewvc/chrome/trunk/src/media/audio/linux/alsa_output_unittest.cc?r1=29299&r2=29298
   M http://src.chromium.org/viewvc/chrome/trunk/src/media/audio/linux/alsa_wrapper.cc?r1=29299&r2=29298
   M http://src.chromium.org/viewvc/chrome/trunk/src/media/audio/linux/alsa_wrapper.h?r1=29299&r2=29298
   M http://src.chromium.org/viewvc/chrome/trunk/src/media/audio/linux/audio_manager_linux.cc?r1=29299&r2=29298
   M http://src.chromium.org/viewvc/chrome/trunk/src/media/audio/mac/audio_output_mac.cc?r1=29299&r2=29298
   A http://src.chromium.org/viewvc/chrome/trunk/src/media/base/media_switches.cc
   A http://src.chromium.org/viewvc/chrome/trunk/src/media/base/media_switches.h
   M http://src.chromium.org/viewvc/chrome/trunk/src/media/media.gyp?r1=29299&r2=29298

Move Alsa device opening into the audio thread, and add in support for multi-channel audio.

Moving the device opening into the audio thread will prevent browser hangs for badly behaving alsa implementations (like pulseaudio) that can hang snd_pcm_open if pulseaudiod is wedged, even if SND_PCM_NONBLOCK is requested.

For multi-channel audio, device enumeration has been added to try and find a multi-channel device with a stable channel mapping.  According to http://0pointer.de/blog/projects/guide-to-sound-apis.html, default should only be used with mono and stereo stream because the channel ordering is not defined by Alsa. To get a well-defined channel ordering, one must use one of the surround40, surround51, etc., device names.  However, these device names do not always allow multiple opens, so a fallback scheme is implemented to use default if necessary.

BUG=20945,17703
TEST=listened with built-in soundcard and USB soundcard with various other audio programs running.

Review URL: http://codereview.chromium.org/275022
------------------------------------------------------------------------

Confirmed working.  Must run like something like this
./chrome --alsa-device=surround51:1

aplay -L
gives a list of drivers

If you only have one audio-device on the system, it should auto-detect.

The device string will be different per computer, but should be some variant of

surroundXX

option with a plug: prefix, or a suffix specifying the exact card.

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