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[Bug 1286021] Re: 2.1 surround support (userspace)


Today's status:

 - alsa-lib patches for adding the surround21:x name is upstream. There has not been an upstream release of alsa-lib since.
 - pulseaudio patches are not yet upstream. But I found that the problem might be more complex than first expected.

 1) It's easy just to add another "Surround 2.1" profile to PulseAudio.
This is just a matter of me resending that patch, we can probably add it
to upstream PulseAudio even though there is no upstream release of alsa-

However, this will just add Surround 2.1 support for stuff that already
is 2.1 in source material (well, 5.1 will be downmixed to 2.1, but
normal stereo will not be upmixed, so no sound will be heard on the

 2) To automatically upmix stereo material to 2.1 (so sound can be heard
on the subwoofer), we can set the key enable-lfe-remixing=yes in
/etc/pulse/daemon.conf. However, this is a very trivial upmixing where
the result sent to the subwoofer is just the same as the average of the
left and right channel.

This might still be a valid approach if the subwoofer hardware has a
lowpass filter in hardware (or possibly, hardcoded in secret registers
on the codec).

 3) A more elegant approach would be to only send LFE sounds to the
subwoofer when upmixing - and only send the remaining non-LFE audio to
the front speakers. However this requires someone to actually write a
lowpass filter, as I don't think we have such an implementation in
upstream PulseAudio currently.

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  2.1 surround support (userspace)

Status in “alsa-lib” package in Ubuntu:
  Fix Committed
Status in “pulseaudio” package in Ubuntu:
  In Progress

Bug description:
  2.1 surround should work as seamlessly as other surround options, such
  as 4.0, 5.1 and 7.1 work today.

  Alsa-lib and pulseaudio patches for 14.04 can be tested here:


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