[LAU] On the fly bass downmixing to mono (mpd or alsa)?

[Matt Garman]

Hi,

I started the thread "How is the bass mixed? Per-channel frequency
analysis? Histogram?" a few weeks ago.  One suggestion that came up a
few times was to simply downmix the bass to mono before sending it
out.

How would I go about this?  Currently, this system doesn't have any
audio.  What I intend to do is send audio out over USB to a DAC.
Ideally, I'd like to do on-the-fly remixing of only "bass frequencies"
to mono.  Ideally the frequency should be configurable, but I think
about 150Hz or below is a good start.

If possible, I'd like to keep things simple, and have only ALSA and
MPD running.  As far as I can tell, I don't think I can do this at the
MPD level.

I saw an example where I can easily downmix all output to mono.  I'm
not opposed to that, but it would be nice to selectively downmix only
the lower (subwoofer) frequencies.

Thanks again!
Matt