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

[Matt Garman]

On Sun, Feb 9, 2014 at 3:27 PM, Fons Adriaensen <fons at linuxaudio.org> wrote:
> On Sun, Feb 09, 2014 at 03:08:41PM -0600, Matt Garman wrote:
>> 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.
>
> You really need to provide a bit more information here.

Sure, no problem.  What I have is:
    (1) Linux server with a media collection in a basement network closet
    (2) A powered subwoofer on the main floor, with only a single coax
run between the sub and network closet in the basement
    (3) In-ceiling speakers on the main floor, all wires terminating
in same network closet

So the fundamental problem is that there's no physical locality
between the server and the speakers.  What I was thinking was to use a
USB DAC that has dual outputs.  One output would go to a power amp,
which in turn would go to a speaker selector (so I can power multiple
sets of speakers).

The DAC's other output needs to go to the sub.  But there's the
problem: I only have a single coax run, so I can only send one
channel.  This means, either pick one and hope most music doesn't pan
the bass (the point of the other thread I started), or come up with
some way to mixdown the signal to mono so I can pass it over the
single conductor.  I thought about doing this in hardware, see this
thread:
    http://www.audiocircle.com/index.php?topic=123314.0
But it occurred to me I could possibly spare some expense by mixing
down the signal at the server level.

In reality, based on the previous thread, I think I could probably
send just one channel and be OK most of the time.  But if I can easily
get it 100% of the time, why not?  :)

Note that in this solution, I'm sending the full-range signal to both
the in-ceiling speakers and the sub.  The sub has a builtin crossover,
and the speakers can handle the full signal.

As a side note, if you read the AudioCircle thread above, you'll see I
already have an AVR that can send a bass signal over coax... my goal
is to get rid of the AVR though, as I have more speakers than it has
outputs (would rather not use the AVR and speaker selector), plus I
want to move control off the AVR and onto the Linux server (mpd).

Let me know if anything is unclear or if you have any other ideas!

Thanks,
Matt