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

[F Tux]

I know you said you wanted to keep it simple, but I'd like to point out
that I have a similar setup running at home. I listen to all my music with
a guitar amplifier for the treble and a bass amplifier for the bass.

Obviously, I have to downmix the signal to mono (I don't miss the stereo
sound, I just mind hearing every single instrument as clear as possible).

The signal coming from my music player is routed into 2 separated EQs which
I use to filter the high frequencies from the bass amp and the low from the
guitar amp. You could use highpass and lowpass filters for this, but using
EQs also serves the purpose of removing troublesome high harmonics in the
guitar amp, and also, equalizing the output.

All this I do with the calf plugins (never bothered to try any other,
because they've suited me good enough so far) but you can also do that with
any gui-less plugin.

If I had to downmix the bass I'd get the player's output through a lowpass
and then downmix it by connecting left and right outputs of the filter to
the channel that's output to the subwoofer. At the same time, you can route
the untouched signal to the treble speakers adding (possibly adding some
plugin in the middle to compensate the latency difference).

I know this is a bit more involved than you originally planned but I doubt
there is a simpler way with the ALSA tools available, and this way you will
end up with a completely customized setup.

Some of the stuff I described is only based on my trial and error
experience. The enlightened people on the list will surely know how to
polish this method though I use what I have described everyday and it just
works.

Sorry for the long text, have a good one!


On Sun, Feb 9, 2014 at 8:51 PM, Matt Garman <matthew.garman at gmail.com>wrote:

> 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
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