[LAU] Fluidsynth: map each channel to individual jack audio output

[Tito Latini]

On Sun, Apr 24, 2016 at 03:13:42AM +0200, Lorenzo Sutton wrote:
> Hi all,
> 
> I have also posted this on the fluid-dev list, but maybe there is some 
> wisdom on LAU about it as well :)
> 
> What is the current status of synth.audio-channels and 
> synth.audio-groups on Linux with jack?
> 
> By starting fluidsynth with something like:
> 
> fluidsynth -a jack -o synth.audio-channels=4 -o synth.audio-groups=4 \
> -o audio.jack.multi=no
> 
> I do get 4 pairs of jack audio outputs.
> 
> However the behaviour isn't as expected at least according to this (very 
> old post):
> 
> https://lists.nongnu.org/archive/html/fluid-dev/2004-02/msg00027.html
> 
> Ideally is there a way to have each channel routed to a separate audio 
> output on jack these days? Any help appreciated.

'-o synth.audio-groups=4' fails: `fluid_settings_getint' returns 1
after `process_o_cmd_line_option' (also tested with a printf).

sh> fluidsynth -a jack -o synth.audio-groups=4 <<< settings 2>/dev/null \
    | grep audio-groups
synth.audio-groups          1

Idem with "-o synth.audio-channels=4 -o synth.audio-groups=4",
but "-G 4" works:

sh> fluidsynth -a jack -G 4 <<< settings 2>/dev/null | grep audio-groups
synth.audio-groups          4

There is a comment in `src/fluidsynth.c:594':

  /* The 'groups' setting is only relevant for LADSPA operation
   * If not given, set number groups to number of audio channels, because
   * they are the same (there is nothing between synth output and 'sound card')
   */

tested with:

sh> fluidsynth -a jack -L 4 <<< settings 2>/dev/null | grep audio-groups
synth.audio-groups          4

The follow example works (audio.jack.multi=no by default):

sh> fluidsynth -a jack -L 4
load /usr/share/sounds/sf2/FluidR3_GM.sf2
select 0 1 0 0
select 1 1 0 20
select 2 1 0 30
select 3 1 0 40