Hi,
In case anyone else was interested in this issue, I wanted to report my
process to successfully get a Faust executable for e.g. a synth to only
listen to one particular MIDI channel (by default, for ON, OFF, CTRL, BEND
messages will respond to ALL MIDI channels).
In short, if you look at what the shell scripts that compile a `.dsp` file
do, they first call `faust` to get a `.cpp` file that wraps an architecture
file. If you recreate this process by seeing what the actual command that
run are with bash debugging on:
bash -x /usr/bin/faust2jackconsole -osc -midi -nvoices 1 your_synth.dsp
...you'll see the `faust` command that spits out a `.cpp` file. Run this
separately, then edit lines that look something like:
if (type == MIDI_NOTE_ON) {
...
}
and change them to:
if (type == MIDI_NOTE_ON & channel == 2) { // if you want to only listen
on channel 3
...
}
Do this for the stanzas for `MIDI_NOTE_OFF`, `MIDI_PITCH_BEND`, etc. and
any message type you want to filter by channel.
Once you've finished editing the `.cpp` file, continue running the `c++`
command you saw the faust wrapper script (e.g. `faust2jackconsole`) spit
out.
When you test it, you'll see that your instrument will properly only
respond to the midi channel you edited it to respond to. Bear in mind, midi
channel 1 is `0` in code, and 2 is `1`, etc. etc. i.e. for MIDI_NOTE_ON:
0x90 // channel 1
0x91 // channel 2
0x92 // channel 3
etc. etc.
Hope this helps someone else wanting to do the same.
Devs: should be a simple matter to make this dynamic from `faust` itself so
that the front-end can make this easy with an extra optional parameter,
like so....?:
faust2jackconsole -osc -midi -nvoices -midi_channel 4 -effect auto
my_synth.dsp
Where `-midi_channel ...` would be the new option that would effectively
alter those intermediate `.cpp` lines I refer to.
Aaron Krister Johnson
Music, etc.:
https://soundcloud.com/aaron-krister-johnson
https://soundcloud.com/filtercreed
https://www.youtube.com/channel/UC_utjGYbSizWE0dNyr0Vdmg
https://aaronkristerjohnson.bandcamp.com/
http://www.untwelve.org
Code:
https://github.com/akjmicro <http://www.untwelve.org>
On Mon, Sep 29, 2025 at 9:27 AM Stéphane Letz <[email protected]> wrote:
> Not much time to work on that right now yes.
>
> Stéphane
>
> > Le 29 sept. 2025 à 18:26, Aaron Krister Johnson <[email protected]> a
> écrit :
> >
> > Stephane et al,
> >
> > Gentle nudge here: any more thoughts on this MIDI channel issue, or is
> the best approach for now to work (or hack) around it?
> >
> > Aaron Krister Johnson
> > Music, etc.:
> > https://soundcloud.com/aaron-krister-johnson
> > https://soundcloud.com/filtercreed
> > https://www.youtube.com/channel/UC_utjGYbSizWE0dNyr0Vdmg
> > https://aaronkristerjohnson.bandcamp.com/
> > http://www.untwelve.org
> > Code:
> > https://github.com/akjmicro
> >
> >
> > On Sun, Sep 21, 2025 at 9:29 AM Aaron Krister Johnson <
> [email protected]> wrote:
> > I guess this is a hack, but for keyOn and keyOff coming from the
> polyphony code (which I traced from looking for `freq` references) I'm
> thinking changing these in `poly-dsp.h` to further filter by channel at
> compile time would be key?
> >
> > // MIDI API
> > MapUI* keyOn(int channel, int pitch, int velocity)
> > {
> > if (checkPolyphony()) {
> > int voice = getFreeVoice();
> > fVoiceTable[voice]->keyOn(pitch, velocity,
> fVoiceTable[voice]->fCurNote == kLegatoVoice);
> > return fVoiceTable[voice];
> > } else {
> > return 0;
> > }
> > }
> >
> > void keyOff(int channel, int pitch, int velocity = 127)
> > {
> > if (checkPolyphony()) {
> > int voice = getPlayingVoice(pitch);
> > if (voice != kNoVoice) {
> > fVoiceTable[voice]->keyOff();
> > } else {
> > fprintf(stderr, "Playing pitch = %d not found\n",
> pitch);
> > }
> > }
> > }
> >
> > Change to:
> >
> > // MIDI API
> > MapUI* keyOn(int channel, int pitch, int velocity)
> > {
> > if (checkPolyphony() & (channel == 1)) { // to respond only
> to MIDI channel 2, actual number is MIDI channel - 1
> > int voice = getFreeVoice();
> > fVoiceTable[voice]->keyOn(pitch, velocity,
> fVoiceTable[voice]->fCurNote == kLegatoVoice);
> > return fVoiceTable[voice];
> > } else {
> > return 0;
> > }
> > }
> >
> > void keyOff(int channel, int pitch, int velocity = 127)
> > {
> > if (checkPolyphony() & (channel == 1)) { // to respond only
> to MIDI channel 2, actual number is MIDI channel - 1
> > int voice = getPlayingVoice(pitch);
> > if (voice != kNoVoice) {
> > fVoiceTable[voice]->keyOff();
> > } else {
> > fprintf(stderr, "Playing pitch = %d not found\n",
> pitch);
> > }
> > }
> > }
> >
> > But yes, would be better to allow the polyphony/voice-table architecture
> to be more standards compliant about channels, I'm thinking. I'll test
> this, for now it will do if it works.
> >
> > Aaron Krister Johnson
> > Music, etc.:
> > https://soundcloud.com/aaron-krister-johnson
> > https://soundcloud.com/filtercreed
> > https://www.youtube.com/channel/UC_utjGYbSizWE0dNyr0Vdmg
> > https://aaronkristerjohnson.bandcamp.com/
> > http://www.untwelve.org
> > Code:
> > https://github.com/akjmicro
> >
> >
> > On Sun, Sep 21, 2025 at 8:26 AM Aaron Krister Johnson <
> [email protected]> wrote:
> > Is this MIDI channel filtering option being considered? If not, would
> the dev team welcome a patch if I tried my hand at it?
> >
> > ~AKJ
> >
> > Aaron Krister Johnson
> > Music, etc.:
> > https://soundcloud.com/aaron-krister-johnson
> > https://soundcloud.com/filtercreed
> > https://www.youtube.com/channel/UC_utjGYbSizWE0dNyr0Vdmg
> > https://aaronkristerjohnson.bandcamp.com/
> > http://www.untwelve.org
> > Code:
> > https://github.com/akjmicro
> >
> > On Fri, Sep 12, 2025, 13:31 Aaron Krister Johnson <[email protected]>
> wrote:
> > Ah, so it seems others have bumped into this limitation.
> >
> > Please, Faust devs, implement this! :D
> >
> > Aaron Krister Johnson
> > Music, etc.:
> > https://soundcloud.com/aaron-krister-johnson
> > https://soundcloud.com/filtercreed
> > https://www.youtube.com/channel/UC_utjGYbSizWE0dNyr0Vdmg
> > https://aaronkristerjohnson.bandcamp.com/
> > http://www.untwelve.org
> > Code:
> > https://github.com/akjmicro
> >
> >
> > On Thu, Sep 11, 2025 at 2:17 AM <[email protected]> wrote:
> > Additional reading here:
> https://github.com/grame-cncm/faust/discussions/696
> >
> > All of this must be carefully considered...
> >
> > Stéphane
> >
> > > Le 10 sept. 2025 à 19:26, Aaron Krister Johnson <[email protected]>
> a écrit :
> > >
> > > P.S. -- and if this is implemented, covering key/vel/velocity, too :D
> > >
> > > Aaron Krister Johnson
> > > Music, etc.:
> > > https://soundcloud.com/aaron-krister-johnson
> > > https://soundcloud.com/filtercreed
> > > https://www.youtube.com/channel/UC_utjGYbSizWE0dNyr0Vdmg
> > > https://aaronkristerjohnson.bandcamp.com/
> > > http://www.untwelve.org
> > > Code:
> > > https://github.com/akjmicro
> > >
> > >
> > > On Wed, Sep 10, 2025 at 10:23 AM Aaron Krister Johnson <
> [email protected]> wrote:
> > > Thanks, Stephane, yes, it would be most useful to have a Faust synth
> only respond on a declared channel, and `0` (or no specification) being
> "listen on all channels".
> > >
> > > Would this be in the "options" header, then?
> > >
> > > Aaron Krister Johnson
> > > Music, etc.:
> > > https://soundcloud.com/aaron-krister-johnson
> > > https://soundcloud.com/filtercreed
> > > https://www.youtube.com/channel/UC_utjGYbSizWE0dNyr0Vdmg
> > > https://aaronkristerjohnson.bandcamp.com/
> > > http://www.untwelve.org
> > > Code:
> > > https://github.com/akjmicro
> > >
> > >
> > > On Wed, Sep 10, 2025 at 9:00 AM Stéphane Letz <[email protected]> wrote:
> > > Hi Aaron,
> > >
> > > MIDI metadata for standard MIDI control can use MIDI channel:
> https://faustdoc.grame.fr/manual/midi/#configuring-midi-in-faust
> > >
> > > But it is currently not possible to set a given channel for polyphony,
> you can see that the channel info is not used in the polyphonic code for
> now:
> > >
> > >
> https://github.com/grame-cncm/faust/blob/f5c23da4fe6e09fa55667ae588f4b6ef9c226cae/architecture/faust/dsp/poly-dsp.h#L900
> > >
> > > Should we extend/revisit the syntax of freq/gate/gain convention for
> something like [freq: chan], [gate: chan], [gain: chan], with the
> convention chan = 0 would mean « receive on all channels», like for other
> MIDI messages ?
> > >
> > > Stéphane
> > >
> > > > Le 10 sept. 2025 à 17:35, Aaron Krister Johnson <[email protected]>
> a écrit :
> > > >
> > > > I notice in the manual, certain MIDI slider metadata operations can
> be filtered by a certain MIDI channel, like `keyon` uses the velocity of a
> given key on a given channel.
> > > >
> > > > But IMO, these kinds of operations seem geared towards a
> non-standard way of using a MIDI signal. Most useful mono- or poly- synths
> would want to use the general
> > > > keywords with special meaning: `freq/key`, `gain/vel`, and `gate` to
> drive the usually needed MIDI parameters in, for instance, a synthesizer
> signal chain.
> > > >
> > > > The problem I am having is: I'd like to pair a custom Faust synth
> with an outboard analog monosynth. Ideally, I could assign Faust to listen
> only on a certain MIDI channel.
> > > > As it is, it seems like I have to do the opposite: Faust will
> respond to _all_ channels (not ideal) and I make the hardware synth respond
> to MIDI channel 2. They are isolated only
> > > > by the fact that they are listening on different MIDI ports
> (otherwise, the Faust synth would be "hearing" both channels and trying to
> respond to both). What I have works for now,
> > > > but let's say I want 2 Faust synths -- they'd both be responding to
> all channels that particular virtual port is listening on.
> > > >
> > > > So, in short: is there an easy way to make a running Faust synth
> instance listen only to a particular MIDI channel when it's designed to use
> the keywords `freq`, `gain`, and `gate`,
> > > > per the docs? How easy would it be to hack Faust or add a feature
> for users for this if not?
> > > >
> > > > Aaron Krister Johnson
> > > > Music, etc.:
> > > > https://soundcloud.com/aaron-krister-johnson
> > > > https://soundcloud.com/filtercreed
> > > > https://www.youtube.com/channel/UC_utjGYbSizWE0dNyr0Vdmg
> > > > https://aaronkristerjohnson.bandcamp.com/
> > > > http://www.untwelve.org
> > > > Code:
> > > > https://github.com/akjmicro
> > > > _______________________________________________
> > > > Faudiostream-users mailing list
> > > > [email protected]
> > > > https://lists.sourceforge.net/lists/listinfo/faudiostream-users
> > >
> >
>
>
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