Faust: MIDI


Using MIDI in Faust requires only minor additions to the code and compiler arguments. For first steps it can be helpful to control single synth parameters with MIDI controllers. This can be configured via the UI elements. The following example uses MIDI controller number 48 to control the frequency of a sine wave by adding [midi:ctrl 48] to the hslider parameters.

// midi-example.dsp
// Control a sine wave frequency with a MIDI controller.
// Henrik von Coler
// 2020-05-17


freq = hslider("frequency[midi:ctrl 48]",100,20,1000,0.1) : si.smoo;

process = os.osc(freq) <: _,_ ;

CC 48 has been chosen since it is the first slider on the AKAI APC mini. If the controller numbers for other devices are not known, they can be found using the PD patch reverse_midi.pd.

Compiling with MIDI

In order to enable the MIDI functions, the compiler needs to be called with an additional flag -midi:

$ faust2xxxx -midi midi_example.dsp

This flag can also be combined with the -osc flag to make synths listen to both MIDI and OSC.

Note Handling & Polyphony

Typical monophonic and polyphonic synth control can be added to Faust programs by defining and mapping three parameters:

  • freq

  • gain

  • gate

When used like in the following example, they will be linked to the parameters of MIDI note on and note off events with a frequency and a velocity.

// midi_trigger.dsp
// Henrik von Coler
// 2020-05-17

freq    = nentry("freq",200,40,2000,0.01) : si.polySmooth(gate,0.999,2);
gain   = nentry("gain",1,0,1,0.01) : si.polySmooth(gate,0.999,2);
gate   = button("gate") : si.smoo;

process = vgroup("synth",os.sawtooth(freq)*gain*gate <: _,_);

Compiling Polyphonic Code

$ faust2xxxx -midi -nvoices 12 midi_trigger.dsp

MIDI on Linux

Faust programs use Jack MIDI, whereas MIDI controllers usually connect via ALSA MIDI. In order to control the synth with an external controller, a bridge is nedded:

$ a2jmidi_bridge

The MIDI controller can now connect to the a2j_bridge input, which is then connected to the synth input.