MIDI filter/processor sketch

Projects Programming
4

You could use something like this:

Control Surface: MIDI Tutorial (see the MIDI routing section at the bottom)

#include <Control_Surface.h>

HardwareSerialMIDI_Interface midi = Serial;
// For testing, use:
//    USBDebugMIDI_Interface midi;

// A unidirectional pipe that connects a MIDI source to a MIDI sink.
// Messages coming from the source are handled by the `mapForwardMIDI()`
// function, where they can be altered or filtered out, and then sent
// to the MIDI sink.
struct MyMIDIFilter : MIDI_Pipe {
  // Pass on only Note On/Off messages, changing the channel to 5 and
  // transposing all notes an octave down.
  void mapForwardMIDI(ChannelMessage msg) override {
    switch (msg.getMessageType()) {
      case MIDIMessageType::NOTE_OFF:  // fallthrough
      case MIDIMessageType::NOTE_ON:
        msg.setChannel(CHANNEL_5);
        if (msg.data1 >= 12)
          msg.data1 -= 12;
        sourceMIDItoSink(msg);
        break;
      default: break;
    }
  }
  // Do not pass on System Exclusive messages.
  void mapForwardMIDI(SysExMessage) override {}
  // Do not pass on System Common messages.
  void mapForwardMIDI(SysCommonMessage) override {}
  // Pass on Real-Time messages without changes.
  void mapForwardMIDI(RealTimeMessage msg) override {
    sourceMIDItoSink(msg);
  }
};

MyMIDIFilter filter;

void setup() {
  // (MIDI input) -> (MIDI filter) -> (MIDI output)
  // (source)     ->    (pipe)     ->        (sink)
      midi        >>    filter     >>         midi;
  midi.begin();
}

void loop() {
  midi.update();
}

See also: Control Surface: ChannelMessage Struct Reference

If you use the USBDebugMIDI_Interface, open the Serial monitor (at 115200 baud) and type 98 3C 7F (Note On, channel 9, C4, full velocity). When you press Enter, you'll get back:

Note On    Channel: 5    Data 1: 0x30    Data 2: 0x7F

If you try to send a Pitch Bend message, for example, E0 12 34, you'll get nothing back, because the code filters them out.