framework-arduinoteensy-cxx20/libraries/MIDI/examples/AltPinSerial/AltPinSerial.ino

#include <MIDI.h>

// Simple tutorial on how to receive and send MIDI messages
// on a different serial port, using SoftwareSerial.
// Here, when receiving any message on channel 4, the Arduino
// will blink a led and play back a note for 1 second.

#if defined(ARDUINO_SAM_DUE) || defined(SAMD_SERIES)
   /* example not relevant for this hardware (SoftwareSerial not supported) */
   MIDI_CREATE_DEFAULT_INSTANCE();
#else
   #include <SoftwareSerial.h>
   using Transport = MIDI_NAMESPACE::SerialMIDI<SoftwareSerial>;
   int rxPin = 18;
   int txPin = 19;
   SoftwareSerial mySerial = SoftwareSerial(rxPin, txPin);
   Transport serialMIDI(mySerial);
   MIDI_NAMESPACE::MidiInterface<Transport> MIDI((Transport&)serialMIDI);
#endif

void setup()
{
  pinMode(LED_BUILTIN, OUTPUT);
  MIDI.begin(4);                    // Launch MIDI and listen to channel 4
}

void loop()
{
  if (MIDI.read())                  // If we have received a message
  {
    digitalWrite(LED_BUILTIN, HIGH);
    MIDI.sendNoteOn(42, 127, 1);    // Send a Note (pitch 42, velo 127 on channel 1)
    delay(1000);                    // Wait for a second
    MIDI.sendNoteOff(42, 0, 1);     // Stop the note
    digitalWrite(LED_BUILTIN, LOW);
  }
}