Help creating MIDI to pulse output generator for DCO

[thunderthedog]

Hi. I'm trying to make a arduino based DCO. the first step I need to do it make code that takes a midi input, converts the MIDI noteByte to a frequency, then sends out a 100uS pulse on a digital out pin at the frequency determined by the noteByte. This sounds simple but i'm having a lot of trouble doing it. The main issue I come up with is I can't get the arduino notebyte variable to be carried over to the output. I feel like my code needs a lot of work. the noteByte to frequency equation I know is right and works (i tried it with a dummy AnalogRead pot variable and the whole DCO output worked but the arduino lags and I don't get the desired frequencies). I've also heard timer interrupts are a better more accurate way to do this sort of thing but I haven't been able to understand how they work yet. If someone can help me make code that works that would be great. Here's my code:

float commandByte;
float noteByte;
float velocityByte;
byte noteOn = 144;
float frequency;

void setup() {
  Serial.begin(31250);
  pinMode(13, OUTPUT);
  pinMode(12, INPUT);
}



void checkMIDI() {

  do {
    if (Serial.available()) {
      commandByte = Serial.read();//read first byte
      noteByte = Serial.read();//read next byte
      velocityByte = Serial.read();//read final byte
      if (commandByte == noteOn) {
        frequency = 440 * pow(2, (noteByte - 69) / 12);
        float highTime = 100;
        float lowTime = ((1000000 / frequency) - highTime);
        digitalWrite(13, HIGH);
        delayMicroseconds(highTime);
        digitalWrite(13, LOW);
        delayMicroseconds(lowTime);
      }
    }
  }
while (Serial.available() > 2);//when at least three bytes available
}


void loop() {
  checkMIDI();
  
  //for debugging
  if (digitalRead(12) == HIGH) {
    Serial.print("MIDI BYTE: ");
    Serial.print(noteByte);
    Serial.print("    FREQUENCY: ");
    Serial.println(frequency);
  }
}

[PieterP]

@thunderthedog

void checkMIDI() {
  ...
}

The code you posted does not correctly parse MIDI input. It also tries to read more Serial input than might be available.

See my answer in another recent thread on the topic:

@PieterP

You need a state machine that keeps track of where you are in the message, and you need to check whether the bytes you've received are status bytes or data bytes. MIDI messages can consist of any arbitrary number of bytes, so you cannot naively process the incoming data in chunks of 3 bytes. If you design this state machine correctly, you should also be able to handle running status. The MIDI standard document (see https://midi.org) contains the necessary information and diagrams to implement this.

If you don't want to use any libraries and really want to write the parser yourself, you can find some inspiration in existing libraries:

• Control-Surface/src/MIDI_Parsers/SerialMIDI_Parser.cpp at 3ae248994d478a6aa3da7fdc199c8890ed5bb9aa · tttapa/Control-Surface · GitHub
• arduino_midi_library/src/MIDI.hpp at 2d64cc3c2ff85bbee654a7054e36c59694d8d8e4 · FortySevenEffects/arduino_midi_library · GitHub

This should make it clear that it is not a trivial task. Be prepared to study the MIDI standard carfully, and write a lot of tests to make sure your code handles the different types of messages correctly, e.g.

• Control-Surface/test/MIDI_Parsers/tests-MIDI_Parsers.cpp at 3ae248994d478a6aa3da7fdc199c8890ed5bb9aa · tttapa/Control-Surface · GitHub
• arduino_midi_library/test/unit-tests/tests/unit-tests_MidiInput.cpp at 2d64cc3c2ff85bbee654a7054e36c59694d8d8e4 · FortySevenEffects/arduino_midi_library · GitHub

Your digitalWrite calls should not be inside of the MIDI parsing code, now they're only executed once for each MIDI message, which is not what you want. Consider using PWM on a hardware timer instead.