Working on a polyphonic interrupter for atmega328p / Arduino UNO

[Zipdox]

Code: [Select]

#include <MIDI.h>

#define samplingRate 62500
#define maxPlayingNotes 2

MIDI_CREATE_DEFAULT_INSTANCE();

float midiNotes[] = {8.18, 8.66, 9.18, 9.72, 10.3, 10.91, 11.56, 12.25, 12.98, 13.75, 14.57, 15.43, 16.35, 17.32, 18.35, 19.45, 20.6, 21.83, 23.12, 24.5, 25.96, 27.5, 29.14, 30.87, 32.7, 34.65, 36.71, 38.89, 41.2, 43.65, 46.25, 49, 51.91, 55, 58.27, 61.74, 65.41, 69.3, 73.42, 77.78, 82.41, 87.31, 92.5, 98, 103.83, 110, 116.54, 123.47, 130.81, 138.59, 146.83, 155.56, 164.81, 174.61, 185, 196, 207.65, 220, 233.08, 246.94, 261.63, 277.18, 293.66, 311.13, 329.63, 349.23, 369.99, 392, 415.3, 440, 466.16, 493.88, 523.25, 554.37, 587.33, 622.25, 659.26, 698.46, 739.99, 783.99, 830.61, 880, 932.33, 987.77, 1046.5, 1108.73, 1174.66, 1244.51, 1318.51, 1396.91, 1479.98, 1567.98, 1661.22, 1760, 1864.66, 1975.53, 2093, 2217.46, 2349.32, 2489.02, 2637.02, 2793.83, 2959.96, 3135.96, 3322.44, 3520, 3729.31, 3951.07, 4186.01, 4434.92, 4698.64, 4978.03, 5274.04, 5587.65, 5919.91, 6271.93, 6644.88, 7040, 7458.62, 7902.13, 8372.02, 8869.84, 9397.27, 9956.06, 10548.08, 11175.3, 11839.82, 12543.85};

typedef struct {
    byte pitch;
    int onTime;
    int offTime;
    int hasBeenOn;
} Note;

Note getNoteTimes(byte pitch, float dutyCycle){
  float period = samplingRate/midiNotes[pitch];
  Note finalNote = {pitch, round(dutyCycle*period), round((1-dutyCycle)*period), 0};
  return finalNote;
}

volatile Note playingNotes[maxPlayingNotes];


void handleNoteOn(byte channel, byte pitch, byte velocity){
    for(int i = 0; i < maxPlayingNotes; i++){
        if(playingNotes[i].onTime == 0){ // 0 onTime means this note isn't in use
            playingNotes[i] = getNoteTimes(pitch, 0.1);
            break;
        }
    }
    digitalWrite(13, HIGH);
}

void handleNoteOff(byte channel, byte pitch, byte velocity){
    for(int i = 0; i < maxPlayingNotes; i++){
        if(playingNotes[i].pitch == pitch){
            playingNotes[i] = {0,0,0,0}; // 0 onTime means it doesn't play
        }
    }
    digitalWrite(13, LOW);
}

void handlePitchBend(byte channel, int bend){
  float bendfactor = (1+ (float) bend/8190);
  float dutyCycle = 0.1;
  for(int i = 0; i < maxPlayingNotes; i++){
      if(playingNotes[i].onTime == 0) continue;
      float period = samplingRate/(bendfactor * midiNotes[playingNotes[i].pitch]);
      playingNotes[i].onTime = round(dutyCycle*period);
      playingNotes[i].offTime = round((1-dutyCycle)*period);
  }
}




void setup(){
    for(int i = 0; i<(sizeof(playingNotes)/sizeof(playingNotes[0])); i++){
        playingNotes[i] = {0,0,0,0};
    }

    pinMode(9, OUTPUT);
    pinMode(13, OUTPUT);

  cli(); //stop interrupts
  // set Timer/Counter Control Register to 0
  TCCR2B = 0;
 
  OCR2A = 0; // = 16000000 / (freq*prescaler) - 1 (must be <256)
  // turn on CTC mode
  TCCR2A |= (1 << WGM21);
  // Set CS01 and CS00 bits for 64 prescaler
  TCCR2B |= (1 << CS20);
  // enable timer compare interrupt
  TIMSK2 |= (1 << OCIE2A);
  sei(); //allow interrupts


  MIDI.setHandleNoteOn(handleNoteOn);
  MIDI.setHandleNoteOff(handleNoteOff);
  MIDI.setHandlePitchBend(handlePitchBend);
  MIDI.begin(MIDI_CHANNEL_OMNI);
}

void loop(){
    MIDI.read();
}

volatile bool shouldPlay;
ISR(TIMER2_COMPA_vect){

  shouldPlay = false;
 
  for(int i = 0; i < maxPlayingNotes; i++){
    if(playingNotes[i].onTime == 0) continue;
    playingNotes[i].hasBeenOn++;
    if(playingNotes[i].hasBeenOn > playingNotes[i].onTime){
      playingNotes[i].hasBeenOn = -playingNotes[i].offTime;
    }
    if(playingNotes[i].hasBeenOn >= 0){
        shouldPlay = true;
    }
  }

  if(shouldPlay){
    PORTB |= 1 << 1;
  }else{
    PORTB &= ~(1 << 1);
  }

}

Currently it can only play 2 notes at the same time or else it gets stuck unable to receiver MIDI commands to turn off. I'm nut sure why but I'm guessing because the lack of CPU time to process Serial data because of interrupts.

[Netzpfuscher]

Your code is full of float calculations. It's possible that you ran out of CPU time. Float calculations on a 8bit micro without floating point unit is extremely expensive.
If you make your calculations with integer arithmetic you save a lot CPU.

[Zipdox]

Your code is full of float calculations. It's possible that you ran out of CPU time. Float calculations on a 8bit micro without floating point unit is extremely expensive.
If you make your calculations with integer arithmetic you save a lot CPU.

The float calculations are for higher accuracy so it's not as out tune. But I'll try and see if I can get better performance with int.

[Netzpfuscher]

You simply need fixed point 

Take a look at the function after line 130:
https://github.com/Netzpfuscher/UD3/blob/master/common/ud3core/tasks/tsk_midi.c

[Zipdox]

You simply need fixed point 

Take a look at the function after line 130:
https://github.com/Netzpfuscher/UD3/blob/master/common/ud3core/tasks/tsk_midi.c

I'm not very experienced with C, what do you mean?

[Netzpfuscher]

To make it simple.
Let's use your MIDI-Table and pick a number: 18.35 you also can represent it with 1835 if you multiply it by 100 or 12543.85 which is the "same" as 1254385. Both numbers fit in a uint32. And calculations with int/uint are fast on a Atmel. Floats needs emulation in Software which is extremely slow. So you can multiply all of your numbers by 100 make your calculations and at the end you can divide by 100 to get the integer part and make modulo operation to get the fraction. You need to care yourself not to overflow the variables
But this is not so fast than another way (which I use) the first example is decimal, a decimal division or multiplication on a system without a hardware multiplier/divider is slow. But a binary multiplication/division is very fast. It's a shift operation. Divide by 2 is a right shift multiply by 2 is a left shift which takes only one clock cycle

Lets use the number 12543.85 you can use 16 bits to store the 12543 and 16 bits to store the .85.
fixed_point_number = (12543<<16) + ((85*65535)/100) = 822073752 = ‭‭0011000011111111 1101100110011000‬

The conversion only needs to be done once at the beginning and at the end of the calculation. Or use precalculated values for the frequency in this format And is has another advantage. Lets say you need to put the result in a 16 bit PWM Block or a 16 Bit DAC. Then you need no back calculation to decimal. You take your 32bit fixed point value shift it right value = value >>16 and you have it for your PWM block  But that everything works right all numbers in a calculation needs to be in this format. For integers it is simple just shift left and right and you have it.

For all of this there are premade librarys just google a bit for fixed point math.

[Zipdox]

To make it simple.
Let's use your MIDI-Table and pick a number: 18.35 you also can represent it with 1835 if you multiply it by 100 or 12543.85 which is the "same" as 1254385. Both numbers fit in a uint32. And calculations with int/uint are fast on a Atmel. Floats needs emulation in Software which is extremely slow. So you can multiply all of your numbers by 100 make your calculations and at the end you can divide by 100 to get the integer part and make modulo operation to get the fraction. You need to care yourself not to overflow the variables
But this is not so fast than another way (which I use) the first example is decimal, a decimal division or multiplication on a system without a hardware multiplier/divider is slow. But a binary multiplication/division is very fast. It's a shift operation. Divide by 2 is a right shift multiply by 2 is a left shift which takes only one clock cycle

Lets use the number 12543.85 you can use 16 bits to store the 12543 and 16 bits to store the .85.
fixed_point_number = (12543<<16) + ((85*65535)/100) = 822073752 = ‭‭0011000011111111 1101100110011000‬

The conversion only needs to be done once at the beginning and at the end of the calculation. Or use precalculated values for the frequency in this format And is has another advantage. Lets say you need to put the result in a 16 bit PWM Block or a 16 Bit DAC. Then you need no back calculation to decimal. You take your 32bit fixed point value shift it right value = value >>16 and you have it for your PWM block  But that everything works right all numbers in a calculation needs to be in this format. For integers it is simple just shift left and right and you have it.

For all of this there are premade librarys just google a bit for fixed point math.

I will try to convert it to fixed point.