ESP32 Digital Piano

// ESP32 Digital Piano - Advanced (8-voice polyphony + ADSR + MIDI serial)
// MIDI output via Serial at 31250 baud (connect to MIDI DIN socket or USB-MIDI)
// For native USB MIDI use ESP32-S2/S3 with USB_MIDI library
#include "driver/i2s.h"
#include <Wire.h>
#include <Adafruit_SSD1306.h>
#include <math.h>

Adafruit_SSD1306 oled(128,64,&Wire,-1);

const int TOUCH_PINS[8]={4,15,13,12,14,27,33,32};
const float FREQS[8]={261.63f,293.66f,329.63f,349.23f,392.0f,440.0f,493.88f,523.25f};
const int MIDI_NOTES[8]={60,62,64,65,67,69,71,72}; // C4-C5
const int SAMPLE_RATE=22050, I2S_NUM=0;
const int TOUCH_THRESH=40;

struct Voice {
  float phase,freq,amp;
  int state; // 0=off 1=attack 2=decay 3=sustain 4=release
  float env;
};
Voice voices[8]={};

// ADSR times in samples
const float ATTACK_S=0.01f, DECAY_S=0.05f, SUSTAIN_L=0.7f, RELEASE_S=0.1f;

void midiNoteOn(int note, int vel){ Serial1.write(0x90); Serial1.write(note); Serial1.write(vel); }
void midiNoteOff(int note){        Serial1.write(0x80); Serial1.write(note); Serial1.write(0); }

void processEnv(Voice &v, float dt){
  switch(v.state){
    case 1: v.env+=dt/ATTACK_S; if(v.env>=1){v.env=1;v.state=2;} break;
    case 2: v.env-=dt*(1-SUSTAIN_L)/DECAY_S; if(v.env<=SUSTAIN_L){v.env=SUSTAIN_L;v.state=3;} break;
    case 3: break;
    case 4: v.env-=dt/RELEASE_S; if(v.env<=0){v.env=0;v.state=0;} break;
  }
}

void audioTask(void*){
  int16_t buf[512];
  const float dt=1.0f/SAMPLE_RATE;
  while(1){
    for(int s=0;s<256;s++){
      float L=0;
      for(int i=0;i<8;i++){
        if(voices[i].state==0) continue;
        processEnv(voices[i],dt);
        float samp=sinf(voices[i].phase)*voices[i].env*voices[i].amp;
        L+=samp;
        voices[i].phase+=2.0f*M_PI*voices[i].freq*dt;
        if(voices[i].phase>2.0f*M_PI) voices[i].phase-=2.0f*M_PI;
      }
      int16_t s16=(int16_t)constrain((int)(L*8000),-32767,32767);
      buf[s*2]=s16; buf[s*2+1]=s16;
    }
    size_t w; i2s_write((i2s_port_t)I2S_NUM,buf,sizeof(buf),&w,portMAX_DELAY);
  }
}

void initI2S(){
  i2s_config_t cfg={
    .mode=(i2s_mode_t)(I2S_MODE_MASTER|I2S_MODE_TX),
    .sample_rate=SAMPLE_RATE,.bits_per_sample=I2S_BITS_PER_SAMPLE_16BIT,
    .channel_format=I2S_CHANNEL_FMT_RIGHT_LEFT,
    .communication_format=I2S_COMM_FORMAT_I2S,
    .intr_alloc_flags=0,.dma_buf_count=4,.dma_buf_len=512,.use_apll=false
  };
  i2s_pin_config_t pins={.bck_io_num=26,.ws_io_num=25,.data_out_num=22,.data_in_num=-1};
  i2s_driver_install((i2s_port_t)I2S_NUM,&cfg,0,NULL);
  i2s_set_pin((i2s_port_t)I2S_NUM,&pins);
}

void setup(){
  Serial.begin(115200);
  Serial1.begin(31250,SERIAL_8N1,16,17); // MIDI baud rate on Serial1
  Wire.begin(21,23); oled.begin(SSD1306_SWITCHCAPVCC,0x3C); oled.setTextColor(WHITE);
  initI2S();
  xTaskCreatePinnedToCore(audioTask,"audio",4096,NULL,5,NULL,0);
}

bool wasDown[8]={};
void loop(){
  for(int i=0;i<8;i++){
    bool down=(touchRead(TOUCH_PINS[i])<TOUCH_THRESH);
    if(down&&!wasDown[i]){ // key press
      voices[i]={0,FREQS[i],1.0f,1,0};
      midiNoteOn(MIDI_NOTES[i],100);
    }
    if(!down&&wasDown[i]){ // key release
      if(voices[i].state>0&&voices[i].state<4) voices[i].state=4;
      midiNoteOff(MIDI_NOTES[i]);
    }
    wasDown[i]=down;
  }
  oled.clearDisplay(); oled.setCursor(0,0); oled.setTextSize(1);
  oled.print("Active: ");
  for(int i=0;i<8;i++) if(voices[i].state>0) oled.printf("%d ",i+1);
  oled.display();
  delay(10);
}