ESP32 I2S microphone level meter

// INMP441 sound level meter on an ESP32.
//
// The INMP441 is a MEMS microphone with an I2S output: the ESP32 drives the
// bit clock (SCK) and the word select (WS), and the microphone answers with
// 24-bit samples on SD, most significant bit first. L/R to GND puts it in the
// left slot, so the driver reads ONLY_LEFT. Each sample arrives in a 32-bit
// slot, left-aligned, so shifting right by 8 gives the 24-bit value.
//
// Every 512 samples (32 ms at 16 kHz) the sketch prints the RMS level and
// the peak in dBFS, decibels below a full-scale 24-bit sine, and a bar of the
// level from -90 to 0 dBFS. The numbers are labeled for the Serial Plotter. The microphone sends zeros for its first 2^18
// clocks after power-up (about a quarter of a second of reading), as its data
// sheet says, so the first few lines read -inf.
//
// Wiring: VDD to 3V3, GND to GND, L/R to GND, SCK to GPIO 32, WS to GPIO 25,
// SD to GPIO 33.

#include <driver/i2s.h>
#include <math.h>

#define I2S_PORT I2S_NUM_0
#define I2S_SCK 32
#define I2S_WS 25
#define I2S_SD 33

const int kSampleRate = 16000;
const int kSamples = 512;
int32_t samples[kSamples];

void setup() {
  Serial.begin(115200);
  Serial.println("INMP441 level meter");

  i2s_config_t i2s_config = {
    .mode = i2s_mode_t(I2S_MODE_MASTER | I2S_MODE_RX),
    .sample_rate = kSampleRate,
    .bits_per_sample = I2S_BITS_PER_SAMPLE_32BIT,
    .channel_format = I2S_CHANNEL_FMT_ONLY_LEFT,
    .communication_format = I2S_COMM_FORMAT_STAND_I2S,
    .intr_alloc_flags = ESP_INTR_FLAG_LEVEL1,
    .dma_buf_count = 4,
    .dma_buf_len = 256,
    .use_apll = false,
    .tx_desc_auto_clear = false,
    .fixed_mclk = 0
  };
  i2s_pin_config_t pin_config = {
    .bck_io_num = I2S_SCK,
    .ws_io_num = I2S_WS,
    .data_out_num = I2S_PIN_NO_CHANGE,
    .data_in_num = I2S_SD
  };

  if (i2s_driver_install(I2S_PORT, &i2s_config, 0, NULL) != ESP_OK) {
    Serial.println("i2s_driver_install failed");
    while (true) delay(1000);
  }
  i2s_set_pin(I2S_PORT, &pin_config);
  i2s_zero_dma_buffer(I2S_PORT);
}

void loop() {
  size_t bytes_read = 0;
  i2s_read(I2S_PORT, samples, sizeof(samples), &bytes_read, portMAX_DELAY);
  int n = bytes_read / sizeof(int32_t);
  if (n == 0) return;

  // Integer work per sample; the floats come once a buffer.
  int64_t sum = 0;
  int64_t sumSquares = 0;
  int32_t peak = 0;
  for (int i = 0; i < n; i++) {
    int32_t s = samples[i] >> 8;  // the 24-bit sample
    sum += s;
    int32_t a = s < 0 ? -s : s;
    if (a > peak) peak = a;
  }
  int32_t mean = (int32_t)(sum / n);  // take out any DC offset
  for (int i = 0; i < n; i++) {
    int64_t d = (int64_t)((samples[i] >> 8) - mean);
    sumSquares += d * d;
  }

  const float fullScale = 8388607.0f;  // 2^23 - 1
  float rms = sqrtf((float)sumSquares / n);
  float level = 20.0f * log10f(rms / fullScale) + 3.01f;  // dBFS, full-scale sine = 0
  float peakDb = 20.0f * log10f(peak / fullScale);

  Serial.print("level:");
  Serial.print(level, 1);
  Serial.print(" peak:");
  Serial.print(peakDb, 1);

  // The bar: one # for every 2.25 dB above -90 dBFS.
  int bars = 0;
  if (level > -90.0f) bars = (int)((level + 90.0f) / 2.25f);  // not for -inf
  if (bars > 40) bars = 40;
  Serial.print("  |");
  for (int i = 0; i < 40; i++) Serial.print(i < bars ? '#' : ' ');
  Serial.println("|");
}