ESP32 I2S: digital audio in from a microphone and out to an amplifier

// MAX98357A 440 Hz tone on an ESP32.
//
// The MAX98357A is an I2S amplifier: it takes the bit clock (BCLK), the word
// select (LRC) and the samples (DIN) and drives a speaker directly. With SD
// left open it plays the average of the left and right channels; GAIN left
// open is 9 dB.
//
// This sketch uses ESP_I2S.h, the I2S library of Arduino-ESP32 3.x. In
// setup() it fills a table with one cycle of a sine; loop() keeps writing the
// next block of samples, the same value to the left and the right slot, so
// the speaker hears a steady 440 Hz. At 16 kHz, 440 Hz is 36.36 samples a
// cycle, so the table is read with a 16.16 fixed-point phase rather than a
// whole number of samples, and no float is touched per sample.
//
// Wiring: VIN to 5V (VIN on the DevKit), GND to GND, BCLK to GPIO 26,
// LRC to GPIO 25, DIN to GPIO 22, a speaker across SPK+ and SPK-.

#include <ESP_I2S.h>
#include <math.h>

#define I2S_BCLK 26
#define I2S_LRC 25
#define I2S_DIN 22

const uint32_t kSampleRate = 16000;
const uint32_t kFrequency = 440;
const int kTable = 256;
const int16_t kAmplitude = 8000;  // about a quarter of full scale

I2SClass i2s;
int16_t sine[kTable];
int16_t block[2 * 128];  // 128 stereo frames: left, right, left, right ...
uint32_t phase = 0;
// Table steps a sample, in 16.16 fixed point: 440 * 256 / 16000.
const uint32_t kStep = (uint32_t)(((uint64_t)kFrequency * kTable << 16) / kSampleRate);

void setup() {
  Serial.begin(115200);
  for (int i = 0; i < kTable; i++) {
    sine[i] = (int16_t)(kAmplitude * sinf(2.0f * (float)M_PI * i / kTable));
  }
  i2s.setPins(I2S_BCLK, I2S_LRC, I2S_DIN);
  if (!i2s.begin(I2S_MODE_STD, kSampleRate, I2S_DATA_BIT_WIDTH_16BIT, I2S_SLOT_MODE_STEREO)) {
    Serial.println("Failed to initialize I2S!");
    while (true) delay(1000);
  }
  Serial.println("Playing 440 Hz");
}

void loop() {
  for (int i = 0; i < 128; i++) {
    int16_t s = sine[(phase >> 16) & (kTable - 1)];
    block[2 * i] = s;      // left
    block[2 * i + 1] = s;  // right
    phase += kStep;
  }
  i2s.write((uint8_t *)block, sizeof(block));
}