I2S.h Transmission on core0, ADCInput on Core1 not working as expected

[Geekachuqt]

Hi, I'm facing an issue where sampling the ADCs and transmissing to I2S on different cores is not working as expected.

I have set up a program which uses while(I2S.availableForWrite(){} to write to an I2S codec. If I include ADCInput.read() calls inside this loop, the program works as expected, and pushes audio through the ADC to the I2S codec.

However, if I instead add a boolean flag to core0, which triggers a case on core1 to remove the flag and execute ADCInput.read(), the audio gets severely distorted, as if the two cores not executing in sync anymore.

The main reason this confuses me is that by adding Serial.println(millis()) at the start of each of the two loops, I can see that they are executing in tandem, but the result isn't as I'd expect.

Is there something more that needs to be done in order to offload processing in this manner from processor one to processor two?

#include <ADCInput.h>
#include <I2S.h>

#define I2S_WORDS 64
#define I2S_DATA_PIN  9
#define I2S_BCLK_PIN  10
#define I2S_BUFFERS   4
#define SAMPLE_RATE 48000

I2S i2s_output(OUTPUT);

ADCInput adc(A0,A1,A2,A3);

int16_t left_buffer[I2S_WORDS];
int16_t right_buffer[I2S_WORDS];

int16_t A0Buffer[I2S_WORDS];
int16_t A1Buffer[I2S_WORDS];
int16_t A2Buffer[I2S_WORDS];
int16_t A3Buffer[I2S_WORDS];

int16_t pos = 0;
float counter = 0;
float second = 0;
float localPos = 0;
bool process = false;

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  digitalWrite(LED_BUILTIN, HIGH);
  Serial.begin(115200);

  i2s_output.setFrequency(SAMPLE_RATE);
  i2s_output.setDATA(I2S_DATA_PIN);
  i2s_output.setBCLK(I2S_BCLK_PIN);
  i2s_output.setBitsPerSample(16);
  i2s_output.setBuffers(I2S_BUFFERS, I2S_WORDS);
  i2s_output.begin();

  for(int16_t i = 0; i < I2S_WORDS; ++i){
    left_buffer[i] = 0;
    right_buffer[i] = 0;

    A0Buffer[i] = 0;
    A1Buffer[i] = 0;
    A2Buffer[i] = 0;
    A3Buffer[i] = 0;
  }
}

void loop() {

  while(i2s_output.availableForWrite()){
    process = true;
    i2s_output.write(A0Buffer[pos]);
    i2s_output.write(A0Buffer[pos]);

    counter++;
    if(counter == 48000){
      counter = 0;
      Serial.println(millis());
    }

    pos++;

    if(pos == I2S_WORDS){
      pos = 0;
    }
  }
}

void setup1(){
  adc.setFrequency(SAMPLE_RATE/4);
  adc.setBuffers(I2S_BUFFERS, I2S_WORDS);
  adc.begin();
}

void loop1(){

  if(process){

    process = false;

    second++;

    A0Buffer[localPos] = adc.read();
    A1Buffer[localPos] = adc.read();
    A2Buffer[localPos] = adc.read();
    A3Buffer[localPos] = adc.read();

    localPos++;

    if(localPos == I2S_WORDS){
        localPos = 0;
    }

    if(second == SAMPLE_RATE){
      second = 0;
      Serial.println(millis());
    }
  }
}