Does readBytes() have delay?

[hexer97149]

I currently have an ESP32 that initially receives from the I2S microphone and sends the data via Serial to a Python program using Pyaudio for playback. However, I'm using readBytes() to read 2048 bytes of raw data, then keeping one channel's data and using a for loop to separate the data before sending it. I'm not sure where the delay is causing the playback to sound like it's slightly under-sampled. Can you tell me what I might have done wrong?

and this is my arduino code:

/**
   @file streams-i2s-serial.ino
   @author Phil Schatzmann
   @brief see https://github.com/pschatzmann/arduino-audio-tools/blob/main/examples/examples-stream/streams-i2s-serial/README.md

   @author Phil Schatzmann
   @copyright GPLv3
*/


#include "AudioTools.h"

AudioInfo info(16000, 2, 16);
I2SStream i2sStream; // Access I2S as stream
//CsvOutput<int32_t> csvStream(Serial);
//StreamCopy copier(csvStream, i2sStream); // copy i2sStream to csvStream
int16_t samples[1024];
uint8_t DataBuffer[1536];
int sb_index = 0;

// Arduino Setup
void setup(void) {
  Serial.begin(115200);
  //AudioLogger::instance().begin(Serial, AudioLogger::Info);

  auto cfg = i2sStream.defaultConfig(RX_MODE);
  cfg.copyFrom(info);
  cfg.i2s_format = I2S_STD_FORMAT; // or try with I2S_LSB_FORMAT
  cfg.bits_per_sample = 16;
  cfg.is_master = true;
  // this module nees a master clock if the ESP32 is master
  cfg.use_apll = false;  // try with yes
  //cfg.pin_mck = 3;
  i2sStream.begin(cfg);

  // make sure that we have the correct channels set up
  //csvStream.begin(info);

}

// Arduino loop - copy data
void loop() {
  //copier.copy();

  size_t bytes_read  = i2sStream.readBytes((uint8_t*) samples, 512 * sizeof(int16_t));// 512 * sample_size (2 bytes) = 1024 Bytes
  size_t samples_read = bytes_read / sizeof(int16_t);// numbers of samples = bytes_read / sample_size 
  //Serial.print("samples_read:");
  //Serial.println(samples_read);
  if (samples_read) {

    for (int i = 0; i < samples_read; i = i + 1) { //512 samples(2chs) -> 256 samples(1ch)
      if (i % 2 != 0) {
        DataBuffer[sb_index] = '\n';
        sb_index ++;

        DataBuffer[sb_index] = (samples[i] >> 8) & 0xFF; //(samples[i] >> 8) & 0xFF
        sb_index ++;

        DataBuffer[sb_index] = (samples[i] & 0xFF) ; //(samples[i] & 0xFF)
        sb_index ++;
        //Sending data via SerialUSB
        if (sb_index >= 1536) {

          Serial.write(DataBuffer, sb_index); // 256 samples (512 Bytes) + '\n' * 256 (256 Bytes) = 768 Bytes

          sb_index = 0;

        }
      }


    }
    // Clear the read count
    samples_read = 0;

  }

}

2024-09-23.07-44-48.mp4

[pschatzmann]

Why so complicated ? your code is quite difficult to understand: If I get your logic right, you just want to write out binary data delimited with a line feed.

I suggest to do something like this instead:

loop() {
    int16_t sample;
    size_t bytes_read = i2sStream.readBytes((uint8_t*) &sample, sizeof(int16_t));
    assert(bytes_read==sizeof(int16_t)); // i2s uses blocking read, so this should always be treu
    Serial.write((uint8_t*)&sample, sizeof(int16_t));
    Serial.print('\n'); // for cr or Serial.println() for cr lf
}

There is still one issue however: you generate 2 * 3 (=2 bytes + delimiter) * 16000 * 8 bits per second = 768000 but your Serial transmission rate is much lower. This can't work....