Syncing of buffer-transmission with ESP32, I2S MEMS-mic and SD-card (FreeRTOS, PlatformIO, ESP-PROG)

Question

i know this forum dislikes "open" questions like this, nevertheless i'd like somebody to help untie the knot in my head, much appreciated.

The goal is simple:

• read a stereo 32bit 44100 S/s I2S signal from 2 adafruit sph0645 mics
• create a wav-header and store the data onto an SD-card

I've been at this for a few days now and i know that this will be much more complicated than i originally thought. Main reason: signal quality. Like most tutorials on this subject the simplest "hello world" for these mics is a looped polling for I2S-samples. Poll, fill buffer, output via serial or write to SD-card. This returns a choppy, noisy, sped up version of RL-audio. The filling of the internal DMA-buffers can be seen as constant, but the rest is mostly chaos, so

how to i sync these DMA-buffers with the rest of my code?

From experience with the STM32 HAL i'd imagine some register which can be set to throw an interrupt whenever a buffer is full, or an event which can be sent between tasks via queues. Examples on this subject either poll in a main loop with mono an abysmal sample-rate and bit depth or use pages of overkill code and never adress what it does, "just copy and it works", not good. Does the ESP32-Arduino framework provide some way to to this properly? The espressif-documentation isn't something to look forward to, since some of their I2S interface functions don't even work (if you are researching this topic as well, you too might have noticed that i2s_read only returns zeros). Just a hint into the right direction would help, i'm writing my own code anyway. Interrupts? Events? Timers? Polling for full buffers? Only you might know.

have a good one, thx

Answer 1

Thanks to https://github.com/atomic14/ i now have an answer for a syncing-method which works very well. This method has been tried by https://esp32.com/viewtopic.php?t=12546 who also didn't fully understand what was going on: the espressif i2s-interface offers a flag stored in an event which is triggererd every time one of the specified dma-buffers has received a full set of data, ergo, is full. It looks like this:

while(<your condition>){

i2s_event_t evt;
if (xQueueReceive(<your queue>, &evt, portMAX_DELAY) == pdPASS){
  if (evt.type == I2S_EVENT_RX_DONE){
    size_t bytesRead = 0;
    do{

      //read data via i2s_read or i2s_read_bytes


    } while (bytesRead > 0);

No data is stored in this queue, but rather a flag which can then be used to synchronize dma-filling and further buffering/calculating/sending the read data.

HOWEVER this only works if you install the i2s driver in a specific setup. Instead of using

i2s_driver_install(I2S_NUM_0, &i2s_config, 0, NULL);

in your setup, you can activate the "affinity" for events by passing a queue-handle and a lenght:

i2s_driver_install(I2S_NUM_0, &i2s_config, 4, &<your queue>);

hope this helps getting started, it sure did help me.