Conversion to PDM to PCM using FIR and decimation

[ATeli.1]

I am trying to decimate a PDM signal in order to obtain a PCM signal. Is this code correct?

void LowPassFilterWithDecimation(uint8_t *pdm1, uint8_t *pdm2, int32_t *pcm1, int32_t *pcm2, int pdm_buffer_size, int decimation_factor){
  char str[100];
 
  // initialize buffers
  for(uint16_t i = 0; i < pdm_buffer_size/decimation_factor; i++){
    // initialize raw microphones buffers
    pcm1[i] = 0;
    pcm2[i] = 0;
  }
 
  for(uint16_t i = 0; i < pdm_buffer_size; i += decimation_factor){
    for(uint8_t j = 0; j < NUM_TAPS; j++){
      
      pcm1[i/decimation_factor] += pdm1[i + j] * fir_coeff[j];
      pcm2[i/decimation_factor] += pdm2[i + j] * fir_coeff[j];
 
      // trasmit via uart
      sprintf(str, "%ld,%ld\r\n", pcm1[i/decimation_factor], pcm2[i/decimation_factor]);
      HAL_UART_Transmit(&huart2, (uint8_t*)str, strlen(str), 10);
      
    }
  }
}

[PatrickF]

Hi @ATeli.1​ ,

Please provide which STM32 your are using/targeting ?

I'm not an audio expert, but I think using DFSDM (or MDF) present on some STM32 MCU or MPU will make such conversion much simpler and faster as DFSDM embed digital filters in HW.

Regards.