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.