RTOS/CC2650STK: How to increase the PDM audio sampling rate by increasing the sampling points?

Hello,

Currently the PDM microphone is sampled using BCLK at 1.024 Mbps. In order to effectively double the sampling rate, the easiest way is to double the BCLK frequency. This is set in the PDMCC26XX_I2S_open() call.

However, I should warn you that the entire ecosystem is based on a 16kHz sample rate, doubling this could have affects later on during decimation/filtering that occurs inside pdm2pcm16k().

What drives the need for a higher sample rate?

Hi, I have tested with 17Khz sine wave. My main requirement is time accuracy, now it seems this is fine.

But I found that the audio signal sampled by sensortag has severe distortion when it comes to the audio wave shape. I plotted the sampled audio signal by Matlab, it can be seen clearly the audio wave shape distortion, this will affect further DSP process such as corss-correlation algorithm

Hi Jason,

I have reached out to our expert for advice. I will continue to look into this.

However, I feel that there is a possibility that the filtering inside the pdm2pcm16k() function may be affecting your waveform.

What is the use case that requires a higher than 16kHz sample rate?

thank you for ur kindness help.

I know that the PDM driver was originally designed for human speech processing, but I wanna use it for distance measurement.

Thank you

Through my studies,I found that the following coefficients matter decimation/filtering(different freq),is it right?
const int32_t PDMCC26XX_aBqCoeffs[] = {
  //--v--  Adjust overall gain by changing this coefficient
    331,     0, -1024, -1356,   342,     // DC-notch, halfband LP filter (@32 kHz)
    200,   789,   934,  -994,   508,
    538,   381,   944,  -519,   722,
    732,   124,   987,  -386,   886,
    763,    11,  1014,  -386,   886,
    0, // Terminate first filter
    // Insert optional second filter here (@16 kHz). Some examples:
    //1147,-1516,   522, -1699,   708,    // +5dB peak filter (F0=500 Hz, BW=3 octaves)
    //1313, -565,    -6,  -725,   281,    // +5dB peak filter (F0=2.5 kHz, BW=2 octaves)
    //1335,  532,   -66,   694,   225,    // +5 dB peak filter (F0=5.5 kHz, BW=1 octave)
    0, // Terminate second filter
};

P.S. I found a phenomenon that once I add the PDMCC26xx.c driver file to my project tree, the time the PDM takes to sample a buffer(for example 192 sampling points) will change( In the past, the PDM takes relatively constant time to sample one buffer, but now the time it takes can vary, I am really confused since I didn't change anything to my driver file)