nRFx PDM driver data buffer handling gives noise samples at every memory block of defined size

[ShalinDoshi]

I am trying to capture audio PDM samples at every 5s interval blocks, but at the time of "pevt->buffer_released" event when i am giving the new buffer to PDM driver it results in some noisy data at every block of my buffer size(PDM_SAMPLE_SIZE). I have set the PDM driver at default sample rate 16KHz, Mono channel (right), 16 bit PCM encoding. I am getting noise while fetching DMA data.

Questions:

Why i am getting noise at every memory block (attached binary file of 100K data samples around 5 seconds of voice data) ?

Is there any document which shows internal easy DMA handling and buffer collection such that i can copy the chunk of buffers to other memory without losing/avoid data corruption on the incoming voice data samples?

Attaching code snippet, on how i handle the PDM data buffers and copy it to RAM(100k buffer) for further processing.

#define PDM_SAMPLE_SIZE     400

void pdm_event_handler (const nrfx_pdm_evt_t* pevt)
{
    if (pevt->buffer_released == g_pdmBuffers[0])
    {
        g_voiceActivityDetected[0] = 1;
        memcpy(&g_FlashStoreSample[g_pdmBufAcqCount * PDM_SAMPLE_SIZE], g_pdmBuffers[0], PDM_SAMPLE_SIZE * 2);
        g_errcode = nrfx_pdm_buffer_set(g_pdmBuffers[0], PDM_SAMPLE_SIZE);
        g_pdmBufAcqCount++;
    }
    else if (pevt->buffer_released == g_pdmBuffers[1])
    {
        g_voiceActivityDetected[1] = 1;
        memcpy(&g_FlashStoreSample[g_pdmBufAcqCount * PDM_SAMPLE_SIZE], g_pdmBuffers[1], PDM_SAMPLE_SIZE * 2);
        g_errcode = nrfx_pdm_buffer_set(g_pdmBuffers[1], PDM_SAMPLE_SIZE);
        g_pdmBufAcqCount++;
    }
    
    if(g_pdmBufAcqCount >= 200)
    {
        g_pdmBufAcqCount = 200;
    }
}

248588.audiodata.bin

Output file converted from binary PDM data file:

Input audio file:

Note: Using ICS-41351 Microphone sensor to capture PDM samples

[Simonr]

Hi

May I suggest taking a look at the Thingy:52 SDK (downloadable here). The Thingy uses a digital PDM microphone, and a part of this SDK is PDM audio drivers with audio codecs. Specifically, take a look at how the buffer handling is done in the Thingy SDK. The m_audio_buffer_handler() and m_audio_process() functions in drv_mic.c should be helpful.

Best regards,

Simon

[ShalinDoshi]

Hi Simonr,

we are using nordic SDK 17 with soft device version s140 for the devkit nRF52840. I saw the code you are pointing out for reference but those APIs will not be used in above SDK and would not be compatible with nRF 52840.

Is there any other method which might help me to mitigate some noise?

[Simonr]

Hi again

You can also check out the Smart Remote 3 DK add-on that has a digital microphone that can do something like this. The firmware can be downloaded here. I'm not too familiar with it, but there should be some noise mitigation code here as well. Alternatively, you might be able to use the buffer handler in the Thingy SDK as reference to see how it's done there and implement something similar in your project.

Best regards,

Simon