Why Thingy52 PDM sample code skip buffer ?

[Louis Wnag]

I am using thing SDK. And,following function is part of thingy SDK.
Could someone can explain why the following function needs skip 31 times?

static void drv_audio_pdm_event_handler(uint32_t *p_buffer, uint16_t length)
{
    ASSERT(length == CONFIG_PDM_BUFFER_SIZE_SAMPLES);

    if (m_skip_buffers)  //m_skip_buffers-->31
    {
        m_skip_buffers -= 1;
    }
    else
    {
        m_buffer_handler((int16_t *)p_buffer, length);//static void m_audio_buffer_handler(int16_t *p_buffer, uint16_t samples)
    }
}

[Louis Wnag]

Hi Overbekk

The microphone I use is STmicro MP23DB01HP (L/R connected to GND as mono Left)

GPIO-40 pull high for the trigger valid data (Once the m_skip_buffers counter reaches 0)

 

But the data received still cannot match the data shown in the scope.

Is there anything miss?

static int buf_cnt = 0;
static void drv_audio_pdm_event_handler(uint32_t *p_buffer, uint16_t length)
{

    ASSERT(length == CONFIG_PDM_BUFFER_SIZE_SAMPLES);

    if (m_skip_buffers)
    {
        m_skip_buffers -= 1;
if(m_skip_buffers == 0){
printf("PULL HIGH\r\n");
nrf_gpio_pin_write(40,1);
}
    }
    else
    {
if(buf_cnt < 1){		
printf("[%d] 0x%08x 0x%08x 0x%08x 0x%08x\r\n", length, p_buffer[0] , p_buffer[1] , p_buffer[2], p_buffer[3]);		
}
        m_buffer_handler((int16_t *)p_buffer, length);//m_audio_buffer_handler(bufer,length)
buf_cnt++;
    }
}

Consol message:

PULL HIGH

[256] 0x00d000cf 0x00cc00d0 0x00ce00d2 0x00c900cf

[ovrebekk]

Hi Louis

The input from the microphone is run through a decimation filter, to make what is essentially a high frequency 1-bit stream into 16-bit PCM samples. 

For this reason the data you see reported from the PDM module  will not match the data you see on the bus. 

There is some more information on the decimation filter here. 

Best regards
Torbjørn

[Louis Wnag]

Hi Overbekk

Sorry I can not understand your mean totally.

May I check with you agin that  *p_buffer of drv_audio_pdm_event_handler() is raw data from digital microphone(PDM interface) ?

If so the single(green line) of  occilloscope must match with *p_buffer . May I right ?

Another question:

drv_mic_data_handler() function will send PCM data(I mean is data be Encode ?) via BLE  to cellphone?

Best regards

Louis

[ovrebekk]

Hi Louis

No, the data from the microphone, and the data output by the PDM peripheral, is not the same. 

The 1-bit PDM data is run through a low pass filter and decimated to provide the 16-bit PCM output. 

For more information about this please refer to this and this. 

Louis Wnag said:

drv_mic_data_handler() function will send PCM data(I mean is data be Encode ?) via BLE  to cellphone?

Yes. The actual sending of PDM data over the Bluetooth link is handled by the ble_tss_mic_set(..) function, which is called from drv_mic_data_handler(). 

Best regards
Torbjørn