Hello newbie here.
I am trying to learn how to convert PDM data to PCM using PDM1_filter() function, I am using GCC.
The first 2 converted PCM data seems to be abnormal as can be seen later.
Firstly, instead of directly interfacing to a microphone or an ADC, I used sine PDM data from:
tone500Hz = full scale single sine PDM data of 2000 bits
tone500HzA4 = lower amplitude single sine PDM data of 2000 bits
These are 2000bits of one sine cycle.
Also to get exactly the whole data of the 2000bits, with a decimation of 64, I multiplied it by 4 cycles, so I would get 8000bits. In effect, for decimator of 64, the PCM data output would be 125 (of 16bits).
I have arranged the PDM data bit in bytes with first data in MSB and have verified the data in excel sheet as shown.
My PDM Filter Init and Config are shown below:
PDM1_filter_handler.bit_order = PDM_FILTER_BIT_ORDER_MSB;
PDM1_filter_handler.endianness = PDM_FILTER_ENDIANNESS_BE;
PDM1_filter_handler.high_pass_tap = 2104533974;
PDM1_filter_handler.in_ptr_channels = 1;
PDM1_filter_handler.out_ptr_channels = 1;
PDM_Filter_Init(&PDM1_filter_handler);
PDM1_filter_config.decimation_factor = PDM_FILTER_DEC_FACTOR_64;
PDM1_filter_config.output_samples_number = 125;
PDM1_filter_config.mic_gain = 0;
PDM_Filter_setConfig(&PDM1_filter_handler, &PDM1_filter_config);First I tried bytes of 0x55 and 0xAA to have a balanced density of 0s and 1s.
// first bit to MSB - 8bit tone "55 or AA"
for(uint16_t i = 0; i < sizeof(tone500HzA4_pdmdata) * 4; i++)
{
pdmBuffer8_8000bits_1000bytes_1[i >> 3] = 0x55;
}
PDM_Filter(&pdmBuffer8_8000bits_1000bytes_1[0],&PCM_outBuffer[0], &PDM1_filter_handler);
printf("PCM_outBuffer \n");
for (uint16_t Index = 0; Index < 125; Index++)
{
printf("%d %d\n",Index, PCM_outBuffer[Index]);
}Then I have this data and waveform shown below. Please notice the first 2 converted PCM data.
Second, I tried the full scale single cycle sine tone500Hz.h
// first bit to MSB - 8bit tone500Hz_pdmdata (full scale single sine PDM data)
for(uint16_t i = 0; i < sizeof(tone500Hz_pdmdata) * 4; i++)
{
pdmBuffer8_8000bits_1000bytes_2[i >> 3] = (pdmBuffer8_8000bits_1000bytes_2[i >> 3] << 1) + tone500Hz_pdmdata[i % sizeof(tone500Hz_pdmdata)];
}
PDM_Filter(&pdmBuffer8_8000bits_1000bytes_2[0],&PCM_outBuffer[0], &PDM1_filter_handler);
printf("PCM_outBuffer \n");
for (uint16_t Index = 0; Index < 125; Index++)
{
printf("%d %d\n",Index, PCM_outBuffer[Index]);
}Then I have this data and waveform. Please notice again the first 2 converted PCM data.
Third, I tried the single cycle sine tone500HzA4.h (a lower amplitude single cycle sine wave)
// first bit to MSB - 8bit tone500HzA4_pdmdata
for(uint16_t i = 0; i < sizeof(tone500HzA4_pdmdata) * 4; i++)
{
pdmBuffer8_8000bits_1000bytes_3[i >> 3] = (pdmBuffer8_8000bits_1000bytes_3[i >> 3] << 1) + tone500HzA4_pdmdata[i % sizeof(tone500HzA4_pdmdata)];
}
PDM_Filter(&pdmBuffer8_8000bits_1000bytes_3[0],&PCM_outBuffer[0], &PDM1_filter_handler);
printf("PCM_outBuffer \n");
for (uint16_t Index = 0; Index < 125; Index++)
{
printf("%d %d\n",Index, PCM_outBuffer[Index]);
}Then I have this data and waveform. Please notice again the first 2 converted PCM data.
Fourth, I tried to combine the 3 PDM data in order stated below:
1. PDM data of 0x55
2. tone500Hz_pdmdata (full scale single sine PDM data)
3. PDM data of 0x55
4. tone500HzA4_pdmdata (lower amplitude single sine PDM data)
PDM_Filter(&pdmBuffer8_8000bits_1000bytes_1[0],&PCM_outBuffer[0], &PDM1_filter_handler);
PDM_Filter(&pdmBuffer8_8000bits_1000bytes_2[0],&PCM_outBuffer[125], &PDM1_filter_handler);
PDM_Filter(&pdmBuffer8_8000bits_1000bytes_1[0],&PCM_outBuffer[250], &PDM1_filter_handler);
PDM_Filter(&pdmBuffer8_8000bits_1000bytes_3[0],&PCM_outBuffer[375], &PDM1_filter_handler);
printf("PCM_outBuffer \n");
for (uint16_t Index = 0; Index < 500; Index++)
{
printf("%d %d\n",Index, PCM_outBuffer[Index]);
}Then I have this data/waveform.
Can you advice on the ff:
1. Are procedures all correct?
2. Is the first 2 PCM-converted data are correct and how to mitigate it?
3. Also on the 2nd red rectangle, I see also somewhat abnormality, how to validate if correct?
4. Do the PDM_Filter_XX still applicable?
I am using STM32F4 discovery board. I think, it would be applicable in any STM32 MCUs.
