nRFX based PDM to PCM implementation (I2S example)

[Nikhil Komalan]

Hello,

I am currently interfacing my nRF52840 DK with i2S based MP34DT05 sensor. I am using nrfx i2s driver's for interfacing with sensor. I am using master clk of 2MHz, LCLK 16KHZ. 

My sensor output PDM data. In my project I am recording audio samples on left channel, so I am currently getting 2*16bit of left data per 32bits as shown in nrf docs. I have got 16bit recorded samples in buffer, but to how to convert this pdm data in pcm format. 

Normally we need to divide the PDM data with a decimation factor, but in I2S we already are dividing the bit sampling rate with ratio value. So is the output from the I2S itself is decimated PCM ?  

Or can anyone give a example of how to convert the PDM to PCM data. Any sort of help is dually appreciated. 

[Audun]

Hello,

first off, I would recommend printing out the entire pdm_buf array and look at the contents. The microphone has a ~10 ms startup time (see below), so it is likely that the first sample in the first buffer will always be the same value.

Second, data on the DIN pin is normally in the same frequency range as the clock signal, but this will vary according to the acoustic input to the sensor.

If you want to validate the DIN signal you see in the logic analyzer, you can try the attached python script. The script parses and filters an exported .csv from Saleae Logic analyzer software. Just make sure the exported csv has DIN as the first exported value. For example:

Time [s],Channel 5,Channel 7
-0.000051150,0,0
0.000000000,1,0
0.000000030,1,1
0.000999980,0,1
0.001000018,0,0
0.001999976,1,0
0.002000010,1,1
0.002999954,0,1
0.002999998,0,0

Here, channel 5 is DIN, and Channel 7 is CLK.

Python script syntax, using 16 kHz as target PCM rate:

$ python pdm_parse.py my_exported_saleae_capture.csv 16000

Fullscreen pdm_parse.py Download

import sys,os,re
import numpy as np
import scipy.io.wavfile as wf
from scipy import signal

#fname = 'pdm.csv'
#Fs    = 16000

if len(sys.argv) < 3:
    print('usage: {} input.csv Fs'.format(sys.argv[0]))
    exit(-1)
    
fname = sys.argv[1]
Fs    = int(sys.argv[2])

print('File: {}'.format(fname))
print('Fs:   {}'.format(Fs))

pattern         = re.compile(r'[-/.0-9]*,[ ]*([0-1]{1}),[ ]*([0-1]{1})')
bitstream_right = []
bitstream_left  = []

print('Note: Make sure PDM DATA is before PDM CLK in trace')

with open(fname, 'rb') as fin:
    prev_clk = None

    for line in fin:
        try:
            data, clk = re.findall(pattern, line.decode('utf-8'))[0]
        except:
            continue

        if prev_clk == '1' and clk == '0':
            # Falling clock edge
            if data == '1':
                bitstream_right.append(32767)
            else:
                bitstream_right.append(-32767)
        elif prev_clk == '0' and clk == '1':
            # Rising clock edge
            if data == '1':
                bitstream_left.append(32767)
            else:
                bitstream_left.append(-32767)

        prev_clk = clk


print('Found {} samples in right bitstream'.format(len(bitstream_right)))
print('Found {} samples in left bitstream'.format(len(bitstream_left)))

y_right = signal.decimate(bitstream_right, 80, ftype='fir')
y_left  = signal.decimate(bitstream_left, 80, ftype='fir')

wf.write(re.sub(r'\.[\w]+$', '_left.wav', fname), Fs, y_left.astype(np.int16))
wf.write(re.sub(r'\.[\w]+$', '_right.wav', fname), Fs, y_right.astype(np.int16))