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,

instead of the NRFX I2S driver, it sounds like you should use the NRFX PDM driver: https://infocenter.nordicsemi.com/topic/sdk_nrf5_v17.0.2/group__nrf__pdm.html

The PDM driver interfaces with the PDM hardware on nRF52840: https://infocenter.nordicsemi.com/topic/ps_nrf52840/pdm.html?cp=4_0_0_5_14

The PDM hardware has the decimation filtering built into hardware, and outputs PCM samples. Two decimation factors are supported, 64 and 80. For example, if you want a 16 kHz PCM rate, you should choose a PDM clock rate of 1.28 MHz, and a decimation factor of 80.

Best regards,

Audun