Decimation of PDM signal on ADAU1467 / EVAL-ADAU1467Z

Hello TKA,

Sorry I missed your question and I need to verify some information but I will give you my initial thoughts for now. I will not be able to verify this until later next week.

The decimation rate will depend on the difference between the clock you are sending out to the microphone and the sampling rate of where you are routing the PDM microphone over to.

So if you send it 768kHz (which seems really low for a PDM mic) and you are routing the PDM data to the DSP core that is running at 48kHz then you will be decimating at a rate of 16x.

The key to controlling the decimation rate will be to use the ASRC. Then the input of the ASRC can be set for 24kHz and the output of the ASRC can be set to the core rate. So this would give you a 32x decimation rate. However, I am not certain where the ASRC obtains its sampling rate clock from when it is set to take its data input from the PDM microphone. It is a small detail I have missed after all these years of working with this part. As an example, I suspect that if you are using the serial input port 3 as the source of the PDM clocks, then the LRCLK of that port should set the rate of the ASRC input so set that LRCLK to master and set it to 24kHz rate and it all should work.

The simplest way to obtain a 32x decimation is to feed it a clock that is 32x the core rate fs and send the PDM microphone directly to the core. This means you would need to send it a 1.536MHz clock signal.

Let me know if you get this to work properly. I will investigate with how to use the ASRC with the PDM microphones.

Dave T

Thank You for reply,
Unfortunately i cannot give any real feedback since priorities shifted a bit, if topic will be back to my department i will certainly return with confirmation.