Pdm clock for bandwidth 20 -2 khz

[manikandan]

Hi,

we are using nrf5340 with infineon IM69D120 mic our bandwidth is 20-2khz,we would need a 4KHZ sampling frequency with a 16 bit. That would be ideal for our use case. 
we received guidance from Infineon on achieving a sampling frequency of 4000Hz. The suggestion is to operate the microphone at a PDM clock frequency of 512 kHz by setting the decimation ratio to 128 and the sampling rate to 4000 Hz, providing an audio bandwidth of 2000 Hz. Is it possible to achieve using nrf52/53 series, if yes kindly guide me to achieve this?

Regards,

manikadan V

[manikandan]

any inputs?

[Karl Ylvisaker]

Hello manikadan V,

we received guidance from Infineon on achieving a sampling frequency of 4000Hz.

I am not sure I understand what this guidance is referring to, could you elaborate? 

The suggestion is to operate the microphone at a PDM clock frequency of 512 kHz by setting the decimation ratio to 128 and the sampling rate to 4000 Hz

This would unfortunately not work with the PDM peripheral of the nRF5340, since this is outside of the possible configurations for the PDM peripheral as described in the Product Specification.

Could you elaborate on the constraints and requirements to your application (not the microphone), so that I may advise on how you could proceed?

Best regards,
Karl

[manikandan]

Hi,

Our requirement is to record the audio (PCM data) and send it in ble while recording (live streaming) right now at 1280 kHz with 80 kHz sampling frequency recording and sending 228 bytes of data per payload up to 256 kb, but audio seems to be distorted.

Kindly check the recorded audio below and give us your feedback.

2) How can we live stream with the possible configuration available in NRF 5340 is any any examples which I can refer?

Regards,

Manikandan V

[Karl Ylvisaker]

Hello Manikandan V,

manikandan said:

Our requirement is to record the audio (PCM data) and send it in ble while recording (live streaming) right now at 1280 kHz with 80 kHz sampling frequency recording and sending 228 bytes of data per payload up to 256 kb, but audio seems to be distorted.

Are these hard requirements for the configuration?
Just to put this in perspective, the sampling rate for CD quality audio is 44.1 kHz, since this is more than double the human ear's frequency range. The maximum supported sampling frequency of the LC3 which is the mandatory codec for LE Audio compliance is also 48 kHz, which means that you will need to use a proprietary secondary codec in order to be able to sample at this rate.
You can read more about the supported features and configurations of the LC3 in the Table 5.1 of Nick Hunns excellent Introduction to LE Audio book, which you can download for free from BT SIG.
I highly recommend looking through this book when familiarizing with the LE Audio specification and all its new features and improvements!

manikandan said:

2) How can we live stream with the possible configuration available in NRF 5340 is any any examples which I can refer?

If the listed requirements are hard constraints then this is unfortunately not possible without also using another codec than the LC3. The LE Audio specification allows for another codec than the LC3 to be used so long as both sides of the link supports it.
In this case, I would be very curious to hear more about the use-case that requires this specific configuration as well, if I could ask.

If the listed requirements are not hard constraints we can make this work with the nRF5340 LE Audio reference application.

Best regards,
Karl