PDM library support for stm32duino

[federica-ven]

Hi all,
Following up on a question asked on a different platform I was wondering if there were resources available for using the PDM2PCM library in the stm32duino environment or any other library that allows to use PDM microphones with an arduino core?

I’m trying to setup my project with a couple of PDM microphones using the existing example Python-Wave-Serial-Converter but it’s been difficult understanding the required changes needed for a setup different than the one used in it. Any chance someone can point me to a guide or documentation as to how to use it properly? I know the libraries available for STM32WB55 have good audio support with cube libraries but I couldn’t find anything like that for the stm32duino.

[fpistm]

Maybe you can find some useful info in this library:
https://github.com/stm32duino/X-NUCLEO-IKS02A1-Audio

[federica-ven]

I was also thinking of trying to integrate what's being done in this function pack: https://github.com/STMicroelectronics/x-cube-memsmic1/tree/main or take some parts of it for my project, do you think it'd be feasible? @cparata @fpistm

[cparata]

Hi @federicaventriglia ,
if the x-cube-memsmic1 example is based on WB55, you can for sure take from that example the MCU clock settings and the audio peripheral (I2S or SAI) settings. For the stereo mic, unfortunately, I cannot help. Consider that the PDM2PCM library is for mono mic, so I'm not sure that it can work also for stereo mic.
Best Regards,
Carlo

[federica-ven]

Hi @cparata the PDM2PCM library in the stm32duino example is for mono only? Meaning it can’t support 2 microphones or just that it needs to be adapted for it from the example?

Maybe I should take a step back and ask a better question: what’s the best way you’d suggest I investigate if I wanted to extract signals at a relatively low rate from a pair of microphones?

[cparata]

Hi @federicaventriglia ,
first of all, you need to find a way to extract the PDM stream (in stereo mode) from the audio peripheral of the WB55. Maybe for this step you can take inspiration from the x-cube-memsmic1 library (at least for the settings of the MCU clock and the Audio peripheral including also DMA); after that you need to find a way to convert the PDM stream to PCM stream. For this step, the PDM2PCM library included into the stm32duino library is used for mono conversion; I'm not sure that it could be used also for stereo conversion, but I'm not an audio expert, so I could be wrong.
Best Regards,
Carlo

[federica-ven]

Hi @cparata I thought the only difference between stereo and mono was the need to have 2 buffers instead of one, since data from the microphones comes in frames that can then be divided based on how many microphones are connected to the data line. I'm no audio expert either, that's mainly why I'm seeking some guidance. I've tried the MEMSMIC Function pack examples and they do work I upload the binary into the exact same hardware they mention (P-NUCLEO and X-NUCLEO extension) but I am not familiar enough with the implementation to reverse engineer it to achieve what I need.