PDM Microphone options for Gem Multi

Rodrigo

So in the Crowd Supply update #6 it was mentioned that the Multi can work directly with PDM microphones.

I've seen some older posts on here where a MEMs microphone was suggested with some additional components, to, presumably turn the signal into "audio" before making it to the (older) Bela. But I guess now you can go directly in with a PDM mic.

So towards that end, I imagine that means something like this:
https://www.adafruit.com/product/3492

Where the PDM mic has a couple passive components, with the I/O still being digital (as far as I can tell).

And NOT something like this:
https://www.sparkfun.com/sparkfun-analog-mems-microphone-breakout-sph8878lr5h-1.html

Which appears to output an actual audio signal.

So with that being said, what is it that one is typically looking for in a PDM mic? I don't see the typical (for microphones) frequency response plots, just SNR and sensitivity, as well as stats like that. Beyond that, I don't really know what differentiates one PDM module to another.

What are some good options for PDM mics to use with the Multi?

max

Rodrigo generally MEMS microphones are available in two types: with analog interface and with a digital interface. The digital interface is usually a PDM format, but I²S exist too.
If you need to pick one of them, considerations are mostly what kind of I/O are available on your board and if you worry about interferences from other signals on the analog lines.
Here is a great overview:
https://www.digikey.co.uk/en/articles/a-comparison-of-digital-pdm-and-i2s-interfaces-in-mems-microphones

Rodrigo

max So in the case of Gem Multi, I suppose I would specifically want ones with a digital interface (if I plan on using the PDM functionality built in).

Beyond that, is the consideration about interference about how things are routed on the MEMS board and/or its proximity to other components on the Multi? Or more specifically, do the fast digital clock signals run the risk of bleeding into the analog lines if nearby the analog I/O?

giuliomoro

Normal signal integrity rules will apply (PDM doesn't elude the laws of physics), but any noise from the high frequency data lines should only affect (if at all) analog i/o lines, not other PDM lines.