WMXZ

The proper operation with MTP is to do either generating files or using mtp-loop. Well, at least that is the mode that works. Also, after writing to uSD, the PC must be told (e.g. by user) to remount/reset the MTP responder (on the Teensy). when...

Add to @MarkT, obviously previous_timestamp needs to declared as unsigned (e.g. uint32_t)

The I2S MEMS microphone (especially the one from Adafruit ( SPH0645LM4H)) can have significant negative offset resulting in always negative values (MSB always set). I use ICS-43434 microphones, that have nearly no offset. You can get them e.g...

Sorry, what is U4? Is that the TLV75733P on the Teensy 4.1?

AFAIK, bootloader update is only done once and only when necessary. Maybe I miss the problem?

The way I did this type of design is to generate me the filter in, say Matlab and then to convert the filter to biquads to be programmed to Teensy. Matlab has tools for that. I have not done it for a while, but I never have seem 20 biquads with...

I think that you are misinterpreting how to use the Teensy biquad filter. The teensy biquad class is not a generic IIR filter, it is a biquad. A biquad is 2nd order. There is no filter order to choose. I'm guessing that you might have thought...

or while(x<5) { ... } which is even less typing