Unstable MODEP - Something keeps breaking

I am using Patchbox/MODEP with a cheap Behringer U2 usb interface pretty much as a practice amp but I’m having what I can only describe as “stability issues”. Sometimes everything runs correctly, and the output sounds great, but after a while something just stops and the output sounds like a helicopter heard through a long air conditioning vent.

If I stop and restart Jack, or change its settings, then sometimes it will go back to sounding right. Sometimes the Pi boots up and sounds good immediately, sometimes it does not and needs to be prodded.

I don’t think that buffer overruns are the problem here - The settings I generally use aren’t ambitious (48k sample rate, 256 samples, 2 sample per period) and MODEP doesn’t report a lot of overflows.

As far as I can tell the problem isn’t an effect module or CPU use either. The same thing happens with zero effects on the pedalboard, and even with my good sounding chain the CPU use is about 18%.

Can anyone suggest what I’m doing wrong?

Welcome to the Blokas community!

As a point of reference, what are you running MODEP on?

Does this happen with any plugins or just a specific combination?

It’s running on a Pi3A, and it seems to be when using any combination of plugins, but it also happens with no plugins at all.

I suspect it might have something to do with the Behringer interface. I would suggest adjusting the sample rate, sample and sample per period and see if that makes any difference.

I’ve tried a lot of different settings, and they all seem to eventually result in something going awry. It’s just about impossible to systematically try things because sometimes it takes minutes to crash, sometimes it up to an hour. Sometimes just restarting Jack will clear the problem, but sometimes changing to a higher sample buffer won’t actually clear it and I have to restart the Pi. Sometimes restarting on previously good settings will just start out already with the problem in place and struggling to clear it just to use MODEP at all.

Everytime I think I have the right settings dialled in, I’ll get a crash and then not be able to find something stable again for ages.

I have also tried using a Presonus StudioUSB instead, and while it seems to be more stable it still eventually ends up in this tinny, echoey mess as well.