I know nothing of this subject but could you not use a clock signal from inside the C64 to keep the drum machine in time with the C64?
retrocomputing
Discussions on vintage and retrocomputing
That was my first thought as well, it seems probable that modern equipment may already have clock syncing capabilities, right?
Where's LMNC when we need him? :D
I think it's that the c64 only has enough juice to do midi follow. The clock on the behringer is known to be pretty crap and that it is what is doing timing in this basic setup. That's why I suggested I might use something like the Tr6 reissue that likely has better midi. It kind of negates the exclusive use of c64 for percussion, but does open the doors to a lot of other interesting uses.
It's basically the symptoms are wandering timing. They always fall a little out of step after a few minutes. I could see if some of that is that specific machine. I have an NTSC bread in that is pretty mint. But this PAL unit has sidFX and a bunch of other mods so.
Does a PS2 count as retro?
I think what you're doing here is really cool.
Fantastic stuff. Unfortunately I don’t know enough to help but you may want to try to catch up with Inverse Phase: https://www.inversephase.com
He is a super nice person and is more knowledgeable about that sort of thing than anyone I’ve met. I’ve run into him a couple of times a VCF Midwest and always learn something new.