qb wrote:You can't as far as I can tell. C64 trackers don't have accurate tempo (it fluctuates over time)
They are rock solid. There is definitely no fluctuation in Goat Tracker (a few microseconds jitter at each tick at worst), but perhaps if you compare the tempo to the closest integer bpm they are drifting by comparison. What makes you say that tempo fluctuates?
Well, I'm no expert, but a tracker composer confirmed this to me months ago. I tried to sample patterns from a tracker module and I couldn't find a matching tempo (this is not a problem with samples that have accurate tempo). I was told that old trackers have tempos like that and basically most modern trackers have this problem too because they are based on old technology.
I tried to put a drum track over it as a start, and it synced for a few seconds, and then it went off-beat. Try it yourself. Sample a whole pattern from a MOD, XM, or even SID song (something reasonably long) and try to find a matching tempo in a DAW (don't adjust the sample with any plugins though). It doesn't work.