jomofo

@Toomerm - I think it's a floating point or some kind of numeric rounding bug that compounds during playback when automating the global tempo track with the curve handles. The tempo operations method I described in my post to workaround the issue isn't all that bad once you get used to how it works. The global tempo track creates prettier curves, but the tempo operations that just makes discrete steps actually works. One thing I remember helps to distinguish this issue from others like it is the drift starts to become noticeable relative to where you start the playhead and then continues to get worse until you stop and reposition the playhead. For example, if you start playhead at time 0, the drift will become noticeable sooner than say if you start with the playhead on bar 10. I tried to log the issue with Apple through a support page, but the barrier to entry was so high that I gave up. They basically want your first born no matter how detailed you are before they'll let a bug report get past their support triage that way. I've seen rumors on Reddit that there's a support line you can call which is a better way to get through to them. Cheers

Wow, okay, so it only took me an hour or so to redo my tempo automation using the Tempo Operations (Edit > Tempo > Tempo Operations...) window. It's not quite as user friendly, but at least I don't have to fight with the tempo curve handles like I do in the global track. I changed most of my curves to just 1/4 steps up and down over a bar. I have a couple of bigger drifts where I felt like it needed 1/8. It actually worked! The clock doesn't drift out of sync now. I speculate there's a bug in whatever they're doing to calculate the tempo from those continuous style curves in the global track that is additive in nature. The longer you playback the larger the error becomes. I don't have a guess as to why certain settings tweaks seem to amplify the error and make it more noticeable sooner in the playback. And I guess bug doesn't exist if you use incremental steps up and down to simulate a curve rather than an actual curve. Anyway, I've burned 3 late night sessions trying to figure this out, so while I'm perturbed, at least I have a workaround. On the bright side, it forced me to understand a ton of settings I never bothered to dig into before.

Hello all, I'm running into what seems to be the same issue and this is the closest thing to explain the behavior I'm seeing. I'm curious, @wjm, if you've tried using the Tempo Operations tool to build your "curve" instead of the global track? It seems like the automation curve in the global track is continuous whereas the curves constructed with Tempo Operations are just incremental discrete steps that mimic a curve. I'm going to play around with this on my track to see if it helps and will report back, but it will be time consuming and I'm half-tempted to give up and just record the MIDI at a constant tempo, bounce the audio and use flex on it to match the tempo drift I need.