Logic Pro X External MIDI slowly drifts out of sync only when Tempo is automated

wjmwpg

New Member
Yesterday I opened a project I've not worked on in about a year to record some basslines with an external hardware synthesizer. The project has tempo automation that varies from 141bpm to 144bpm, and back, throughout the song. I noticed immediately that the tempo of the notes coming into Logic from the external synthesizer were lagging further and further behind the tempo of the rest of the track the longer it recorded. Begging your pardon if I'm long-winded, but I like to be as thorough as possible when asking for help.

The first bassline midi note starts at 5 1 1 1 and the first note of incoming audio from the synth is recorded into Logic at 5 1 1 17, (a perfectly acceptable 17 tick latency if it remains more or less constant). Now by the time the first verse is ending at 21 1 1 1 that lag has only increased to 21 ticks - not a huge deal - but then there hasn't been much tempo change throughout the first verse, and it's this that seems to be the issue. Throughout the chorus the tempo ramps up from 142bpm to 144bpm and then down to 141bpm for the bridge. By the end of the bridge my 17-21 tick lag has now increased to a very audible 179 tick lag - the first midi note of the second verse that starts at 46 1 1 1 doesn't see its audio come in from the external synthesizer until 46 1 1 179. This lag remains more or less constant again throughout the verse (as there's little tempo change happening) and by the start of the second chorus (after a bassline pause from 62 1 1 1to 64 1 1 1 1) that lag has increased to 201 ticks. As the tempo ramps up again the lag starts to increase even more and by 68 1 1 1 its at 240 ticks. Out of curiosity I removed all tempo changes from 68 1 1 1 onward just to see what would happen. Not surprisingly the lag did not get any worse. What did surprise me was that the it actually started to decrease, and by only 5 second later at 71 1 1 1, it was back to its initial latency of 17 ticks and from there until the end of the track it was rock solid (as there was no more tempo automation). So if the tempo changes stop the MIDI will actually drift back into proper sync.

It should probably be noted that I have recorded external midi instruments from this project with all of its automated tempo changes in the past without encountering this issue. At that time I was using the same computer and the same MIDI interfaces. So whatever the issue I can't imagine that it's the hardware. But I'm running out of ideas to rectify the issue . . . and I need to get this song done.


What I've tried to correct this problem:
1. Comp restart - no change
2. Unplug and replug MOTU MIDI interfaces - no change
3. tried two other MIDI interfaces (M-Audio and Alesis) - no change
4. tried two other external hardware synths - no change
5. tried both an old school External MIDI track and a new school External Instrument MIDI Plugin Track - no change
6. removed ALL other tracks from the project and ALL plugins - no change
7. tried creating a new empty project and importing only the midi file and tempo info - no change
8, put Logic in Low Latency Mode - no change
9. tried changing Logic's I/O buffer size - some minor change in the numbers, but exactly the same behaviour

Here are some images to illustrate the issue. First image shows the MIDI regions and the tempo automation. The green boxes show where the punch-in images are on the timeline. On the punch-ins I've place the playhead at the start of the incoming audio note so it's position relative to its MIDI note can be seen.











Any ideas?
 

stevefal

Logician
I'm having the same issue. I have two software instrument tracks that are in sync when play starts anywhere within the song. But if I play the song from the beginning, after a few automated tempo changes, the two tracks have fallen out of sync. Sync between two tracks should be immutable, so this is disconcerting, not to mention a showstopper. Any ideas?
 

CSeye

Logician
If you haven't done so already, send a detailed decription including images to Apple.
Logic Pro X menu>Provide Logic Pro Feedback.
 
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