Here are my results with Johnny’s project file :
I don’t have Analog Techno installed, so since the first track is silent it’s much easier to hear the issue – or, just cut the volume inside the plugin.
I experience the same issue.
However, when it loops around and the sound is gone, I can still see some activity on the VU meters.
If I open a spectrogram and adjust the levels I can clearly see the sound going.
Now, when I increase the gain by any means up to an audible level (you need around +60 dB so put a limiter after the gain plugins, just in case), the sound is indeed still there, although somehow altered (for some unknown reason the kick lacks the pitch decay, instead it is a flat pitch)…
So one sure thing is that the MIDI data still reaches the instruments (MIDI Monitor shows traffic, as stated by Johnny). We can even tweak the Retrologues and hear the sound change, so the plugins seem to be working fine.
Now this is where it becomes funky :
When I replace the Retrologue instance by Padshop or Groove Agent SE, or any other 3rd-party instrument, this weird volume issue is no longer exhibited for this track, so you might think this is a plugin related issue, right?
Well, try with HALion Sonic SE, and you will see that this plugin also exhibits the issue. However, on some patches like Early 70s E-Piano or E-Piano MW Strings, the volume is only slightly reduced and the shape of the sound is altered, and we can still hear it without having to use gain plugins. Weird, ugh? So certain sounds are less affected than others by this issue?
As an additional experiment, I only kept the first Groove Agent track and duplicated it one time, and its MIDI input set to the first Groove Agent output (same setup as when loading the project but with the same sound on the two tracks, and the first track not monitored (the one with the MIDI part on it).
During the first run it will sound twice as loud (normal), but as soon as it starts looping around, there is a MIDI timing issue and the sound will go out of phase. Well, the offset is a bit longer than just a phase shift, so this is more like a very short delay. Note that it does this with any instrument that is inserted on the second track, although if the sound is different it is much less noticeable, but we can still hear the difference between when we first hit play and when it loops again from the start, even with drums + piano or whatever combination.
When using a standard MIDI track instead of the instrument’s MIDI output, there is absolutely no issue whatsoever.
It is therefore almost sure that this weird behavior is linked to the use of the MIDI output from Groove Agent exclusively, because this is none of an issue when using MIDI outputs from other plugins (TAL-Drum for instance, as shown in the video from the first post).
The main issue that it triggers subsequently, that is, the volume drop upon loop, only seems to affect Retrologue and HALion Sonic SE, but to me it is perhaps related to the more global timing issue that it provokes, somehow these specific instruments are particularly sensitive to it.
End of research.