portamento tone strange behaviour on old engine


I’ve noticed a strange behaviour on portamento when playing a song with compatible playback engine (the old module).

This happens using “Pitch effect mode=Renoise”:

I’d expect that U20 bring C note to D, but in fact looks like the playing tone is higher than expected.

This doesn’t happen on standard playback engine, but for my purpose I need to use the compatible playback engine.

How does it works ?

Yeah, that’s weird.