I have a MIDI instrument whose input I'd like to transpose live (ie. so the sound synthesized for it is transposed up a third from the actual input, for example).
I tried adding an AU Pitch Shift effect plugin to the track. This does allow me to do the transposition. But, since this effect works on the output audio signal and not the MIDI input (and does so in software) it is noticeably laggy (to the point where playing is annoying).
I did some investigation and found this article: https://discussions.apple.com/thread/2426268. It's old (2010), but it discusses using MIDI Patchbay to create a fake virtual MIDI input with the transposition. The problem the OP faced there was that Garage Band still picked up the initial untransposed MIDI input (because it listens to all MIDI inputs for the current track; this seems to still be the case as I confirmed this by trying two MIDI instruments simultaneously). Since this question is older, do recent versions of Garage Band allow you to filter out certain MIDI tracks (making this approach possible)?
Ideally, I'd like to do the transpose live from within Garage Band. I also found in the editor (under score) the "Transpose" option (which for some reason is disabled under "Piano Roll"). However, this doesn't seem to have an effect on the live input.
tl;dr Is there a way to live transpose MIDI input that you are synthesizing/monitoring through Garage Band?