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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?

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  • I don't think there is. But you can accomplish that easily with Apple's Logic Pro X (professional version of GarageBand). It's not cheap, but if you're planning on doing more with music/sound on the Mac, I would highly recommend it.
    – YourMJK
    Feb 17, 2019 at 20:21
  • That's what I was afraid of. For just this use case, I think it's not worth it. But your recommendation is duly noted! Feb 17, 2019 at 20:26
  • I just remembered that Apple's MainStage 3, which is optimised for live MIDI performance can also do this of course. It's still not free, but way cheaper than Logic (around 30$ I think).
    – YourMJK
    Feb 17, 2019 at 23:50

2 Answers 2

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I was looking at the exact same problem, and I found a solution: Milky. It's on the Mac App Store, it also runs on M1 Macs, and the free (non-Premium) version does exactly what you need.

https://apps.apple.com/us/app/milky/id1455001521?mt=12

It creates a "virtual" MIDI device that will look to any DAW (including Garage Band) just like your physical keyboard – except that it will take your regular keyboard's input on one end, modify it, and then push it out on its other end.

Milky

I'm currently playing on my Akai LPK25 Wireless, Milky transposed my keys up 3 halftones, and my "DAW" is another free app named "Quick MIDI".

https://apps.apple.com/us/app/quickmidi/id1459790045?mt=12

QuickMIDI

As I said, this is running on my Apple Silicon M1 Mac. I don't notice any lag at all, as this is done on MIDI notes, not on digital audio.

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  • Welcome to Ask Different SE and thank you for your contribution. When you get a chance, please take the tour to understand how the site works and how it is different than others.
    – agarza
    Sep 11, 2021 at 3:49
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Actually, MidiPipe does the same job, and I find it more versatile:

MidiPipe

http://www.subtlesoft.square7.net/MidiPipe.html

The latest version 1.6.2 also runs nicely on M1 Macs. After download and unpacking, you'll need to run the app once with your right mouse button / Open.

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