Hey Eddie,

That should work. I've never used a vocoder but RB will route the midi where you send it. Then you bring it back in as audio through your interface.

FWIW, some people have had decent results using the TCH software that comes with BIAB/RB. I like hardware better myself but just saying...since you're experimenting...
There are some posts on here somewhere if you do a search because there have been several discussions on it in the past.

Maybe that helps ya a little.

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