Yes, I know about the auto correlation trick, especially on the low end of the spectrum. I wrote an MP3->midi convertor that is very fast and does not use the GPU or any kind of deep learning, it also gives results that a year ago were SOTA but in the meantime have been passed by deep learning based techniques.

I will definitely use your software. What would be great is to have a 'virtual midi device'. If it works well enough are you open to licensing it? We will sooner or later hit this problem and such a component would make our lives a lot easier, it would also allow me to practice on my real piano using the software, which if I didn't want to play the piano I'd give my left arm for.

On another note: when you know what should be played it is actually easier: you only need to output a '1' or a '0', what is played is what is expected vs what is played is not what is expected.

This may be a simpler problem to solve, and in that context it might even be able to detect chords.