TL;DR: I trained a 125M-parameter transformer to autocomplete piano performances in real time (~108 notes/sec on an iPhone 15). The biggest improvements came from finding the right MIDI representation, cleaning the training data aggressively, and adding DPO post-training.

Almost a year ago, I started tinkering with an idea: connect my MIDI piano to my phone, play something, and have AI autocomplete the song for me. Think GitHub Copilot, but for piano.

It turned out to be a deeper rabbit hole than I expected. Fourteen experiments later, it is finally at a point where I am happy enough with it to write about.

Your browser does not support the video tag. Potato-quality video because the good phone was busy running the MIDI model.

The app, RollTab, is available for free here if you have a MIDI keyboard and an iPhone/iPad.…