Copilot autocompletes your code. RollTab autocompletes your piano playing. Developer simedw spent a year and fourteen experiments training a 125M-parameter transformer that continues whatever you play on a MIDI keyboard — entirely on-device, about 108 notes per second on an iPhone 15. The Show HN post scored 452 points and topped HackerNews.
How a tiny model learned piano
Three things mattered more than scale. A smarter MIDI representation: one NOTE token carrying pitch, timing, duration and velocity, instead of four separate events per note. Aggressive data cleaning: expanding the dataset 5x actually made the model worse. And DPO post-training with Gemini 3.5 Flash as judge, which pushed preference scores from 24.6% to 69.1%. The training set: a few hundred thousand public-domain classical MIDI files, roughly 300 million notes.
Free app, real instrument
RollTab is a free iOS app, not an API. Plug a MIDI keyboard into an iPhone or iPad, play 8 or more notes, and the model takes over. No cloud, no subscription.
Small model, on-device, one vertical scene — that playbook keeps producing the best AI demos right now.
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