Show HN: OnKey – I'm a bad singer, so I built an app to show where I'm off-key

I'm not a good singer, but strangely enough I still like singing. I've improved a little over time with singing apps, but their song lists are limited by licensing, so I'd often open a song expecting the original and find a cover I didn't want to sing to.

So I built OnKey: a free, open-source (MIT) Mac app that listens to whatever you're playing in Spotify, Apple Music or YouTube, learns the singer's melody, and shows you live where you're sharp or flat, with a score at the end. Nightingale did something similar years ago for your own audio files. OnKey works with the music you already stream.

Demo (1:35, sound on): https://singonkey.app

The first time you play a song, you can sing along while it listens, or play a whole playlist and let OnKey learn the songs in the background before you come back to sing them. When a song finishes it learns the notes on your Mac (a couple of minutes), and from the second play the notes scroll towards you before you sing them.

Learning the song and scoring you both happen on your Mac. No account, no analytics, and the music is never saved or sent anywhere. Your takes stay there too. To check the score means something, I ran 20 recordings of singers, from poor to professional, that music experts had already scored. OnKey ranked them in nearly the same order as the experts did.

It's still rough in places: busy mixes and harmonies can confuse which voice is the tune, and now and then a note lands an octave off. Scoring is forgiving on purpose for now, set for beginners like me, so stricter modes for better singers are next. And it's Apple Silicon and macOS 14.2+ only.

I'd love to hear where it scores you unfairly or the notes go wrong, and I'm happy to answer questions about how it hears the music and learns a song.