Less guessing. More playing.

The situation

You are learning a song. You want its key and tempo, and you would rather not open a recording session or trust your ear on a bad day. The existing tools work, but they are a detour. I wanted the answer to live where the music is already playing: in the browser tab.

What I built

The Chrome extension listens to the audio in a tab, no microphone, and runs the analysis on the device. It shows the key on an interactive piano and, in Studio Mode, transposes YouTube in real time so a singer or a guitarist can practice in a key that fits. The website carries a free analyzer for audio files with the same engine underneath.

Around the engine I designed the brand, wrote the site in English, Spanish and Brazilian Portuguese, set the pricing, and answer the support email. I build it as a solo founder with AI engineering assistants, and I document each version: what worked, what did not, and why.

The benchmark that humbled version 1

Users trusted KeyDetector 1.x because it said 94 to 98% confidence. In September I tested it properly, against the GiantSteps dataset instead of my own tracks. It got the exact key right 21.8% of the time, and it was wrong with the same confidence as when it was right.

So I rebuilt the engine and measured every change. Engine 2.0 reaches 59.8% exact key on GiantSteps, a hard electronic-music set. On worship music, validated against 229 real songs from church services, it gets the key right 94.9% of the time and the tempo 83.3%. Different music needs different assumptions, so there are style presets, each one measured: drum and bass tempo went from 43.9% to 80.6% with its own. And the confidence figure is now honest.

Version 2.0: from detector to practice studio

The same method, applied to chords. A learned chord recognizer went from 65.8% to 74.7% on GuitarSet and from 73.3% to 87.6% on AAM, and it only shows sevenths when it is sure (88.8% precision on the ones it shows). Around it:

Chords over YouTube in time with the video, with A-B loops, a printable and correctable chord chart, and a practice mode that ramps the speed up. On-device stem separation with HTDemucs, so nothing is uploaded. Natural-voice transposition that keeps a singer sounding like themselves in another key. A song library with versions, clips and sequences, MIDI control, a metronome and a free tuner.

149 automated tests and a 54-page user guide in three languages hold it together. Version 2.0 is built and in final testing in real Chrome before it goes to the store; the website already runs the 2.0 engine.

Listening to the people who pay

In August a Pro user wrote that the transposed key was sometimes unreadable. My first diagnosis was wrong. I assumed a missing feature. It turned out to be an initialization bug hiding behind a silent catch. The lesson went straight into the project notes: reproduce first, promise second.

Where it stands

Version 1 is live, growing, and paying for itself in a modest way, with a 14-day Pro trial and a lifetime price validated by real purchases. AI assistants already send almost five percent of new visitors, which changes how the site should be written. Next: validate 2.0 in real Chrome and publish it.