A key is the chord that feels like sit down. People also call it home. You can hunt it with your voice, with one string, or with a page in the Songbook.
Catch, in Settings then Lab, is experimental. It may mark a chord unsure. It does not promise a key, and it does not claim to hear every song. Treat it as a sketch, then edit.
The honest method is slow. Hum the last note of a phrase. Find that pitch on the pad. Try major and minor on that tonic. One of them will settle.
Play a recording from another speaker into the room. Do not expect this browser to tap the other phone's audio graph. Use the mic path you already have, or just listen.
When the song rests, match that rest with a single pad chord. If C feels closed and Am feels closed too, you may be on a relative pair. C major and A minor share letters.
G major and E minor are the same kind of pair. If a folk tune keeps landing on Em but uses C and D, you may still be in G with a sad lean.
Open Amazing Grace and sing the other song against those three chords. If it almost fits, you are close. Move the key chip until the clashes shrink.
Ode to Joy and Silent Night are also small maps. A small map is kinder than a jazz page when you are still guessing.
Three-chord Song and The Four Chords are even smaller. If the unknown song is a three-chord or four-chord loop, those patterns will lock sooner than a long hymn.
Lab Catch listens on this device. Audio is not uploaded. A noisy room, a small speaker, and a mixed band all make the sketch worse. That is expected.
If Catch writes a chart, read it like notes from a hurried friend. Delete a flicker. Merge a held chord. Replace a letter that your ear rejects.
The chord progression generator is the place to type the letters you actually believe. Save that. Play it. If it supports the melody in your head, you found a working key.
You will be wrong sometimes. Being wrong and editing is the skill. A stamp that says sure would be a different product, and this is not that product.