Change the key of a song. It finds the key, you pick the new one. The tempo stays. Free, in your browser.

Singers move a song to fit their range: a key or two down for a lower voice, up for a higher one. Players move a backing track to the key they know, or to match a capo position. DJs move a track to a compatible key on the Camelot wheel so two songs mix without clashing. Producers move a sample to the key of the project.

The key comes from chroma analysis: the energy in each of the twelve pitch classes is measured across 45 seconds of the song, past the intro, and matched against major and minor key profiles. Songs that sit between a major key and its relative minor (C major and A minor share every note) can read as either; both need the same shift, so the result is right either way.

The shift is Signalsmith Stretch, the pitch and time engine the SoundGen studio uses. It moves every note by the same number of semitones and keeps the length exactly the same, so the tempo doesn't change.

Two or three semitones either way usually sounds natural. Past that, voices change character, because the resonances of the throat move with the pitch: a big shift up sounds younger and thinner, a big shift down darker and heavier. Instruments tolerate more than voices. Drums are untuned, so they just sound brighter or duller.