Change the key of a song

First it finds the key, the same way the key finder does: it listens to which notes the song uses most and matches them against every major and minor key. Then you pick the key you want, and every note moves up or down by the same number of semitones, the shortest way there (never more than six). The tempo and the length stay as they were.

A major song stays major and a minor one stays minor: moving every note by the same step can't turn one into the other.

Played faster, a record also plays higher; slowed down, it plays lower. AudioRinse keeps the two apart with Signalsmith Stretch, an open-source pitch and time-stretching library made for music with many instruments at once. It cuts the sound into short overlapping slices, moves each slice's frequencies or its timing, and joins them back up, here in your browser.

You hear every change as the song plays, and the download is made the same way, faster than real time. On test tones every pitch we checked, from an octave down to an octave up, landed within 2 cents of the target, and every speed gave exactly the expected length.

Finding a key isn't certain. On songs with hand-labelled keys, the key finder names the exact key on about 6 songs in 10, and that key or a close neighbour on about 3 in 4; the usual miss is the relative key (A minor for C major), which uses the same notes.

The change itself doesn't depend on it: the shift is counted in semitones, and moving a song up two semitones is the same move whatever its key. If you know the key, count from there, or set the semitones directly with the pitch changer.

• Singers, to bring a song into their range for practice, an audition or a karaoke night.
• Players, to match a recording to a part written in another key, or to an instrument that's easier in it.
• DJs, to mix two songs in key: the Camelot wheel shows which keys blend.