Key lock

Without key lock, changing a track's speed changes its key as well: it is what happened with vinyl, and it is physics rather than a design decision. Raise the pitch and everything rises in tone.

Key lock decouples the two. With it on you can move the tempo while the key stays put, which is what makes harmonic mixing possible when two tracks need to meet in the middle on speed: without key lock, a tempo adjustment of several per cent is no longer the same track in key terms, and the compatibility you checked no longer holds.

It has a cost. Holding key steady at a changed tempo is signal processing, and past a point you hear it: metallic artefacts appear, on vocals above all. How far you can stretch before it becomes a problem depends on the material and the system, and it is worth testing with your own tracks rather than trusting a number.

Common practice: leave it on by default and switch it off deliberately when you want the pitch-shift effect. Our checker has a key lock switch for exactly this reason: with it off, it tells you which key the track will actually play in rather than the one you matched on paper.