Tempo & Pitch Changer

Why speed and pitch don't have to move together

In the analogue world, speed and pitch are locked to the same control — play a record faster and everything rises in pitch because you're physically compressing the waveform in time. Digital audio inherits that behaviour: reading samples faster produces the same effect, which is why a raw speed change turns voices into cartoon characters.

Decoupling the two requires resynthesis. This tool relies on WSOLA — waveform similarity overlap-add — which slices the audio into overlapping windows of around sixty milliseconds and reassembles them at a new spacing to reach the target duration. Before writing each window, it searches nearby positions for the one whose waveform best matches what's already been written and splices there, which is what keeps periodic sounds — a sustained vowel, a bass note, a held chord — from tearing into phase cancellation at each join.

Pitch shifting builds on the same machinery. To raise a track two semitones without altering its length, the audio is first resampled so it plays higher and shorter, then time-stretched back to the original duration — pitch moves, tempo doesn't. Using both controls together simply combines the two ratios.

How far you can push it before it sounds processed

Time stretching degrades gradually rather than falling apart at a hard line. Between roughly 85% and 120% of original speed, almost no one notices any processing at all. From 70% to 85% the result stays clean enough for practice and most creative work. Below 60% the artefacts become part of the sound, which is exactly why the slowed-down aesthetic leans on them deliberately.

Pitch behaves the same way. Shifting two or three semitones in either direction is transparent on most material. Push past about five semitones and the formants — the resonances that make a voice sound like a specific human throat rather than a synthesised one — start moving along with the pitch, giving the voice an artificial edge. That's not a flaw in this implementation; it's what happens to any pitch shift that doesn't model formants separately.

• Practice: 70–85% speed, pitch unchanged
• Key matching for a singer: ±2 to 3 semitones, speed unchanged
• DJ tempo matching: adjust speed to the target BPM, leave pitch alone
• Creative slowdown: below 70%, where the artefacts become the point

Working out the right ratio

Matching a target tempo is simple arithmetic: divide the target BPM by the original BPM. A 128 BPM track that needs to land at 120 becomes 120 ÷ 128 = 0.9375, or 93.75% speed. If you don't already know the source tempo, run the file through the Key & BPM Finder first and bring that figure back here.

Key matching follows the same logic in semitones. Work out the interval between the source key and the target — A minor to C minor is three semitones up — and enter that number directly. The Camelot code from the analyser makes this easier too: adjacent numbers on the wheel sit a fifth apart, and the letter tells you whether you're in the major or minor ring.

You might also need

• Slowed + Reverb MakerStretch a song to a slower tempo and wrap it in reverb for the ambient remix look.
• Key & BPM FinderWork out a track's tempo, musical key and Camelot code for harmonic mixing.
• Audio Cutter & TrimmerSelect a region of a track, keep or delete it, and smooth the edges with fades.