Tempo and Pitch

Why speed changes pitch at all

Pitch is frequency, and frequency is events per second. Play a recording faster and every waveform cycle in it arrives sooner, so the frequency of everything rises together. Speed and pitch are not two effects that happen to be linked — on a physical playback medium they are the same thing.

Doubling the speed doubles every frequency. A doubling of frequency is the definition of an octave. So double speed is exactly one octave up, always.

The formula

semitones = 12 × log₂(new tempo ÷ old tempo)

The logarithm is there because pitch perception is logarithmic. Going from 100 to 200 Hz and from 200 to 400 Hz are both one octave, even though one is a jump of 100 Hz and the other 200 Hz. An octave holds twelve semitones, hence the multiplier.

What common shifts actually do

The standard ±8% turntable range is worth internalising: it moves pitch by a little over a semitone in either direction. That is enough to take a track noticeably out of key with another, which is the whole reason harmonic mixing exists as a discipline.

Where this still applies

• Turntables — the pitch fader changes platter speed, and pitch follows.
• Tape and varispeed — physically coupled, by definition.
• Samplers in fixed-ratio mode — playing a sample back at a different rate transposes it. This is how the classic sampler sound was made.
• Repitching a loop with warping switched off.

Where it does not

Time-stretching algorithms break the link deliberately. They chop audio into short grains and redistribute them in time, so the playback rate changes while the grain contents — and therefore the pitch — stay put. Every modern DAW does this by default when you drop audio onto a timeline at a different tempo.

That is why dragging a 90 BPM loop into a 120 BPM project no longer makes it sound like a chipmunk. It also means that if your DAW is warping, this calculation describes what would have happened without warping, not what you are hearing.

The audible cost of stretching

Time-stretching is not free. Push it far enough and you hear artefacts: a metallic, flanged quality on sustained material, and smeared transients on percussion. How far you can go before it becomes obvious depends on the algorithm and the source, but a 10–15% change is usually transparent and 30% usually is not.

For large tempo changes, repitching often sounds better than stretching — you accept the pitch shift and transpose it back with a separate, cleaner tool if you need the original key.