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Time stretching explained
Gazillion Industries ·
The sample is 94 bpm. The beat is 88. You drag the corner of the region,
the numbers agree with each other, and the room tone behind the singer starts to sound like it
is being spoken through a fan.
Time stretching changes the length of audio without changing its pitch. It works by cutting
the audio into short windows and either repeating or dropping them, or by re-synthesizing the
sound from an analysis of its frequencies. Both approaches are guesses about material the
recording never contained, so the artifacts are structural rather than a sign you did it wrong.
Small stretches, monophonic material and transient-aware modes hide them. Big stretches on a
full mix do not.
The short version
- What
- length changes, pitch stays
- How
- overlapping windows, or analysis and resynthesis
- Safe
- within about 10% on a full mix
- Further
- monophonic material and drums take more than mixes do
- Artifacts
- smearing, flanging, doubled transients, metallic tails
- Alternative
- repitch instead, and let the key move
The two families of algorithm
The time-domain method chops the audio into small grains and overlaps them, adding repeats to
make it longer or throwing some away to make it shorter, aligning each splice so the waveform
matches where it joins. It is cheap, it keeps transients crisp, and when it fails you hear
stuttering or a comb-filtered flutter on sustained sound.
The frequency-domain method analyzes the signal into frequency bands and phases, advances
them at a different rate, and builds new audio from the result. It handles sustained material far
better and blurs attacks, which is the "phasiness" people describe as underwater or metallic.
Modern implementations detect transients and switch strategies at each hit, which is why one
algorithm can hold a drum loop together and still stretch a pad.
How to stretch without wrecking it
- Get the source tempo right first: a wrong tempo means the stretch is fighting the grid.
- Pick the mode that matches the material: transient-aware for drums, spectral for pads and
vocals, monophonic for a single line.
- Keep the ratio inside 10 percent on a full mix if you want it transparent.
- Stretch, then bounce, then listen to the bounce rather than the live playback.
- If the attacks doubled, slice at the transients and move the pieces instead.
- If the tails swirl, shorten the analysis window or use a different mode.
- When both fail, repitch instead and accept the key change.
SKRUU keeps
pitch and time as separate knobs: TIME runs from −50% to +25% and moves nothing but the
length, and PITCH LOCK holds the pitch while you do it.
Stretching versus repitching
Repitching is what a tape machine or a turntable does: speed and pitch move together, no
analysis, no artifacts, and a key change you have to live with. Producers chose it for decades
because it was the only option, and the sound of a sample repitched down a tone is baked into
entire genres. What is varispeed covers the
mechanism, and how to change pitch without changing
tempo covers the other direction.
The rule of thumb: if the sample is close to your tempo, stretch it. If it is far away, or if
the artifacts are audible, repitch it and build the beat around the new key. A sample pitched to
fit sounds like a decision. A sample stretched 20 percent sounds like a plugin.
Where it costs you
Stretching a full mix stretches the reverb, the room and the noise floor, and those are
exactly the parts your ear uses to judge whether something is real. Drums lose the hard front
edge of the transient first. Vocals lose breath, then diction. Every stretch also costs CPU
while it plays, so a session with twenty stretched regions behaves worse than a session with
twenty bounced ones, and bouncing is free.
Questions people ask
How do you time stretch audio in Logic Pro?
Turn on Flex Time for the track, choose the algorithm that suits the material, and drag the
region's end or the flex markers. To fit a region to the project tempo, use the region's Flex
& Follow Tempo setting rather than stretching by hand.
How do you time stretch audio in GarageBand?
Enable Follow Tempo & Pitch on the region so it conforms to the project tempo, or hold
Option while dragging the region's lower right edge to stretch it manually.
What is the best time stretching software?
Every major DAW licenses a capable algorithm, and the mode you pick matters more than the
brand. Dedicated stretching tools pull ahead on extreme ratios and on material with both
transients and long tails, where a single mode has to compromise.
How much can you time stretch before it sounds bad?
About 10 percent on a full mix, more on a single instrument, and a great deal on a pad, where
there is nothing sharp to smear. Drums are the tell: if the transients start to double, you have
gone past what the algorithm can hide.
What SKRUU does
SKRUU is two knobs that do not touch each other. PITCH moves the pitch and
TIME moves the length, so you can speed it up while pitching down, or the other
way round. PITCH runs from −20% to +10% in 128 states and reads out in semitones as well as
speed; TIME runs from −50% to +25%. PITCH LOCK and TIME
LOCK hold one while the other moves, DRAG sets how long the new speed
takes to arrive, and there is zero latency.
The fan noise behind the singer was never in the room. It arrived when you dragged the
corner.
Keep reading
SKRUU — varispeed pitch and time stretch for Mac and Windows. Pitch and
time move independently, with sixteen elastic return shapes.
Hear it — $29