Time Stretching and Pitch Shifting

Stretching changes duration, shifting changes pitch, and both leave traces on the material.

Related

PitchOctaveTimbre

The problem

When a recording is slowed down, why does the pitch stay put while the drums turn mushy?

How it works

Time stretching changes duration while holding pitch; pitch shifting changes pitch while holding duration. Common methods include the phase vocoder, granular synthesis and transient preservation. A phase vocoder cuts the signal into frames and moves phase in the frequency domain; it suits sustained tones but smears transients, so drums spread out. Granular synthesis rearranges small grains, producing repetition and a metallic edge at large ratios. Pitch shifting can resample or move formants. Keeping formants makes a transposed voice sound like the same person; dropping them sounds like a sped-up tape.

Practice and settings

Keep stretching within about 15 percent; beyond that, re-record or replay. Use transient-preserving modes for drums and bass, and formant preservation for voice. For large stretches, slice and align transients first, then stretch the slices. Above five semitones of shift, check sibilance and breath. Match loudness before and after, so louder is not mistaken for better.

What to listen for

Hear a slowed drum loop: the snare turns from a hit into a smear. Hear a voice shifted down without formant preservation and it sounds like slow tape. Hear a piano shifted up: the low resonance moves up and thins. Hear sustained strings, where heavy stretching adds periodic wobble.

Common misconceptions

That stretching and pitch shifting are the same operation; that any material can be stretched far; that formant preservation is only a colour preference.

Sources

• Curtis Roads, The Computer Music Tutorial — 相位声码器与颗粒合成的说明核对，未抄原文
• Jean Laroche, Mark Dolson, Improved Phase Vocoder Time-Scale Modification — 瞬态处理的结论核对，未抄原文

Used for fact-checking only; all prose is original.