@audio/shift-psola

import psola from '@audio/shift-psola'

PSOLA time-stretch + sinc resample. Autocorrelation detects pitch periods; two-period Hann grains are placed at pitch-synchronous intervals, reducing the grain-boundary artifacts plain WSOLA gets on voiced monophonic material. The final resample rescales the whole spectrum by ratio, so formants move with f0 exactly as they do for wsola — this is a smoother stretch, not formant preservation.

psola(audio, { ratio: 0.75, sampleRate: 48000 })
psola(audio, { ratio: 1.5, minFreq: 100, maxFreq: 400 })

Preserves waveform-per-period shape, voiced-speech naturalness.
Destroys formants (same resample-driven shift as wsola), polyphony (assumes single pitch contour), unvoiced regions (pitch-mark jitter); falls back to plain WSOLA internally when no reliable pitch period is found.

Phase coherence 0.998 — pitch-synchronous grains align with the waveform period almost perfectly, just short of formant's 1.000. Lower attack corr (0.941) from pitch-mark jitter on non-periodic onsets.

Use when: Monophonic speech, solo voice, single melodic instrument.
Not for: Polyphonic material, chords, or anywhere formants must stay put (use lpc).

Stream

let write = psola({ ratio: 1.5 })
let out = write(inputBlock)
let tail = write()  // flush

psola buffers the whole input; write(chunk) accumulates and the batch algorithm runs once on write() (flush, no argument), returning everything at once.

Data

Input is a Float32Array (mono) or an array of Float32Array channels ([left, right, ...]) — anything else throws TypeError. ratio here is a fixed number for the whole call — a function or Float32Array throws.

Part of @audio/shift — the shift family umbrella. This README is generated from the umbrella docs.

MIT © audiojs