time-stretch

Time Stretch

Change playback speed without changing pitch (and optionally vice-versa). rubberband is the quality engine — ffmpeg's atempo works for mild changes but artefacts at extremes; rubberband holds up to ~0.5×–2× cleanly.

When to use

• Podcast post: tighten an episode by 5–15% to match a pace target.
• Recover a slow recording without making the speaker sound chipmunked.
• Fit an audio file to a target duration (e.g. ad slot).
• Pitch-shift without changing duration (less common — singing/voice acting work).

Do not use this skill for:

• Beat-aligned tempo changes — that's a music-production concern not in scope.
• Sample-rate conversion — use convert-format.

Inputs

1. Input file — required.
2. Mode — one of:
   • tempo — change speed, preserve pitch. Takes a ratio (1.10 = 10% faster, 0.95 = 5% slower).
   • duration — fit to a target duration in seconds; the skill computes the ratio.
   • pitch — shift pitch in semitones, preserve duration. Takes a number (+2 = up two semitones, -3 = down three).
3. Output path — defaults to <input-stem>.stretched.<ext>.

Procedure

1. Detect engine. Prefer rubberband if on PATH; else fall back to ffmpeg atempo (and warn if the requested ratio is outside [0.5, 2.0] — atempo requires chaining beyond that range, which compounds artefacts).

2. rubberband (preferred):

   # tempo: ratio 1.10 = 10% faster
   rubberband --tempo <ratio> "<input>" "<output>"
   
   # duration: fit to N seconds
   #   ratio = orig_duration / target_duration
   rubberband --tempo <ratio> "<input>" "<output>"
   
   # pitch: shift by N semitones
   rubberband --pitch <semitones> "<input>" "<output>"
   

   Add --fine for high-quality (slower) processing on critical material; default mode is fine for most podcast use.

3. ffmpeg fallback (atempo, tempo mode only):

   ffmpeg -i "<input>" -filter:a "atempo=<ratio>" "<output>"
   

   For ratios outside [0.5, 2.0], chain: atempo=2.0,atempo=<remainder> (or the inverse for slow-down). Warn the user that quality degrades.

   ffmpeg has no clean equivalent of --pitch without changing duration; if pitch mode is requested and rubberband is missing, tell the user to install rubberband — don't fake it.

4. Verify output: ffprobe -i "<output>". Report old vs new duration and the ratio applied.

Output

• Stretched/shifted audio file at the resolved output path.
• One-line summary: <input> (<orig-duration>s) → <output> (<new-duration>s, ratio <ratio>, engine <rubberband|atempo>).
• For pitch mode: <input> → <output> (pitch <±N> semitones, engine rubberband).

Notes

• rubberband-cli is apt install rubberband-cli — tiny footprint. Add via install-deps.
• For very long files, rubberband processes single-threaded and is slower than atempo. Acceptable trade for quality on episode-length material.
• Tempo changes alter LUFS slightly; if the file was previously loudness-normalised, re-run normalize after.