Time and pitch — stretching without the chipmunk effect

Two of the most-wanted edits sound like they should be the same thing, but they’re opposites:

• Change the speed — make a recording play faster or slower.
• Change the pitch — make it higher or lower.

On a tape deck or a turntable they’re welded together: spin the reel faster and everything gets shorter and higher — the “chipmunk” effect. Digitally we can pull them apart. Cathar gives you three commands, and the difference between them is exactly which of those two things you hold still.

  speed  : faster  → shorter AND higher   (like a tape deck)   cathar speed
  tempo  : faster  → shorter, SAME pitch                        cathar tempo
  pitch  : higher  → SAME length                                cathar pitch

Speed — the honest tape deck

cathar speed --factor 1.5 simply plays 1.5× faster: the file gets shorter and the pitch rises, exactly like spinning a reel faster. Under the hood it’s just resampling — read the samples at a different rate (see the resampling chapter). It’s the right tool when you want the tape-deck behaviour, or to nudge a recording that was captured at a slightly wrong speed.

Tempo — change the length, keep the pitch

This is the clever one. You want a podcast 10% shorter without everyone sounding like a chipmunk. The trick is granular overlap-add: chop the sound into short overlapping grains and lay them back down closer together (to speed up) or farther apart (to slow down). Do that naïvely and you get clicks and warbles at every join. The fix is to slide each grain a few milliseconds so its waveform lines up with the one before it — the joins become invisible. That method is called WSOLA (waveform-similarity overlap-add), and it’s Cathar’s default:

cathar tempo lecture.wav --factor 1.25     # 25% faster, voices unchanged
cathar tempo song.wav    --factor 0.8      # 20% slower

For sustained, tonal material (strings, pads) there’s a second engine, the phase vocoder, which works in the frequency domain and keeps long notes smooth. Pick it with --mode pv; stick with the default wsola for speech and anything percussive.

Pitch — change the note, keep the length

Pitch-shifting is tempo and speed working together. To raise the pitch a semitone without changing the length, Cathar:

1. time-stretches the audio longer by the pitch ratio (pitch unchanged), then
2. resamples it back down to the original length — which speeds it up, and that is what raises the pitch.

cathar pitch vocal.wav --semitones -2      # down a whole tone
cathar pitch vocal.wav --semitones 7       # up a fifth

A tone shifted up an octave (--semitones 12) comes out exactly twice the frequency, in the same number of seconds.

When it sounds artificial

Time-stretching is never free. Push tempo or pitch past roughly ±20–30% and you’ll start to hear it: a slight metallic “phasiness” on the phase vocoder, or a faint fluttering on WSOLA. That’s the algorithm inventing information that was never recorded. Small moves are transparent; big ones are an effect, not a repair. Use the mode that suits the material, and reach for the smallest factor that gets the job done.