Wow and flutter — when the pitch won’t sit still

Play an old cassette or a warped record and listen to a held note — a piano chord, a sustained vowel. It doesn’t sit still. It drifts and wavers, sagging flat and creeping sharp. That seasick wobble is wow and flutter, and it’s the signature flaw of anything with a spinning part.

• Wow is the slow drift — under a few times per second. A slightly eccentric record, a belt that’s stretched, a motor that surges.
• Flutter is the fast wobble — tens of times per second. A pinch roller with a flat spot, a bearing with grit in it.

Both come from the same cause: the medium didn’t move past the head at a constant speed. And because speed and pitch are welded together on analog gear (see the previous chapter), a speed wobble is a pitch wobble.

The key insight: everything wobbles together

Here’s what makes it fixable. A speed error doesn’t shift one note — it scales every frequency by the same amount at that instant. When the tape runs 1% fast, the whole recording is 1% sharp: the bass, the voice, the cymbals, all of it, together.

So if you can measure how the pitch of any one steady thing moves over time, you’ve measured the speed error for the entire recording.

   ideal:     a steady 440 Hz note ────────────────────────────
   wowed:     440 → 448 → 435 → 443 → …   (the note "breathes")
              └─ that breathing IS the speed curve ─┘

How cathar dewow does it

cathar dewow warped-tape.wav

Cathar finds a strong, sustained tone in the recording and watches its instantaneous frequency — literally, how fast its waveform is turning, moment to moment. That traces out a speed curve: 1.0 where the tape ran true, 1.01 where it ran fast, 0.99 where it dragged.

Then it time-warps the audio to undo that curve — stretching the fast bits back out and squeezing the slow bits back in — so the note stops breathing and sits at a constant pitch. The overall length is preserved; only the wobble is removed.

The catch: Cathar needs something steady to lock onto. On a solo piano note or a sustained vocal it works beautifully. On a dense, constantly-changing mix with no stable pitch, there’s nothing to measure, and dewow sensibly leaves the audio alone rather than guessing.

A cousin problem: channels out of step

Tape has a second timing flaw. If the playback head is tilted a hair — its azimuth is off — the left and right channels arrive slightly out of step, smearing the stereo image and thinning the sound when you sum to mono.

cathar azimuth measures the tiny delay between the two channels (by sliding one against the other until they line up best) and nudges the right channel back into alignment — down to a fraction of a sample.

The same trick aligns separate recordings of the same moment — two mics, or a reference track — with cathar align --reference good-take.wav. When the takes differ a lot in level or room sound, try --method gcc-phat (generalised cross-correlation); the default time-domain correlation is fine on clean, similar signals. Line up the takes first, and everything you do afterwards (mixing, comparing, noise profiling) gets easier.