What is interlacing?

If moving objects in a clip have fine horizontal stripes along their edges while everything standing still looks perfectly normal, you're looking at interlacing. It isn't damage, and it isn't a bad transfer. It's the footage working exactly as designed, on a kind of display that no longer exists.

A bandwidth trick from the 1930s

Broadcast television couldn't carry enough bandwidth to send complete frames at a smooth rate. The workaround was to send half a picture at a time: all the odd-numbered lines, then all the even-numbered lines, each one a fraction of a second apart. Each half is called a field. A cathode ray tube drew them in sequence fast enough that your eye combined them, and you got the motion smoothness of fifty or sixty pictures per second for the bandwidth of twenty-five or thirty.

It was a genuinely clever piece of engineering, and it survived into DV camcorders, DVDs and early digital broadcast. Which is why so much family footage from roughly 1980 to 2010 is interlaced.

Why it looks wrong now

Modern displays don't work that way. They show whole frames. So the two fields, captured at genuinely different moments, get displayed at the same instant. Anything that moved in the gap between them appears twice, offset, one stripe apart. That's the comb.

Stationary parts of the frame look fine because nothing moved between the two fields, which is why the effect appears to cling to moving objects specifically.

Why it has to be fixed first

This is the part people get wrong, and it's expensive. An AI upscaler has no concept of fields. Hand it a combed frame and it sees a regular, high-contrast horizontal pattern, exactly the sort of fine structure it was trained to preserve and sharpen. So it preserves and sharpens the comb. You end up with crisp, high-resolution interlacing artefacts, which are far harder to remove than the originals and often impossible to fully undo.

The order that works is deinterlace, then denoise, then upscale, then colour. Every step makes the following ones harder if you take it out of turn, and interlacing is the least forgiving of the lot.

Telling interlacing apart from things that look similar

Comb teeth only appear on movement, and they're perfectly horizontal and evenly spaced. Rolling shutter skew leans vertical edges over but doesn't stripe them. Compression blocking shows up as square patches, usually in dark or busy areas, and it doesn't care whether anything moved. Ghosting from a bad tape transfer smears rather than combs.

If you're unsure, step through a few frames one at a time on a shot with fast motion. Interlacing is unmistakable frame by frame, and almost invisible at normal speed on some displays, which is how it survives into finished edits without anyone noticing.

Doing it on a Mac

Crisp detects interlaced sources and handles the deinterlace pass on-device, before any enhancement runs, so the ordering problem above is taken care of without you having to think about it. Nothing is uploaded, which tends to matter for exactly the sort of irreplaceable family footage that's most likely to be interlaced in the first place.

Interlacing, in detail

Can I just upscale interlaced footage and fix it afterwards?

No, and this is the single most expensive mistake with old video. The upscaler treats comb teeth as real detail and sharpens them. Deinterlacing afterwards then has to work against artefacts that have been reinforced and enlarged. Deinterlace first, always.

Does deinterlacing lose quality?

It costs something, unavoidably, because you're reconstructing whole frames from half-pictures. A good deinterlacer blends or interpolates intelligently rather than simply discarding one field. Throwing away every other line is the fast method and it halves your vertical resolution.

How do I know whether my footage is interlaced?

Find a shot with fast horizontal movement and step through it a frame at a time. If moving edges break into fine horizontal stripes while static areas stay clean, it's interlaced. Footage from DV camcorders, DVDs and pre-2010 broadcast very often is.

Is 1080i actually 1080p?

Not in terms of motion. 1080i carries the same line count but delivers it as two half-height fields per frame, so a moving subject never has a full 1920x1080 sample at a single instant. Deinterlaced 1080i and true 1080p are not equivalent.

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