Weird TV Effect

Why is there a weird rainbowy distortion when someone on TV wears a houndstooth jacket or a similar pattern?

It’s called a moiré pattern. It happens when two regular patterns are superimposed on each other and they don’t match exactly. The pattern in the jacket causes interference patterns with the discreet scan lines and tension mask holes in your TV set. You’ll get a similar effect when scanning a halftone print as from a magazine on a flatbed scanner.

You can also see this effect with two chain link fences, with alternating bands where they match/don’t match.

Or two home window screens.

Nope, it’s not a moire pattern, not quite.

It’s called “cross color.” Try searching on those keywords. It occurs because the color information in NTSC (american) television is hidden within the picture. It takes the form of very fine vertical stripes. Stripe-y objects in the real world can get past the TV camera, mimic the encoded color patterns, and fool your TV set into thinking it’s the color signal.

When an actual image contains similar vertical stripes (as with pinstripe or herrinbone patterns in Jonny Carson’s suitcoat) the TV camera is supposed to deal with it. In theory, the filter in the camera removes that information so it cannot fool your TV into producing false colors. In theory, your TV set is supposed to figure out the difference between the color signal, versus striped patterns which are nearby in frequency. In practice, an inexpensive TV set will display horrible “cross color” while a more-expensive set will not. The difference is in the amount of filtering, an an expensive set might have lots of extra components in the color circuitry.

Tech info: the “chroma” information takes the form of 3.57MHz modulated signal called the “color subcarrier” which looks like 460 horizontal pixels on the TV screen. The hue of the colors is encoded in slight phase shifts, and the saturation (pastel-ness) is encoded as the 3.57MHz signal strength.