Certains patterns/colors look weird on TV

I’m having a hard time figuring out how to describe what I want to ask here.

You know how sometimes on the idiot box a person’s tie or dress will look very strange and cause the TV to flicker over the tie/dress?

What is that? What causes it? Why?

Thank you in advance, smart Dopers, for fighting my ignorance.

You’re looking at a TV screen in which the picture is made of lots of little pixels. If a pattern is nearly at the same scale as the pixels on your screen, you get an interferometer effect. Similar to the wavy patterns you see when you look through two mesh screens.

It’s called moire (or maybe moiré) patterning - sometimes called “herring-bone”

It’s at it’s most impressive on “composite” signals (NTSC or PAL)- where the colour signal is mixed in with the luminance signal (the “black and white” bit) so the whole thing can be sent as one.

In PAL and NTSC signals the luminance and chrominance bits are given different frequencies to keep them seperate - but with fine detail (a pattern on a tie or dress) the luma signal can produce high frequencies that get interpreted as colour by the decoder in your goggle-box

It’s one of the reasons to avoid PAL or NTSC and use S-Video or RGB connections if possible - the effect Lumpy mentions can still happen, but it’s much less common