[old_tv_nut]

NTSC rejected color-alternating at field rate because of flicker problems. I can't recall if PAL was rejected for line flicker or not. Of course, line-delay memory was not available at the time.

If output luminance were constant with chroma amplitude and phase changes, 30 Hz flicker would be invisible. However, two things make this impossible, because they change the luminance of the reproduced color when the chroma phase or amplitude changes:

1) CRT gamma - saturated colors crank up the dominant primary color beam current more than they turn down the less-used gun beam currents, so luminance is not constant vs. chroma signal level (color saturation).
2) the scaling of the R-Y and B-Y axes to prevent overmodulation except in extreme cases like 100% amplitude color bars, so luminance is not constant vs. phase.

Luminance flicker threshold is somewhere between 50 and 60 hz for normal TV picture brightness, but more like 15 Hz for chroma-only changes with constant output luminance.

By the way, this is why you can see luminance flicker on the COL-R-TEL rotating wheel converter, but see correct colors without visible switching from R to G to B. [Strictly true for stationary objects and stationary eyeballs, otherwise you see color fringing/breakup, but that is not exactly the same thing as flicker.]