Quote:
Originally Posted by cbenham
CPA works at the vertical rate, not horizontal. One field is one color phase and the second field is the other color phase. CPA was dropped because in it's simple form it produced bad flicker at the vertical rate. There were no 'one field' video delay lines available to solve the problem at that time.
Cliff
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Very true. The problem was that the NTSC system was not true constant-luminance because of the CRT non-linearity. (Current digital TV systems are not constant-luminance either.) What this means is that the chroma signal can affect the brightness of a color (more so for saturated colors, not at all for grays).
It's the chroma flicker translating into luminance changes that is visible. During the development of HDTV, Dr. Schreiber at MIT proposed going to a true constant luminance system, and demonstrated that 30 Hz true chroma-only flicker (or even 15 Hz!) is invisible. This could hypothetically be used to reduce the frame rate of chroma to save bandwidth, but would run into problems when the camera pans or the eye tracks a moving object. It also could be used to more crudely encode the chroma without producing visible artifacts.