As Mark Schubin wrote, the I axis represents the color to which the human eye is most sensitive.
The time when the ideas concerning quadrature modulation phase angles were being thought out came a while after the work by Hazeltine that made color television compatible with exsisting monochrome receivers.
RCA had at first used equal amounts of R,B,G, to make the luminance signal. This worked, but there were discrepancies in the way objects shot with a color camera appeared on a B&W TV monitor.
The gray scale seemed wrong.Hazeltine looked at the luminance values of red, green and blue and came up with 'the Constant Luminance Principle' in which ~59% Green, ~30% Red and ~11% Blue were used to constitute the luminance signal.
This meant that the luminance portion of the color broadcast signal would have the same expected gray scale values
that video from a monochrome camera would exhibit when viewed on a B&W TV.Another really interesting and useful characteristic of the NTSC color system is the fact that in NTSC, a pure white video signal and a pure black video signal will have no chroma sidebands at all.
This is because there is no color information in Black or White, or if you wish to view it another way, the color values are equal in Black or White and therefore cancel resulting in no modulation of the color subcarrier.
The only time chroma sidebands are present in the composite video signal is when the camera is shooting an object with color in it. It took the engineers of about a dozen separate really big electronics companies to invent NTSC and it took about 10 years to get to the point on Decemebr 17, 1953, that a color signal containing an image of the NBC Chimes was patched into the WNBT transmitter for a few seconds at 5:32 PM. RCA had just received word that the FCC had approved the NTSC color system.
Dale Kelly wrote:
OK, that makes sense. Do you perhaps know the answer to the question I had asked in the body of the message below.Actually, the YUV matrix seems more useable to me and I don't know whyRCA chose to use YIQ, any idea on your end? Perhaps it was an IP thing?<If you want to read a comprehensive Color Television History, go here: http://novia.net/~ereitan