To give a simple explanation:

The 'Y' component is the same as in the S-video 'Y/C', which refers to the luminance, or black & white, portion of the NTSC signal, and it's what provides the basic picture information.

By the way, the 'C' refers to the chrominance, or color information, which was added later as a separate signal for B&W/color compatibility. Many people believe that the compromise met by the 'Y/C' method of signal transmission is the main flaw in the NTSC standards.

The Pb and Pr are the signals that form the three primary colors, 'b' being blue, and 'r'; being red. The two signals are combined mathematically in various ways to form the green component. This should not be confused with the computers' RGB video signal, which forms all of the combinations from the three primaries themselves.

The theory is that the more that the different video signal components are transmitted independently, the greater the signal purity, partly because less filtering is required to separate the signals, which results in a picture with better color control, less dot crawl, sharper images, etc.

Larry
www.fineelectricco.com