Color came along after television had established itself as a black and white medium. Therefore video color space was developed to carry the black and white picture and color separately from the earliest days of color television. This basic approach also made color television signals inherently backwards compatible with black and white televisions as the older black and white models could simply ignore the color component of the signal and still show a serviceable picture.
Most users who look at the notations for color difference systems try to apply it to what they know about RGB. I’ve had users look at the “Y’/B-Y’/R-Y’” designation on their Betacam deck or their DVD player and ask if the green signal is buried in the “Y’” channel because they see the “R” and the “B” which obviously correspond with “Red” and “Blue”.
Well...not exactly.
The “Y’” represents the luma, or black and white portion of the video signal. The “R-Y” and “B-Y” in the case above represent the two color difference signals. (Color difference systems do vary and there are several non-interchangeable notations which I'll cover in another post. ) In any case, the two color difference values can be demonstrated with one illustrated concept as they are all applied the same way with the exception of the scaling factors
On a vectorscope, the further from the center the display shows a given signal, the more saturated with color it is. The “compass” direction it extends out from the center determines its “hue” or what “color” the color is. Upon looking a bit closer at that same vectorscope, we see that there is a vertical and a horizontal axis that run through the very center. The vertical and horizontal axis might be designated as R-Y’ and B-Y’, or PB and PR, or you even may see perpendicular axes tilted 33 degrees counter-clockwise from vertical and horizontal labeled “I” and “Q” .
My sort of quirky take on this to help people visualize the concept is that one could think of these color difference signals as the axes of “latitude” and “longitude” creating the “coordinates” that tell the video system where to find the proper hue and saturation to create the designated color for a pixel that already has brightness or luma information supplied by the Y’ component.
(As with all things technical about digital video, my personal reference bible on the subject is Charles Poynton's "Digital Video and HDTV Algorithms and Interfaces." I highly recommend it as a source of unbiased and thorough information.)
TimK
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