The digital luminance and color difference signals in component video including ITU-R BT.601 (SD), ITU-R BT.709 (HD) ITU-R BT.2020 (UHD) coding. For SD with 4:2:2 sampling, the Y luminance signal is sampled at 13.5 MHz and the two color difference signals are sampled at 6.75 MHz co-sited with one of the luminance samples. Cr is the digitized version of the analog component (R-Y), likewise Cb is the digitized version of (B-Y). For BT.709 HD , sampling rates are 5.5 times greater – 74.25 MHz for Y and 37.125 MHz for Cr and Cb. Finally for BT.2020 4K UHD the frequencies are four times greater again at 297 and 128.5 MHz respectively for the luminance and chrominance.
These are the analog luminance, Y, and color difference signals (R-Y) and (B-Y) of component video. Y is pure luminance information whilst the two color difference signals together provide the color information. The latter are the difference between a color and luminance:
red minus luminance and blue minus luminance. The signals are derived from the original RGB source, usually a camera.
The Y, (R-Y), (B-Y) signals are fundamental to much of television. For example in ITU-R BT.601 it is these signals that are digitized to make 4:2:2 component digital video, in the PAL and NTSC TV systems they are used to generate the final composite coded signal, and in DTV they are sampled to create the MPEG-2 video bitstream.
Convenient shorthand commonly, but incorrectly, used to describe the analog luminance and color difference signals in component video systems. Y is correct for luminance but U and V are, in fact, the two subcarrier modulation axes used in the PAL color coding system. Scaled and filtered versions of the B-Y and R-Y color difference signals are used to modulate the PAL subcarrier in the U and V axes respectively. The confusion arises because U and V are associated with the color difference signals but clearly they are not themselves color difference signals. Or could it just be because YUV trips off the tongue much more easily than Y, R-Y, B-Y?
See also: PAL