The color information minus the luminance information is known as the color difference signal. Mathematically, this would be shown as Red-Y (Red minus luminance), Green-Y, and Blue-Y, or R-Y, G-Y, and B-Y. Based on the Pythagorean theorem, only two signals are needed to calculate the color information. R-Y and B-Y were chosen because they contain the least amount of luminance information and therefore conserve bandwidth.
The designations R-Y and B-Y were later changed to Pr and Pb for analog signals and Cr and Cb for digital signals. Thus, Y, R-Y, B-Y would be designated either as Y, Pb, Pr or as Y, Cb, Cr. As analog video is replaced by digital, the Pb and Pr designations are no longer in common use and are being replaced by the Cb, Cr designations.
NOTE: It is not uncommon to see these signals also represented as YUV. Here the color difference signals are NOTE: d as U and V. This is not strictly correct, as that designation is specific to analog signals only.
The color difference signals for transmission are created by measuring the output of the red and blue chips and the luminance signal. This results in vectors which appear 901º apart from each other, creating two sides of a right triangle-on a vectorscope, the R-Y axis gs straight north and the B-Y axis gs straight east (Figure 6.5).
Once the color information from the red and blue outputs has been measured, simple arithmetic dictates what the green output is without actually having the information on the received signal. This process is known as encoding and decodingcolor.
At any given instant, knowing the vector length and angle, circuits in the receiver can reconstruct the original strengths of the R-Y and B-Y signals that produced it.