The traditional color signal specification defined primary colors and a white point using chromaticities based on red (R), green (G) and blue (B) phosphor emissions in a cathode ray tube (CRT). The ITU-R BT.709 picture signal standard for digital (HDTV) broadcasting was adopted in 1990, while the sRGB (IEC61966-2-1) standard for displays used with personal computers and other equipment was adopted by the International Electrotechnical Commission in 1999.
Except for the fact that there is no overwhite in picture signals based on the sRGB standard, the two standards have very similar color gamut. In CRTs, R, G and B phosphors are used to represent a triangular gamut encompassed within the color points for the primary colors on an x,y chromaticity diagram.
Expanding the Color Space by Increasing Primary Color Purity and Enlarging the Triangle
A wide range of colors can be represented in television and video picture signals by mixing the intensities of the three primary colors (RGB) as color signals. Because these composite colors can be used to produce every color contained within a triangle defined by the primary colors, the color space that can be represented can be expanded by raising the purity of the primary colors and enlarging the triangle (Figure 1.1).
Expanding the Color Space Using Negative Color Signal Values
By using negative values for the RGB color signals, colors outside the triangle can be represented without changing primary color chromaticities. Figure 1.2 shows how cyan, which is outside the triangle, can be represented using a negative value for R and positive values for G and B. In an color system of light (additive color mixing), a color that is exactly the same as one represented by mixing G and B can be obtained by mixing cyan, which is outside of the triangle, and R. Other colors outside of the triangle can be represented by mixing negative G and B signals. This characteristic of the xvYCC standard is used to expand the color space for video applications.