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.

The existing standards for digital video are ITU-R BT.709 for HDTV (1920x1080) and ITU-R BT.601 for SDTV (720x480). The xvYCC standard (IEC 61966-2-4), adopted in January 2006, expands the color gamut by introducing negative color signal values for these video signals. This standard encompasses xvYCC709, which is upwardly compatible with the B7.709 standard, and xvYCC601, which is upwardly compatible with BT.601. The four characteristics of these standards are shown in Figure 2.

1. The chromaticity coordinates of RGB primary colors and the reference white (D65) are the same as for the existing BT.709 standard.
2. The opto-electric transfer characteristics of signals not less than 0 and not greater than 1 are defined using the same formula as for BT.709, and signals above 1 are also defined using the same formula. Signals below 0 are defined to produce a transfer curve with origin symmetry.
3. The conversion matrix from RGB (color signals) to YCC (luminance/color-difference signals) for xvYCC709 is the same as for BT.709, and that for xvYCC601 is the same as for BT.601.
4. With 8-bit quantization of luminance and color-difference signals, definition formula are set and the color gamut is expanded by using values between 1 and 15 and between 241 and 254 as picture signals. Definitions over 8 bits are also used to support precise gradation.

The Munsell Color Cascade provided by Michael R. Pointer is a highly saturated color chart consisting of 48 hues and 16 levels of lightness for a total of 768 colors. Figure 3.1 provides two-dimensional and three-dimensional representations of chromaticity points in the actual color chart and the color chart for the color space. Figure 3.2 uses this data to show the color representation capacity of the xvYCC standard. It is difficult to represent the xvYCC color space on a two-dimensional x,y chromaticity diagram, which is an x,y projection of the three-dimensional color space. We will therefore explain it in three-dimensional space, using x,y chromaticity and luminance (z axis). Chromaticity points (black dots) in this color chart form a triangular pyramid, with color points spreading out where luminance is low.

Figure 3.2 shows the relationship between the color space in the existing BT709 standard and color points in the color chart. Here, some color points in the color chart are excluded from the color space in the BT709 specification. The enclosed area is equivalent to 55% of the 768 colors in the color chart. The gray area in the color chart denotes colors that cannot be represented under the BT709 standard.

Figure 3.3 provides a three-dimensional image of the color space of the xvYCC standard. There are four peaks representing areas of high luminance, while the width of the graph expands as luminance decreases. This means that the entire color chart can be encompassed, and that 100% of the color chart representing highly saturated object colors can be represented under the xvYCC standard.

As described above, a CRT TV based on the BT709 standard can only represent 55% of the Munsell Color Cascade (see Figure 4.1). In 2006, Sony launched BRAVIA, the world's first LCD television based on the xvYCC standard. An LCD TV with LED backlighting can represent 82% of the Munsell Color Cascade, while a Sony GxL projector with a laser light source can represent up to 97% of the colors (*3) (see Figure 4.2 and 4.3). This is indicative of the performance that can be achieved when the xvYCC standard is applied to extended-gamut display devices.

*3 As of November 2005, based on Sony research.

In 2007, Sony also launched the world's first digital video camera capable of recording xvYCC color signals. This allows images of objects with highly saturated colors, such as flowers, cars and stained glass, to be captured more realistically then ever before.

Sony has announced the designation "x.v.Color" for products that can process color signals based on the xvYCC standard to represent a wider color range or "gamut" than is possible under the existing standard (BT709). This logo is applied to products that have this capability. Other manufacturers have also started to use this logo, indicating that the use of the xvYCC standard in consumer electronic products is increasing. The DCDM digital cinema standard has already been formulated for video content with a wider color gamut than BT709, and content is now being produced. The shift to extended-gamut display devices and the increasing use of the xvYCC standard are likely to drive the shift toward extended-gamut content. It is hoped that the xvYCC standard will also be applied to technology for the equipment needed to produce and distribute movies, broadcast content, packaged media, and network content.