System Colorimetry for Super Hi-Vision (SHV)

We are currently developing Super Hi-Vision (SHV) as a future television system that will provide a significantly better viewing experience than the systems used today. There is no doubt that demand exists for higher image quality in television broadcasting and that color reproduction is one of the fundamental elements determining image quality. System colorimetry refers to the expression of color primaries and the color chosen for reference white. This article describes a set of RGB primaries and reference white for SHV that enables wide-gamut color reproduction.

A color TV reproduces colors by combining the three primary color signals of red (R), green (G), and blue (B). Figure 1 shows the chromaticity coordinates of the RGB primaries for HDTV (Rec. ITU-R BT.709) in the xy chromaticity diagram. The horseshoe-shaped curve is called the spectrum locus, representing all the visible monochromatic light sources. The straight line that connects the ends of the spectrum locus is referred to as the purple boundary, representing the color purple, which is not included in the spectrum locus. The range of visible color includes those inside the horseshoe-shape curve, with the color saturation increasing from the center to the edge. Colors reproducible with the HDTV system colorimetry are limited to the area inside the triangle connecting the RGB points. The reference white in the diagram is the chromaticity point for R, G, and B signals having identical levels, defined as D65^*1 for the HDTV system colorimetry.

The conventional television colorimetry was standardized on the basis of CRT display devices — in particular, the characteristics of their phosphors — that had been prevalent for many years. However, with the advent of non-CRT technologies and wide-gamut displays such as LED-backlit LCDs and laser displays, the constraints inherited from CRTs are no longer relevant to the system colorimetry. This freedom led us to develop a new system colorimetry for SHV with the RGB primaries on the spectrum locus, thereby harmonizing the needs for compatibility with other media, reproduction of real object colors, cost and performance, and device feasibility.

We have submitted this new set of RGB primaries and reference white for Ultra High Definition Television (UHDTV) to the ITU-R^*2. Figure 2 shows the chromaticity coordinates of the primaries and Pointer's colors^*3. While HDTV could not produce certain object colors, the proposed one can reproduce almost all the colors. Further study is underway on color encoding for luminance and color difference components and a camera system conforming to the proposal.