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The development of a new High Definition Television system for use within Europe has provided a unique opportunity to depart from standard television colorimetry and coding which has been used worldwide since colour television began.

All television systems in operation at present use coding systems which allow substantial proportions of the perceived luminance to travel via narrow band colouring channels, thus reducing luminance detail in coloured areas. This is known as the "failure of constant luminance". Also, the gamut of colours reproducible by existing television systems and equipment does not do justice to saturated colours encountered in real life.

A new system has been devised which greatly expands the colour gamut that can be reproduced, and uses constant-luminance coding to avoid loss of detail in coloured areas. This has been achieved without compromising compatibility with the existing television system; the proposed system produces signals which can be decoded by a conventional receiver while still producing satisfactory colour reproduction and sharpness. The enlarged colour gamut of the proposed system will allow future displays to provide continuing improvements in colour performance as new types of display (eg laser projectors are developed with increasingly saturated display primaries.

The constant-luminance decoding receiver is necessarily more complex than a conventional receiver, and may not be achievable without resort to digital processing in the domestic receiver. Studio monitoring using constant-luminance decoding could also be prohibitive in both cost and complexity. To overcome both of these problems, a modified form of standard decoder has been developed; it almost completely removes the residual colour errors due to compatible decoding while retaining the colour sharpness advantage of constant-luminance coding.