The Clean PAL concept and its history

The PAL system of colour coding, whereby the luminance, Y, and chrominance components, U and V, are coded into a single signal, has served us well for many years. Unfortunately, however, it does suffer from impairments, caused by the fact that the Y and U,V signals occupy the same spectral space. These manifest themselves as cross-colour, which is luminance wrongly interpreted as chrominance, and cross-luminance, which is chrominance wrongly interpreted as luminance. The former appears as rainbow patterns in areas of high-level periodic luminance detail whilst the latter appears as dot patterns on coloured edges. Various attempts to overcome these deficiencies have been previously proposed, usually involving modification of the standard PAL decoder and often of the coder as well. A coder which enables luminance and chrominance to be successfully separated became known as a Clean PAL coder, whereas a decoder that does the same became known as a Clean PAL decoder. A particular clean coder will be designed to work with a corresponding clean decoder, the pair forming a Clean PAL system. It follows, therefore, that clean decoding of standard PAL is harder than clean decoding of cleanly coded PAL