5.5 Colour difference signals

An effective colour television system can be made in which only three pure or single wavelength colours or primaries can be generated. The primaries need to be similar in wavelength to the peaks of the three receptor responses, but need not be identical. Figure 5.12 shows a rudimentary colour television system. Note that the colour camera is in fact three cameras in one, where each is fitted with a different coloured filter. Three signals, R, G and B must be transmitted to the display which produces three images that must be superimposed to obtain a colour picture.

Figure 5.12: (a) A simple colour system uses three primaries and transmits a complete picture for each. This is incompatible with monochrome and uses too much bandwidth. Practical systems use colour difference and luminance signals which are obtained by a weighted calculation as shown in (b).

A monochrome camera produces a single luminance signal Y whereas a colour camera produces three signals, or components, R, G and B which are essentially monochrome video signals representing an image in each primary colour. RGB and Y signals are incompatible, yet when colour television was introduced it was a practical necessity that it should be possible to display colour signals on a monochrome display and vice versa. Creating or transcoding a luminance signal from R, G and B is relatively easy. Figure 5.11 showed the spectral...