2.4.1 Introduction

The signals produced by colour television cameras are in the form of red, green and blue (RGB) colour signals. Cathode ray tube colour displays also operate with RGB. Although RGB signals can be of high quality, the three channels required represent an inefficient use of bandwidth and circuitry. Furthermore, impairments can arise if the three channels are not accurately matched. More efficient and rugged methods of colour encoding are therefore required at other points in the signal chain, such as for studio processing, recording, distribution and broadcast emission.

Several additional methods of colour encoding have been developed. Some produce composite signals, such as PAL, NTSC and SECAM, which retain compatibility with monochrome receivers. Such systems have been in use for many years. Now, however, component coding methods such as 4:2:2 digital components have been introduced. These methods sacrifice direct compatibility for advantages such as improved signal processing and picture quality.

Compatibility between systems is increasingly a problem in television broadcasting, especially for the new digital media: satellites, cable and new terrestrial services. This is particularly so in the case of the coding systems now in use. Here, all the main systems are presented in common terms, making the similarities and differences more apparent.

2.4.2 Colour Signal Relationships

All the colour systems described here are based on encoding of a luminance signal and two colour-difference signals, instead of the red,...