conversion, etc., and the Nicam tibility of the error rate to all forms of 'echoes' and multipath 728 dual channel digital system for television sound. propagation. Initially, the introduction of digital processing was applied to Digital systems are inevitably subject to quantising noise, which encoded 'composite' waveforms with input and output from each depends upon the number and arrangement of the levels at which unit in analogue form. However, it was soon appreciated that digital the original analogue signal is digitised, and also to aliasing fol- processing offered an opportunity to improve picture quality by dover distortion. Aliasing represents spectral components, arising eliminating the degradations inherent in the PAL, NTSC and from the process of sampling, not in the original signal; when these SECAM colour encoding systems due to 'mixed highs' (frequency fall within the spectrum of the sampled signal they result in foldover multiplex) insertion of chrominance information within the lumin- distortion. Aliasing can be minimised by effective filtering, al- ance baseband. Attention was therefore turned to use of 'compo- though the ease and cost with which such filtering can be accom- nent' signals with luminance and chrominance signals kept plished is very much a factor of the sampling rate. separate, although this involves an extension of the baseband of the The increasing use of digital equipment based on the CCIR analogue signals and consequently higher digital bit rates. Recommendation 601 (1982) as given in Table 57.4 created the need Advantages of handling signals in digital form include the more to interconnect equipments with outputs in either digital or analogue rugged nature of a digital signal, which requires a much lower signal form. With the introduction of digital equipment it is easier to build to noise ratio, and its greater immunity to many forms of phase and a completely new centre rather than modify an existing complex. amplitude distortion and interference. The ability to make use of 