18.8.1 Transmission Performance of Media

One of the consequences of removing redundancy from television images is that, when transmitted by digital means, the images become more susceptible to channel errors. Some media are much cleaner than others, in that they generate fewer errors and the rates are consistently low, although some have high rates and are not consistent. One example of a clean medium is optical fibre as used for telecommunications links where the guided nature of the medium virtually guarantees consistent error rates as low as perhaps one error in 10 ¹⁰ per link; the statistical distribution of these errors is usually far from ideal and gaussian, as assumed by most textbook approaches.

An example of a medium that has an error rate which is inconsistent over time is that which would be used for terrestrial digital television in the UHF spectrum. The presence of noise, multipath and other disturbances made worse by the poor antenna performance possible in the environment, where portability is important to the user, make modulation design for this medium very difficult. The failure characteristic of complex digital modulation will be very different from the graceful failure of existing analogue systems, hence the search for a scheme which is scalable to provide a hierarchy to soften the failure mode if required.

18.8.2 Harmonisation of Standards

Although it is possible to design in isolation for digital television transmission over different media, there is a need to consider the harmonisation of standards so...