mbiners for any reverse traffic up the 57.3.7.1 More channels network). Whether fibre will ever sensibly be taken through to the home is a matter of fierce debate. Many see coaxial cable surviving Proposals for pushing the coaxial network technology to beyond for the final feed, both because optical equipment costs will not be 1GHz means 100+ channels becoming possible. Some of these may low enough on a per customer basis and to avoid powering and be available then for narrowcasting to one or a few individuals. housing it in the home. Another alternative to remember is the possibility of radio as that final feed, probably at high microwave frequencies. 57.3.7.2 Enhanced TV formats A number of higher quality formats for the video signal have been Digital video coding proposed round the world, and cable TV is having to start to think about catering for them. One basic point is that a channel may have 57.4.1 to be transmitted in both the present standard (NTSC, PAL, SECAM) and the enhanced form. Thus it is another driver towards Pulse Code Modulation (PCM) encoding of a video signal com- the need for more channel capacity. prises the steps of sampling, quantisation and allocation of code True high definition TV (HDTV) with a base bandwidth of circa words. Sampling converts the continuous analogue representation 30MHz would present a real problem, except for advanced fibre into a series of picture elements though this term and its short forms, systems. More modest enhancements, and hence bandwidth re- pixel and pel, are also used to refer to the digital versions. The quirements, arise from systems like the Japanese MUSE, the Ameri- Nyquist criterion of the sampling rate being at least twice that of the 57/24 Video transmission highest frequency present leads to minimum rates in the region of a notable example being medical scans, have a wide dynamic range 10MHz to 12MHz for conventional 525 and 625 line television and require significantly more bits. signals. Though these values are satisfactory for monochrome or CCI