ly improved noise and interference immunity at the ex- channels, and provide a set top unit to descramble those that the pense of bandwidth (30MHz per channel typically). It has been customer has paid extra to watch. A simple unit would have fixed used quite extensively on longer super trunk links over both capability, but it then needs changing out if the customer needs a coaxial cable and fibre. different service. Addressable units change their capability on com- Digital modulation. This similarly gives improved noise and mands from codes sent in a data stream over the network from the interference immunity, but with the additional advantage of no head end. The disadvantages of scrambling are that it involves a degradation with repeatering. It is more suited to fibre than relatively expensive unit in the home, that it affects video quality, coaxial cable (the latter having considerable equalisation prob- and that it is liable to programme theft from pirate versions of the lems). It has had limited use to date because of cost, but there set top unit. are signs now of it making a bigger impact. There is a strong desire nowadays that at least the basic pro- gramme tier should be deliverable without the need for a unit in the For audio service the standard FM radio band (88MHz to home. Hence it should be placed in the spectrum SO as to be able to 108MHz) has been relayed over the cable, perhaps repositioning the feed directly into the TV set. carriers and adding channels in on new carriers. However the FM The dominant feature of the network design is the amplification improvement is very low for a stereo broadcast. This then presents need. Loss for a coaxial cable is proportional to the square root of almost as severe a problem for fibre as mentioned above for AM frequency, SO it is an increasing problem with modern systems. transmission of video. Digital audio transmission is now receiving Amplifiers are required for two reasons however, to allow for interest because of its impact in the domestic equipment area; i