A proved the expected problem. It was clear that the dedicated link cable run of 500m therefore needs an amplifier of 25dB gain at this would have to be kept short to reduce per customer cost; how short frequency to restore the signal. A splitter in the run would add depended on technical solution, for u.h.f. delivery 200m and for further loss, around 3.5dB to 4dB for each two way stage in an even v.h.f. delivery 500m maximum to avoid amplification on a coaxial split; an asymmetric split can be used to tap off a line at a high loss link; for fibre 1km or more. In nearly all cases the switch was leaving the through loss relatively low. therefore street sited. As the network spreads out cost and convenience means that cable dimensions get smaller and losses higher. Thus at 450MHz a super trunk cable of diameter 39mm might have a loss of 25dB/km, 57.3.4 Transmission whereas the final short underground feed might be only 8mm diameter and 170dB/km loss (overhead feeds being smaller and 57.3.4.1 Modulation techniques even higher loss). Amplifiers are imperfect devices and introduce noise and non-li- For video there are the following main techniques employed: nearity. Thus each time one is used the noise performance will deteriorate and cross modulation between the channels will occur; Amplitude modulation, the standard method matching the TV hence the 'run' of amplifiers is limited. The manufacturer's specifi- receiver input format. To reduce bandwidth (to between 6MHz cation will indicate a noise figure and cross modulation perfor- to 8MHz) vestigial sideband (VSB) is normally used on cable mance, which enables the effects to be calculated. Given that the networks, as done over the air. Although simplest and cheapest, equalisation process is also imperfect, and there are other impair- it is not a robust technique, being prone to noise as it builds up ments, the run may be less than this basic calculation indicates. through amplifier stages and to interfering signals (external or There are different designs of amplifie