the gain and certainly could increase impairments. Additionally coaxial networks, particularly Most existing cable TV systems do not apparently incorporate joints, are not perfectly immune from the outside world. Too low a switching. In fact the effective switch is the TV set tuner, selecting level signal could be subject to external interference; too high a a channel by filtering it out from the incoming multiplex, demodu- level could cause radiation beyond national regulations. lating it and routeing it to the set's output stages. A similar function It is useful to define a zero reference level in dB terms, rather than will also be done within a set top unit, ending up with frequency voltage or wattage terms, since cable loss and amplifier gain are in translation of the selected channel to a convenient position in the this form. Thus 0dBmV is defined as the level corresponding to spectrum (there may be intermediate demodulation to carry out 1mV across 75 ohms (the characteristic impedance of all coaxial descrambling), cable used in cable TV). Video switching within the network is of course a central feature of switched star systems. Two basic techniques are used, space switching and frequency agile switching. 57.3.4.3 Optical 57.3.5.1 Space switching Except for supertrunk links, optical transmission is in its infancy for cable TV application, but is rapidly becoming a major issue. In This is essentially the familiar crosspoint matrix (Figure 57.22) terms of network design the task is relatively straightforward be- where an input is physically linked to an output line when a cross- cause the long range of an optical link often means an external point is made. For a cable TV application a broadcast switch is repeater can be avoided. The essential features in selecting an needed, i.e. one where one input might be connected to several optical system are: outputs. Thus the loading of an input signal is variable from 1 to n outputs; hence there are buffer amplifiers needed as shown in Figure 1. Channel capacity per f