ng end of the transmission link. This form of partial response often employed. operation is called duobinary coding (Lucky et al., 1968; Taub and A common form of time domain equaliser is the transversal Schilling, 1986; Dorward, 1991). equaliser (TVE) shown in Figure 19.1. This consists of a delay line tapped at intervals equal to the intersymbol interval T. Each tap is 19.2.2 Effects of noise connected to an amplifier (which may be an inverter to obtain negative gain). The output of the equaliser is the sum of the outputs of these amplifiers. It is possible to adjust the gains of the amplifiers The principal advantage of PCM and other forms of digital trans- (in magnitude and sign) to cancel ISI by adding appropriately mission is that it is possible to obtain satisfactory transmission in the weighted versions of preceding and following pulses at the time of presence of very severe crosstalk and noise. In the case of binary each symbol, and thus cancel interference between them. transmission, it is only necessary to detect the presence or absence A TVE can be adjusted manually. However, if the characteristics of each pulse. Provided that the interference level is not so high as of the transmission path change with time, it is preferable for the to cause frequent errors in making this decision, the output signal will be almost noise free. equaliser to be adjusted automatically. A TVE which does this for itself during normal operation is called an adaptive equaliser (Lucky Consider an idealised train of unipolar binary pulses, as shown in et al., 1968; Clark, 1985). Adaptive equalisers are used in data Figure 19.2(a). If the symbols '0' and '1' are equiprobable, i.e. as in modems which may be used on a variety of different connections in Equation 19.11 the mean signal power corresponds to Equation a switched telecommunication network. They are also used for 19.12. Thus, the signal to noise ratio is given by Equation 19.13, digital radio links, which must cater for varying propagation condi- where o is the r.m.s. n