3.1 Introduction

The generic term for the path down which information is sent is the channel . In a transmission application, the channel may be a point-to-point cable, a network stage or a radio link.

In digital circuitry there is a great deal of noise immunity because the signal has only two states, which are widely separated compared with the amplitude of noise. In transmission this is not always the case. In real channels, the signal may originate with discrete states which change at discrete times, but the channel will treat it as an analog waveform and so it will not be received in the same form. Various frequency-dependent loss mechanisms will reduce the amplitude of the signal. Noise will be picked up as a result of stray electric fields or magnetic induction and the receiving circuitry will contribute some noise. As a result, the received voltage will have an infinitely varying state along with a degree of uncertainty due to the noise. Different frequencies can propagate at different speeds in the channel; this is the phenomenon of group delay. An alternative way of considering group delay is that there will be frequency-dependent phase shifts in the signal and these will result in uncertainty in the timing of pulses .

In digital circuitry, the signals are generally accompanied by a separate clock signal that reclocks the data to remove jitter as was shown in Chapter 1. In contrast, it is generally not feasible to provide a separate clock in transmission applications. A separate clock line would not only raise cost, but would be impractical because at high frequency it is virtually impossible to ensure that the clock cable propagates signals at the same speed as the data cable except over short distances. Such timing differences between parallel channels are known as skew.

The solution is to use a self-clocking waveform and the generation of this is a further essential function of the coding process. Clearly, if data bits are simply clocked serially from a shift register in so-called direct transmission this characteristic will not be obtained. If all the data bits are the same, for example all zeros, a common code in audio and video, there is no clock when they are serialized.

It is not the channel which is digital; instead the term describes the way in which the received signals are interpreted . When the receiver makes discrete decisions from the input waveform it attempts to reject the uncertainties in voltage and time. The technique of channel coding is one where transmitted waveforms are restricted to those that still allow the receiver to make discrete decisions despite the degradations caused by the analog nature of the channel.