ortion. (10.24) H2 + H4 10.7.1.5 Intermodulation distortion Intermodulation distortion is a form of non-linear distortion whereby the amplitude of a signal of one frequency is affected by the amplitude of a simultaneously applied signal of a lower fre- FREQUENCY quency. Combination frequencies are produced which have values equal to the sum and difference of the two applied frequencies. The non-linear transfer characteristics reduce the amplitude of the Figure 10.12 Attenuation distortion voice channel higher frequency signals at times when the lower frequency signal is near to maximum and minimum voltages (Figure 10.13.) 10.7.2 Distortion of digital signals 10.7.1.2 Delay distortion 10.7.2.1 Attenuation distortion The velocity with which a signal travels through a medium is a The attenuation of signals in the voice frequency range increases function of the velocity of propagation, which varies between me- with an increase in frequency and SO the various harmonics con- diums. The velocity tends to vary with frequency, with an increase tained in a digital waveform will be attenuated to a greater extent towards the band centre and a decrease towards band edge. The than the fundamental frequency. The greater attenuation on the finite time the signal takes to pass through the voice channel of the higher order harmonics means the rectangular waveshape will grad- Input signal Figure 10.13 Illustration of intermodulation distortion 10/10 Signals and noise ually be lost. The effect is accentuated by the length of the line and SO pulses will be more rounded. 10.7.2.2 Synchronisation distortion RLOAD Synchronisation is the principal whereby the receiver will at all times sample the incoming bit at the correct instance in time. Noise generator Distortion can occur if the sampling drifts from the correct sample period. The two methods of synchronisation, isochronous and anio- chronous, the latter often being referred to as start-stop, can both suffer from distortion. Figure 10.14 Equivalent circuit for resistor noise Types o