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Various schemes for specifying the degree of distortion in terms
that can be measured have been proposed, but the nearer they
approach to a fair judgment the more complicated they are; so the
admittedly unsatisfactory basis of percentage harmonic content is
still commonly used. Preferably the percentage of each harmonic is
specified, but more often they are all lumped together as 'total
harmonic distortion'.
19.8 Phase Distortion
The main result of passing a signal through any sort of filter, even if
it is only a resistance and a reactance in series, is to alter the
amplitudes of different frequencies to different extents (Fig. 18.9).