An undesired change in waveform, as opposed to desired changes in waveshape like those from modulation. (a) Amplitude distortion is caused by undesired amplitude-versus-frequency characteristics. (b) Harmonic distortion is a nonlinear distortion characterized by the generation of harmonics of an input frequency. The percent harmonic distortion is a measure of fidelity; if Ef is the rms voltage of the fundamental and En is the rms voltage of the nth harmonic, the percent harmonic distortion is
(c) Intermodular distortion is a nonlinear distortion characterized by the appearance in the output of frequencies equal to the sum and difference of integral multiples of the component frequencies present in the input. (d) Nonlinear distortion is caused by a deviation from a linear relationship between input and output. (e) Phase distortion results when phase shift is not linear with frequency over the bandpass or where it is linear but where the zero frequency intercept is not a multiple of π. Where the intercept is a multiple of nπ, the waveshape is not changed in n is even and is inverted if n is odd, although the entire waveform may be delayed. See linear-phase filter.