= Signal distortion
<Signal distortion> is an unwanted change in a signal's shape or information-bearing relationships. A nonlinear response $y=ax+bx^2$ driven by a sinusoid produces DC and a second harmonic, introducing <frequencies> absent from the input. A linear system can also distort a waveform when its gain or phase response varies across the signal bandwidth, changing the relation between existing components. Distortion differs from additive noise, although both can degrade a measurement. The relevant comparison is to the intended signal transformation; an intentional filter is not faulty merely because it changes the waveform.
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