A form of distortion that results in a digital synthesizer, digital signal processor, or soft synth when the signal being processed exceeds the Nyquist frequency of the system.The result is the introduction of tones whose frequency is the difference between the original frequency and the Nyquist frequency. It is considered a particularly unpleasant form of distortion; it can account for the "hard" or "edgy" sound often attributed to digital synthesis or processing systems. One of the difficulties in writing code for a soft synth is ensuring that aliasing cannot occur in any part of the signal processing chain under any combination of patch parameters.
Aliasing is sometimes produced intentionally as an effect, by means of downsampling a signal without applying anti-aliasing first to reduce the signal bandwidth below the Nyquist frequency. The effect is somewhat similar to that produced by amplitude modulation.