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When, in movies, a car accelerates, something strange happens with its wheels. At a certain speed, they start to slow down or even go backwards. This is due to the alias effect. In digital audio, it is similar. You can experience the effect by sending a simple sine sweep through a (alias-producing) harmonic-distortion plugin. Some strange, chaotic tones will occur (listen to the sample files below).

This is, because the generated tone is higher than the maximum frequency of the digital audio signal, the so-called Nyquist frequency, which is half the samplerate. The Illustration shows, how a high-frequent signal is detected as a lower-frequent alias. Aliasing is an exclusively digital problem. As analog devices do not have a Nyquist frequency, they do not generate aliasing. It is an unwanted sideeffect, as it stands in no harmonic relation to the initial signal. Harmonic distortion will add overtones with higher frequencies, causing the aliasing. The only way to avoid aliasing is using a higher internal samplerate and filtering, and that is exactly what Reviver does.

Harmonically distorted sine sweep with alias.

This is a frequency-time plot of a simple sine sweep which was harmonically distorted using Reviver's serial mode with the alias filter turned off. You can see how the aliases are bouncing between zero and Nyquist.

Harmonically distorted sine sweep without alias.

This is the same sine sweep sent through Reviver, all the settings are the same, but now the alias filter is turned on. We see that there are no aliases in the audible area (below 20kHz).