Anharmonic distortion involves the introduction of frequencies not musically related to the source signal, usually through some form of modulation process, such as aliasing in digital equipment which is overloaded or has poor anti-alias filtering, or scrape flutter in analogue tape machines. 

Some form of corruption — wanted or unwanted — of the source signal. All forms of distortion are formed by non-linearities in an electronic circuit or an audio transducer. Harmonic Distortion is typically caused by over-loading to some degree components in the signal pathi, resulting in the addition or stengthening of odd and/or even harmonics of fundamental frequencies present in the source signal. Anharmonic distortion involves the introduction of frequencies not musically related to the source signal, usually through some form of modulation process, such as aliasing in overloaded digital equipment, or scrape flutter in analogue tape machines. Some forms of distortion are often added intentionally to a musical source to alter it's character in a pleasing way — such as on an electric guitar, for example.