Distortion refers to any alteration or deformation suffered by an audio signal's waveform relative to its original source. Although technically a corruption of the signal caused by non-linearities in electronic circuits or transducers, its use can be either an unwanted defect or a purposeful aesthetic effect. There are two main categories. Harmonic distortion usually occurs when equipment is overloaded, generating new frequencies that are musical multiples of the fundamental note, which is the basis of guitar amplifier sound and analog saturation. Anharmonic distortion, on the other hand, introduces frequencies that have no musical relationship with the original sound, resulting in dissonant and unpleasant noises, as occurs in digital aliasing or intermodulation defects.