distortion & coloration

Anything that changes the original sound (like adding 'color' to it) is a form of distortion. However, we do not refer to it as distortion because when a device only colors (adds small changes to) the original sound the distortion is very low. 

Simple harmonic distortion occurs when an audio processing device adds low level signals of  harmonically related frequencies to the original (input) signal.  If for instance the frequency of the input signal is 1kHz and the device adds a small amount (say 1% of the output) of a 2kHz signal onto the original (signal), which is twice the original frequency,  we can then say that the device produces 1% of second (even) harmonic distortion. 

If the device adds a 3kHz signal (x3 the input signal frequency) to the original then the output signal will contain third (odd) harmonic distortion products, and so on.   

So when for instance a speaker is connected to a hi-fi monitor the  sound coming out of it may be clean and transparent but this amp/speaker  system still 'colors' (distorts very,  very slightly) the signal. READ MORE

distortion unit