Distortion is used a term used to describe any changes made to a waveform. These changes can be made in many ways, but most commonly we associate distortion with a device that has been overdriven with amplitude. Every device has a threshold for how much amplitude it can handle. If a device, such as a compressor, is pushed with more amplitude than it can handle (clipping) then it will begin to change the shape of the waveform. This will add new harmonics to the waveform, which changes the timbre of the waveform. Of course, every device will produce different types of distortion. It is also worthy of mention that most devices will manipulate a waveform without out clipping the amplitude.
Below are several examples of a signal being distorted with several different compressor plugins and the types of distortion that they produce when overdriven. I have used a signal generator to run a 200Hz sine wave through the compressors and captured the waveform with a frequency analyser at both the mellowest setting and most overdriven. Figure 1 shows the waveform of the pure sine tone at 200Hz.
Figures 2 and 3 show this compressor at its mellowest setting. Unusually this plugin doesn't affect the waveform prior to compression.
Figures 4 and 5 show the BF-76 at full compression. Several harmonics have been added to the waveform initially at every octave and then doubled as high frequencies are added.
Figures 6 and 7 depict the FG-Grey compressor at its mellowest compression. It can be observed that this compressor has already begun to add harmonics.
Figures 8 and 9 show the FG-Grey at full compression. The harmonics that have been added are similar to that of the BF-76, however the amplitude of the harmonics is much more consistent in the FR-Grey.
Virtual Tape Machine
Figures 10 and 11 show the Virtual Tape Machine at its mellowest compression. It can be observed that there is a significant change in the waveform already. Many harmonics have been added yet only at small amplitudes.
Figures 12 and 13 show the Virtual Tape Machine at full compression. The frequency analyser shows that there has not been much of a change in comparison to the other compressors. There has only been a slight amplitude change in the first couple of added harmonics.
Pro Tools 12 [Image] (2015). Avid. Retrieved August 15th.