Every oscillator contains a distortion section which provides both harmonic and random distortion, as well as overdrive, phase doubling and phase drift.
Fathom does not insert distortion as an effect late in the signal flow. Instead, distortion is applied directly to the oscillator to mimic the behavior of real analog electronics.
Distortion is created with a random signal applied to the oscillator frequency.
The algorithm implements harmonic distortion by spacing the random points using intervals related to the voice frequency which creates the same effect as an electric guitar being over-driven through an amplifier.
Points in the distortion pattern are connected with a bicubic spline function to create a smooth continuous signal which reduces aliasing.
All distortion parameters can be modulated.