teensy-harmonic-distortion

A project for digitally adding harmonic distortion (tube sound?) to audio, built in Arduino for the Teensy 4.0 with the Audio Adapater Board. Full writeup here. Upload as-is to get a ratio of 0.05 between the fundemental and the second harmonic, basically <=5% THD. To adjust it, configure these two lines of code:

#define MAX_ORDER 2
float alpha[MAX_ORDER-1] = {0.05}; // First entry is second-order

like this (example: second harmonic ratio of 0.05 and a third harmonic ratio of 0.005)

#define MAX_ORDER 3
float alpha[MAX_ORDER-1] = {0.05, 0.005}; // First entry is second-order

The code currently inputs 2V (measured) peak-to-peak line signals and a little less than 1.62V peak-to-peak line signals. Configure the in and out levels however you need, but be warned that outputting any higher appears to cause soft clipping (additional third harmonics).