The given rotary encoder gives 2x18 (when computing the rising and falling edge of each encoder turn) steps to set the volume attenuation between 63dB and 0dB.

The avr firmware provides sufficient flexibility to support different characteristics. What I would like to evaluate first is a S-Curve characteristic: Attenuation decreases with a small slope first then slope increases in the middle part and decreases again at the end.

In more detail it is a Sigmoid Curve which is shifted to the right and normalized: