Calibrated for 1 Volt per octave control voltage with a base tone of C1 (33 Hz) at 0V.
The transistor current sink at the bottom of the ladder acts as an exponential voltage to current converter, converting a control voltage which is linear in semitone number/octave number into an exponential current signal. The centre frequency of the filter scales linearly with the bias current. The filter frequency, therefore, scales exponentially with the semitone/octave number in such a way as to track the proper note frequency.
