Step 6: Design: Display
A voltage divider cuts the signal down by 1/2 so it's range is between 0V and 6V. A low-pass RC filter smooths out the very sharp changes in the volume signal (when the level of the nixie tube is changed too quickly, the lit segment jumps out to the middle of the tube, rather than lighting from one end).
The nixie tube has 3 pins, an anode, a control cathode, and an auxiliary cathode. The anode is tied to the output of a high voltage power supply at around 125V. The auxiliary cathode is tied to ground through a 220k ohm resistor. As far as I can tell, the auxiliary cathode acts as a sort of pilot light, forcing the tube to begin to glow at one end first, rather than from the middle or the other end. The control cathode is the main tube, and the current through it determines the length of the tube that is lit. A high voltage NPN transistor modulates the current from the control cathode through a 470 ohm resistor and potentiometer (for tuning purposes). The base-emitter junction of the transistor has a forward bias voltage of about 0.65V that it needs to turn on, so an op-amp is used to adjust the signal for this.