Nixie Tube Music Visualizer

Step 5 Design: Logarithmic to Linear

This is the stage that would be way easier if I could afford the $80 in THAT Corporation's fancy true-RMS and log-calculating chips. The problem is that what we perceive as volume doesn't directly translate to the voltage of an audio signal. When you double the amplitude of a signal, it doesn't sound twice as loud, it sounds Log(2) times louder. Skipping this stage would make the display spend nearly all of the time at the very bottom, and spiking high when the sound gets particularly loud.
One way of solving this is by generating several reference voltages, one at the quietest level that the display should detect, one at +3dB, one at +6dB, and so on. The volume signal can then be compared to each of these references. For every reference voltage the volume signal is greater than, the display lights an additional unit of length.
An array of seven resistors and two potentiometers is arranged as a voltage divider to generate the necessary eight reference voltages. For each band, eight voltage comparators (in the form of two quad-comparator ICs, I used STMicroelectronics TS3704) compare the signal to these references to determine a linear volume level. The eight outputs of the comparators are then averaged with 10K ohm resistors, giving a range of values from 0V to 12V with each step of 1.5V equaling about 2.1dB.

The values of the resistors in the divider from ground to V+ are:
2k potentiometer, 470, 750, 1.1k, 2k, 2.7k, 4.7k, 7.5k, 25k potentiometer