The whole project is divided into three parts :
The power supply which requires 18-24V input and produces all the necessary voltages : 5V , 3.3V and 150V .
Controller - a board containing the preamplifier and the STM32F407 chip , which is responsible for sampling the input , calculating FFT , calculating the logarithm of the signal amplitude , producing 24 PWM signals and the blanking signal . The controller has two modes : stereo mode in which signals from each channels are transformed and calculated independently . In stereo mode each IN9 board displays one channel. In the mono mode signals are added an the FFT is calculated for the sum of the channels . Then the result is displayed using both boards as one , 24 channel display. The controller board contains a microphone preamplifier , so the signal can be either connected to the device from the "line-out" output or picked with the built in microphone
Two IN9 boards. Each board contains 12 constant-current drivers for the neon tubes. The outputs of the drivers can be switched off synchronously with a signal from the controller . I found out , that it is necessary to power the IN9 tubes with puled current if one wants to avoid "floating discharge" in the tubes.
The analyzer in the mono mode :
The analyzer in the stereo mode :