This amazing piece of technology is a 4-in-1 visualizer for audio.  It uses two displays to show you exactly what you want to see. One Screen will always display a Stereo VU meter, analog style. For the other display, you can choose between four options. You can display another VU meter so that both screen show the same information or you can have it show a stereo spectrum analyzer of maximum 69 channels in Stereo. You also have to option to show a digital VU meter of a Correlation meter. If we take the VU meter on one screen ( L and R channel) and the Spectrum analyzer on the other( 69 Left + 69 Right frequency bins) we are actually looking at a 140 channel analyzer. Amazing! It used a Teensy 4.1 with added external PSRAM and a Teensy Audio board to do the actual FFT analysis in real time. Because of this combination of hardware, it is very fast and it is easy for the display to keep up with the audio. 
First of, credit where credit’s due. This design was inspired by and based on a previous design by DIYlab.de. DIYlab did an amazing job designing the graphic layout, the original firmware and the application to configure this device over the USB Serial port. Check out all information about the original firmware here:
https://github.com/donnersm/AudioVisualizer/tree/main
Further more, A big thank you to Arvind Singh who designed a great library to drive ILE9341 displays with a Teensy. https://github.com/vindar/ILI9341_T4
The information there is important as the firmware of the hardware presented by me is bases on the version as described by DIYlab and all basic functions are unchanged. The original software with all functions is intact and has not been changed. The main difference is that my version adds another display showing nothing else then a stereo VU meter. Therefore, for all functionality of the firmware and configuration application, I refer to the Github of DIYlab. ( link is above)
I created a PCB to mount the Teensy, the Audio Board, the Displays and some connectors. It also has an onboard power supply that you can use. However, you can choose to feed all through the USB interface on the Teensy, which is all up to you.  I also added inputs for four Potentiometers and two Pixel ledmatrixes or ledstrips although it hasn’t been implemented in the current firmware. The idea is to drive one matrix in rainbow mode to illuminate any display while the other matrix can be used to display the spectrum analyzer as well. In that case, the potentiometers can be uses to adjust, brightness, sensitivity, peak delay. 
In regards to functionality, in the current version, an extra input switch was added. It can be used to toggle between Line-in (stereo) and Microphone-in (mono).




What you need:
PCB + components
Teensy 4.1
External PSRAM
Teensy Audio Board
2 Displays