Vintage Audio Spectrum Display
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History
After doing some project with vintage tube I got the idea of doing a audio spectrum display with "magic eyes tube". and drive those with a AVR MCU with Fast Fourier Transform
   I had search the net to find some FFT code.  I found  Mr Chan small FFT library,  I download it and at first try it was working, his code is very simple to use.  Thanks Mr Chan.  The way it work,  I pass the audio (line out of your amp) trough a audio compressor, this way if a song is recorded at a low level or at a high level the spectrum will always display the good information.  I had use a ATMega644, because the FFT need a lot of RAM. And I overclock the AVR to 32Mhz with a external full swing oscillator.  You need speed to make all the computation and give the result almost in real time on your tube. The result of the FFT is send to 8 D/A (4 two chanel digital pot) and the analog value is pass trough 2 opamp, one to setup the offset voltage for the tube and the other is there to invert the signal and amplify it.  To get more speed I had the idea to start my 256 A/D convertion when the FFT is computing,  The A/D use almost only hardware to do the job, and in this time the MCU is computing FFT.  At the end of the FFT those value are output and the next data to compute is already there.  If you check on a scoop with a I/O toggeling you will see that the MCU is almost always working the the FFT loop. Like in my nixie clock I had use the same circuit to generate the 250v needed by the magic eyes.
 
Features
8 tube to display audio frequancy 
Audio compressor 
Unique look 
Pictures




Download
Analog Schematic
Digital Schematic
Source Code
 
http://www.microsyl.com/index.php/2010/11/15/vintage-audio-spectrum-display/#more-555