Modified 8bit FFT in c

Forum 2005-2010 (read only) Software Development
2

this is the way I used it:
have fun :slight_smile:

sketch:

char im[128];
char data[128]; 

void loop(){
  int static i = 0;
  static long tt;
  int val;
   
   if (millis() > tt){
      if (i < 128){
        val = analogRead(pin_adc);
        data[i] = val / 4 - 128;
        im[i] = 0;
        i++;   
        
      }
      else{
        //this could be done with the fix_fftr function without the im array.
        fix_fft(data,im,7,0);
        // I am only interessted in the absolute value of the transformation
        for (i=0; i< 64;i++){
           data[i] = sqrt(data[i] * data[i] + im[i] * im[i]); 
        }
        
        //do something with the data values 1..64 and ignore im
        show_big_bars(data,0);
      }
    
    tt = millis();
   }
}

this should give an fft with

  • sampling rate: 1ms
  • frequency resolution: 500Hz
  • lowest frequency: 7.8Hz

there is a lot that can be improved:

  • fix bugs
  • replace FIX_MPY by inline assembler
  • do sampling with timer and interrupts
  • use fix_fftr instead of fix_fft to save ram (i did not get the math right)