Oli Thompson

• We built an Audio Spectrum visualiser and guitar tuner and displayed them through a custom LED matrix on the face of a guitar. I designed, assembled and programmed a custom PCB and used time and frequency domain signal processing.

How it Works

The guitar has two main functions; The spectrum visualiser works by applying a Fast Hartley Transform algorithm to a microphone input and splitting the detected frequencies logarithmically into 8 bins. These are then scaled and displayed on the LED matrix through a look up table.

maparray[5] = map(fht_oct_out[5],35,120,0,15);
  for (int m=0; m <=6; m++){ //iterate through frequency bins
    for(int j=(26-(4*m)); j >= (26-(4*(m+1))); j--){ //iterate across matrix
       for(int i=0; i <= maparray[m]; i++){
        if ((lookuptable[i][j] != 0) && (i < 16) && (j >=0)) { //find corresponding value in looKup table
          strip.setPixelColor(lookuptable[i][j]-1, r,g,b); //set colour
        }
        for(byte k = 15; k > maparray[m]; --k){ //turn all the other LEDs off
          if ((lookuptable[k][j] != 0) && (j >=0)){
          strip.setPixelColor(lookuptable[k][j]-1, 0,0,0);

The guitar tuner takes the analog output from the guitar's pickups and amplifies the signal to 5V. A DC offset of 2.5V is applied to keep the signal positive. This signal can then be read by the 10-bit ADC of the Atmega MCU and an autocorrelation method is used to detect precise frequencies and compare them to the known correct values.

Circuit Schematic