DIY Spectrum Analyser

17 November, 2016

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Alan X has been working on a spectrum analyser project that can show the spectrum visually! He started working with ATTiny85 and kept on updating the project over time.

Alan X used Goertzel’s algorithm with a Hamming window, this algorithm can be used to detect a frequency from sampled data. Here is the preliminary code for a spectrum analyzer:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
// Uses Daniil Guitelson's BGI library
#include "graphics.h" // -lBGI -lgdi32


#define SampleFreq 125000


int main(void)
{
  int N=250;
  double data[N];
  double samples[N];
  double freq;
  double s;
  double s_prev;
  double s_prev2;
  double coeff;
  double magn;
  int i;


  int gd=CUSTOM, gm=CUSTOM_MODE(700,700);
  initgraph(&gd, &gm, "");
  setcolor(WHITE);


  int X1,Y1,X2,Y2;
  double scale,xmin,ymin,xmax,ymax;


  // Find the maximum and minimum data range
  xmin=0;
  ymin=0;
  xmax=50000;
  ymax=N;
  scale=1.1*(xmax-xmin>ymax-ymin?xmax-xmin:ymax-ymin);


  // Generate samples
  for (i=0;i<N;i++) {
    samples[i]=(50*sin(2*M_PI*i*3300/SampleFreq)+50*sin(2*M_PI*i*5700/SampleFreq)+50*sin(2*M_PI*i*25700/SampleFreq)+100);
    // Window the data
    // data[i]=samples[i]; // Straight Goertzel - not great
    // data[i]=samples[i]*(0.5-0.25*cos(2*M_PI*i/N)); // Hanning Window
    data[i]=samples[i]*(0.54-0.46*cos(2*M_PI*i/N)); // Hamming Window
    // data[i]=samples[i]*(0.426551-0.496561*cos(2*M_PI*i/N)+0.076848*cos(4*M_PI*i/N)); // Exact Blackman Window
  }


  // Scan frequencies
  for (freq=100;freq<=50000;freq+=100) {
    coeff=2*cos(2*M_PI*freq/SampleFreq);
    s_prev=0.0;
    s_prev2=0.0;
    for (i=0;i<N;i++) {
      // Goertzel
      s=data[i]+coeff*s_prev-s_prev2;
      s_prev2=s_prev;
      s_prev=s;


    }


    // Get magnitude
    magn=2*sqrt(s_prev2*s_prev2+s_prev*s_prev-coeff*s_prev*s_prev2)/N;
    printf("Freq: %6f Mag: %6.4f\n",freq,magn);


    // Plot data
    X1=(int)((freq-(xmin+xmax)/2)*700/scale+350);
    Y1=(int)((0+(ymin+ymax)/2)*700/scale+650);
    X2=(int)((freq-(xmin+xmax)/2)*700/scale+350);
    Y2=(int)((-magn*700/2+(ymin+ymax)/2)*700/scale+650);
    line(X1,Y1,X2,Y2);


  }
  getchar();
  closegraph();


  return 0;
}

Daniil Guitelson’s BGI library was also used for the graphics.

Output

Here is the output showing the DC, 3300 Hz, 5700 Hz and 25700 Hz signals:

8398261477186172386

The next step is to port the code to a suitable Arduino board and to show the results physically. Thus, he used a MicroView OLED display and here it is listening to a 3v 1kHz square wave:

68801477279127568

#include <MicroView.h>

// Audio Spectrum Analyser
#define SampleInput A0   // Name the sample input pin
#define BandWidth  500   // BandWidth
#define MaxFreq   4000   // Max analysis frequency
#define Trigger     10   // Trigger to synchronise the sampler 2vpp at 1kHz = 32

// Define various ADC prescaler
const unsigned char PS_16=(1<<ADPS2);
const unsigned char PS_32=(1<<ADPS2)|(1<<ADPS0);
const unsigned char PS_64=(1<<ADPS2)|(1<<ADPS1);
const unsigned char PS_128=(1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0);

// Setup the serial port and pin 2
void setup() {
// Setup the ADC
pinMode(SampleInput,INPUT);
ADCSRA&=~PS_128; // Remove bits set by Arduino library
// Set prescaler
// ADCSRA|=PS_64; // 64 prescaler (250 kHz assuming a 16MHz clock)
// ADCSRA|=PS_32; // 32 prescaler (500 kHz assuming a 16MHz clock)
ADCSRA|=PS_16; // 16 prescaler (1 MHz assuming a 16MHz clock)

uView.begin();// Start MicroView
uView.clear(PAGE); // Clear page
uView.println("Spectrum  Analyser"); // Project
uView.println("0-20 kHz"); // Range
uView.println();
uView.println("agp.cooper@gmail.com"); // Author
uView.display(); // Display
uView.clear(PAGE); // Clear page
delay(2000);// Wait
}

void loop() {
static byte *samples; // Sample array pointer
static byte *window; // Window array pointer
static int N=0; // Number of samples for BandWidth
static long sampleFreq; // Sample frequency
long freq; // Frequency of interest
float s; // Goertzel variables
float s_prev;
float s_prev2;
float coeff;
float magn;
int i;

if (N==0) {
// Check sample frequency and set number of samples
samples=(byte *)malloc(100);
unsigned long ts=micros();
for (i=0;i<100;i++) samples[i]=(byte)(analogRead(SampleInput)>>2);
unsigned long tf=micros();
free(samples);
sampleFreq=100000000/(tf-ts);
N=2*sampleFreq/BandWidth+1;

uView.setCursor(0,0); // Set cursor to beginning
uView.print("SI: A"); // Sample input pin
uView.println(SampleInput-14);
uView.print("SF: "); // Sample frequency
uView.println(sampleFreq);
uView.print("MF: "); // Max frequency
uView.println(MaxFreq);
uView.print("BW: ");// andWidth
uView.println(MaxFreq);
uView.print("SN: ");// Number of samples
uView.println(N);
uView.display(); // Display
uView.clear(PAGE);// Clear page
delay(2000);

// Create arrays
samples=(byte *)malloc(N);
window=(byte *)malloc(N);

// Modified Hamming Window
for (i=0;i<N;i++) window[i]=(byte)((0.54-0.46*cos(2*M_PI*i/(N-1)))*255);

// Generate test samples
for (i=0;i<N;i++) {
samples[i]=(byte)(30*(sin(2*M_PI*i*5000/sampleFreq)+sin(2*M_PI*i*2500/sampleFreq)+sin(2*M_PI*i*7500/sampleFreq)+sin(2*M_PI*i*10000/sampleFreq))+127);
}
}

if (true) {
// Sychronise the start of sampling with data slicer
int a0,a1;
a0=1023;
for (i=0;i<N;i+=2) {
a1=analogRead(SampleInput);
a0=(a0*13+a1*3)/16;
if (a1>a0+3) break;
}
for (i=0;i<N;i++) samples[i]=(byte)(analogRead(SampleInput)>>2);
}

// Scan frequencies
for (freq=0;freq<=MaxFreq;freq+=(MaxFreq/40)) {
// Goertzel (https://en.wikipedia.org/wiki/Goertzel_algorithm)
coeff=2*cos(2*M_PI*freq/sampleFreq);
s_prev=0;
s_prev2=0;
for (i=0;i<N;i++) {
s=0.0000768935*window[i]*samples[i]+s_prev*coeff-s_prev2;
s_prev2=s_prev;
s_prev=s;
}
// Get magnitude
magn=2*sqrt(s_prev2*s_prev2+s_prev*s_prev-coeff*s_prev*s_prev2)/N;
// Display on MicroView
uView.line(freq*40/MaxFreq,47,freq*40/MaxFreq,10-(int)(20*log10(magn+0.0001)));
}
// Frequency graduations
uView.setCursor(47,0);
uView.print(MaxFreq/1000);
uView.print("k");
uView.line(0,0,0,5);
uView.line(10,0,10,2);
uView.line(20,0,20,5);
uView.line(30,0,30,2);
uView.line(40,0,40,5);
// Voltage graduations
uView.line(0,40,40,40);
uView.setCursor(47,38);
uView.print("-30");
uView.line(0,20,40,20);
uView.setCursor(47,18);
uView.print("-10");
uView.line(0,10,40,10);
uView.setCursor(47,8);
uView.print("  0");
//Display
uView.display();
uView.clear(PAGE);
}