Data Analysis questions

Projects General Guidance
14

You are correct. The code is missing a line.

Corrected code:

float a[8] = {0, 0, 0, 0, 10, 10, 10, 10}; //example square wave signal, period = 8 x sample time

void setup() {
  Serial.begin(9600);

  //sample time = 10 ms, signal frequency in this case = 1/period = 12.5 Hz.
  //DFT calculated for 0 to 40 Hz in steps of 0.5 Hz, assuming signal repeats 4 times, no window
  // expect peaks at 0Hz (DC offset), 12.5 Hz, 37.5 Hz, ...

  dft (a, 8, 10, 0, 40, 0.5, 4, 0);

}

void loop() {
}

// from https://www.instructables.com/Arduino-Frequency-Transform-DFT/
// some errors corrected SJR 10/2021
/*
  8 TERMS THAT NEED TO BE SPECIFIED
  1: an array of which dft need to be taken
  2:size of the array
  3:time interval between 2 reading in array in milliSECONDS
  4:lower value of frequency range in Hz
  5:upper value of frequency range in Hz
  6:size of steps for frequency range
  7:repetitions of signal array (minimum 1) higher number better accuracy but increased solution time
  8:  0 for no window, 1 for flat-top window, 2 for Hann window, 3 for Hamming window
      (if you do not have any idea about selecting window keep default 3)
  example:
      dft(a,110,0.5,0,30,0.5,10,3);
      here a is an array of size 110 element to be checked for 0 Hz to 30 Hz with 0.5 step (0,0.5,1,1.5, ... ,29,29.5,30)
      10 repetition and hamming window
      by- ABHILASH PATEL
*/

void dft(float a[], int arraysize, float interval, float f0, float fn, float stepsize, int reps, int window)
{
  float mag, sumi, sumr, ti, tr;
  int j, k;
  float twopi = 2.0 * PI;
  Serial.print("data array repetitions = ");
  Serial.println(reps);

  // apply windowing function
  if (window == 1) //flat-top window
  {
    for (int i = 0; i < arraysize; i++)
    {
      float b = PI * i / (arraysize);
      a[i] = a[i] * ( 1 - (1.93 * cos(2 * b)) + (1.29 * cos(4 * b)) - (0.388 * cos(6 * b)) + (0.028 * cos(8 * b)));
      // Serial.println(a[i]);
    }
  }

  if (window == 2) //hann window
  {
    for (int i = 0; i < arraysize; i++)
    {
      float b = twopi * i / (arraysize);
      a[i] = a[i] * 0.5 * (1 - cos(b));
      //Serial.println(a[i]);
    }
  }

  if (window == 3) //hamming window
  {
    for (int i = 0; i < arraysize; i++)
    {
      float b = twopi * i / (arraysize);
      a[i] = a[i] * (0.54 - 0.46 * cos(b));
    //  Serial.println(a[i]);
    }
  }

  // DFT calculation

  for (float f = f0; f <= fn; f = f + stepsize)
  {

    sumi = sumr = 0.0;
    k = 0;
    for (int i = 0; i < (arraysize * reps); i++)
    {
      j = i - k;

      if (j >= arraysize) {
        k = k + arraysize;  //signal repeat index offset
        j = i - k;
      }
      ti = a[j] * (sin(twopi * f * i * interval * 0.001)); //0.001 => time in seconds
      tr = a[j] * (cos(twopi * f * i * interval * 0.001));
      sumi = sumi + ti;
      sumr = sumr + tr;
    }
    mag = sqrt(sumi * sumi + sumr * sumr) / (arraysize * reps);
    Serial.print(f);
    Serial.print("\t");
    Serial.println(mag);
  }

}
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