filter_lowpass.c

#include <stdlib.h>
#include <stdio.h>
#include <math.h>

int fSampleRate;

// Filter parameters, set by init function
float a;
float b;
float c;

// Filter buffer
float buf[3];

// Calculate filter parameters,
// f - filter cutoff frequency, Hz
void init(int sample_rate, float f) {
  fSampleRate = sample_rate;
  float fc = tan(M_PI * f / (float)fSampleRate);
  a = 1.0 / ((1.0 / fc + sqrt(2)) / fc + 1.0);
  b = ((1.0 / fc - sqrt(2)) / fc) + 1.0f;
  c = 2.0 * (1.0 - (1.0 / pow(fc, 2)));
}

// 2-order lowpass filter,
// input, output - data arrays
// count - number of samples to process
void lowpass2(int count, float input[], float output[]) {
  for (int i = 0; (i < count); i++) {
    buf[0] = input[i] - a * (b * buf[2] + c * buf[1]);
    output[i] = a * (buf[0] + buf[2] + 2.0f * buf[1]);

    buf[2] = buf[1];
    buf[1] = buf[0];
  }
}

int main(void) {

  FILE *fp;
  fp = fopen("filtered.txt", "r");
  int r;

  fseek(fp, 0, SEEK_END);
  long fsize = ftell(fp);
  fseek(fp, 0, SEEK_SET);

  float* I = malloc(fsize * sizeof(float));
  int I_head = 0;

  // 250 kHz, cutoff frequency - 300 Hz
  init(250000, 3000.0);

  while(r != EOF ) {
    int t;
    r = fscanf(fp, "%d %f\n", &t, &I[I_head]);
    I_head++;
  }

  float* I_filtered = malloc(I_head * sizeof(float));

  // Order: 2+2+2+2 = 8
  lowpass2(I_head, I, I_filtered);
  lowpass2(I_head, I_filtered, I_filtered);
  lowpass2(I_head, I_filtered, I_filtered);
  lowpass2(I_head, I_filtered, I_filtered);

  for (int i = 0; i < I_head; i++) {
    printf("%d %f\n", (int)(1.0/fSampleRate*1000000.0*i), I_filtered[i]);
  }

  fclose(fp);
}