resample_lagrange_filter_frac_delay.c

This program calculates the characteristics of Farrow filter to compensate the fractional delay from 0 to 1. It uses Farrow filter on the basis of Lagrange polynomial interpolation.

It calculates the filter reaction to one delta-impulse and plots the filter magnitude and the group delay.

Operation results of the program are saved in files.

Files saving samples of filter impulse response characteristic

dat/resample_lagrange_filter_fd_time_0.0.txt
dat/resample_lagrange_filter_fd_time_0.1.txt
dat/resample_lagrange_filter_fd_time_0.2.txt
 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .
dat/resample_lagrange_filter_fd_time_0.9.txt

Impulse response characteristics plotted according to the calculated data are shown in the figure:

Filter magnitude datafiles:

dat/resample_lagrange_filter_fd_mag_0.0.txt
dat/resample_lagrange_filter_fd_mag_0.1.txt
dat/resample_lagrange_filter_fd_mag_0.2.txt
 .  .  .  .  .  .  .  .  .  .  .  .  .  .  
dat/resample_lagrange_filter_fd_mag_0.9.txt

Filter groupdelay datafiles:

dat/resample_lagrange_filter_fd_mag_0.0.txt
dat/resample_lagrange_filter_fd_mag_0.1.txt
dat/resample_lagrange_filter_fd_mag_0.2.txt
 .  .  .  .  .  .  .  .  .  .  .  .  .  .  
dat/resample_lagrange_filter_fd_mag_0.9.txt

Filter magntudes and group delays plotted according to the saved data are shown in the figure:

 
#include <stdio.h>
#include <stdlib.h>
#include <windows.h>
#include "dspl.h"


#define P 1
#define Q 1
#define K 512
#define N 4 
#define FN_LEN 256

int main()
{

 HINSTANCE hDSPL; /* dspl handle */
 
 double s[N] = {0.0, 1.0, 0.0, 0.0};
 
 double *t=NULL; /* time */
 double *h=NULL; /* Frac delay filter impulse response */
 double w[K]; /* Normalized Angular frequency */
 double H[K]; /* Frac delay filter magnitude */
 double GD[K]; /* Frac delay filter group delay */

 int nh; 
 double frac_delay; /* current fractional delay */
 char fn[FN_LEN]; /* file name for result */
 
 /* Load dspl.dll */
 hDSPL = dspl_load();
 if(!hDSPL)
 {
 printf("dspl.dll loading ERROR!\n");
 return 0;
 }

 /* Allocation memory and fill vectors */
 dspl_resample_lagrange(s, N, P, Q, 0.0, &h, &nh);
 t = (double*) malloc(nh*sizeof(double)); 
 dspl_linspace(0, nh, nh, DSPL_PERIODIC, t); 
 dspl_linspace(0, M_PI, K, DSPL_PERIODIC, w);
 
 /* cyycle for fraction delay from 0 to 1 */
 for(frac_delay = 0.0; frac_delay<1.0; frac_delay+=0.1)
 {
 /* current delay filter impulse response */
 dspl_resample_lagrange(s, N, P, Q, frac_delay, &h, &nh);
 
 /* save to file */
 memset(fn, 0, FN_LEN);
 sprintf(fn, "dat/resample_lagrange_filter_fd_time_%.1f.txt", frac_delay);
 dspl_writetxt(t,h,nh,fn);
 
 /* current fractional delay filter magnitude (dB) and group delay */
 dspl_freqz_resp(h, NULL, nh-1, w, K, H, NULL, GD);
 
 /* save magnitude to file */
 memset(fn, 0, FN_LEN);
 sprintf(fn, "dat/resample_lagrange_filter_fd_mag_%.1f.txt", frac_delay);
 dspl_writetxt(w,H,K,fn);
 
 
 /* dave group delay to file */
 memset(fn, 0, FN_LEN);
 sprintf(fn, "dat/resample_lagrange_filter_fd_gd_%.1f.txt", frac_delay);
 dspl_writetxt(w,GD,K,fn);
 }
 
 /* clear dspl handle */ 
 FreeLibrary(hDSPL);
 free(h);
 free(t);
 return 0;
}