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;
}