This program is example of the impulse response and magnitude calculation for the Farrow filter interpolator on the basis of piecewise cubic polynomial.

Result filter impulse response and magnitude is saved to files:

dat/resample_lagrange_filter_time.txt - the Farrow filter interpolator impulse response.

dat/resample_lagrange_filter_freq.txt - the Farrow filter magnitude.

Farrow filter impulse response \(h(n)\) and magnitude \(\left| H\left( \operatorname{e}^{j \cdot \omega}\right)\right|^2\), can be plotted as its showed on the follow figure:

#include <stdio.h>
#include <stdlib.h>
#include <windows.h>
#include "dspl.h"
#define P   10
#define Q   1
#define K   512
#define N   5
int main()
    HINSTANCE hDSPL;        /* dspl handle                  */
    double s[N] = {0.0, 0.0, 1.0, 0.0, 0.0};
    double *t=NULL;     /* time                         */
    double *h=NULL;     /* Filter impulse response      */
    double w[K];    /* Normalized Angular frequency */
    double HR[K];   /* Filter frequency response real part  */
    double HI[K];   /* Filter frequency response image part */
    double H[K];    /* Filter magnitude             */
    int  k, nh;
    /* Load dspl.dll */
    hDSPL = dspl_load();
        printf("dspl.dll loading ERROR!\n");
        return 0;
    dspl_resample_lagrange(s, N, P, Q, 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);
    dspl_freqz(h, NULL, nh-1, w, K, HR, HI);
    for(k = 0; k < K; k++)
        H[k] = 10.0*log10((HR[k]*HR[k] + HI[k]*HI[k]));
    /* clear dspl handle */
    return 0;