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The C Users' Group Library 1994 August
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wc-cdrom-cusersgrouplibrary-1994-08.iso
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vol_300
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308_01
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coeff.c
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1990-06-17
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/* File coeff.c
/* Estimates required number of FIR filter coefficients
/* Based on formula in Rabiner & Gold, p. 156
/* Passband ripple entered in dB instead of %
/* where dB = 20*log10(1+%)
/* Adapted by Bob Briggs
/* 1 December 1989
*/
#include "stdio.h"
#include "math.h"
main(){
char c;
int r;
double a1,a2,a3,a4,a5,a6,d0,d1,d2,d3,d4,f,f1,n;
double fpass,fsamp,fstop;
a1 = 0.005309;
a2 = 0.07114;
a3 = -0.4761;
a4 = -0.00266;
a5 = -0.5941;
a6 = -0.4278;
putchar('\n');
printf("******* FIR filter coefficient number estimator *******\n");
while(1){
putchar('\n');
printf("Enter passband ripple (in dB)\n");
scanf("%lf",&d1);
if(d1 <= 0.0){printf("Error: ripple must be > zero dB\n"); continue;}
printf("Enter stopband attenuation (in dB)\n");
scanf("%lf",&d2);
printf("Enter transition width (normalized Hz, Nyquist = 0 to 0.5 Hz)\n");
putchar('\n');
printf(" [To enter frequencies instead, enter a negative number.\n");
printf(" Program will prompt for frequencies and\n");
printf(" calculate transition width as:\n");
printf(" tw = (fstop - fpass)/fsamp ]\n");
scanf("%lf",&f);
if(f<0){
printf("Enter stopband edge frequency, fstop\n");
scanf("%lf",&fstop);
printf("Enter passband edge frequency, fpass\n");
scanf("%lf",&fpass);
printf("Enter sampling frequency, fsamp\n");
scanf("%lf",&fsamp);
}
putchar('\n');
printf("Ripple = %g dB\n",d1);
printf("Stopband attenuation = %g dB\n",d2);
if(f<0){
printf("fstop = %g\n",fstop);
printf("fpass = %g\n",fpass);
printf("fsamp = %g\n",fsamp);
printf("Transition width = %g\n",(fstop-fpass)/fsamp);
}
else printf("Transition width = %g (normalized Hz)\n",f);
printf("Enter N or n to change data, RETURN if OK\n");
while((c = getchar()) != '\n'); /* flush buffer */
c = getchar();
if(c != 'n' && c != 'N') break;
}
if(f<0) f = (fstop - fpass)/fsamp;
d1 /= 20;
d1 = pow(10,d1);
d1 = (d1-1)/(d1+1);
d3 = log10(d1);
d2 /= -20;
f1 = 11.01217 + 0.51244*(d3 - d2);
d4 = d3*d3;
d0 = a1*d4 + a2*d3 +a3;
d0 *= d2;
d0 += a4*d4 + a5*d3 + a6;
n = d0/f - f1*f + 1;
r = (int)(n + 0.51);
printf("The required number of coefficients is %d\n",r);
}