Variable 2nd order band pass IIR filter

This piece of code implements a variable band pass filter in C language. It is intended to be used to create an array of coefficients sets in runtime, each set of coefficients is mapped to a center frequency by a integer index.

The code also has the function to do the bandpass filtering on a given filter object.

Notice that the code is not optimized to any particular DSP architecture, but you can use it as a reference and do your own optimization.

#include "bp_iir.h"
#include <math.h>
#define BP_MAX_COEFS 120
#define PI 3.1415926

/*This is an array of the filter parameters object
defined in the br_iir.h file*/
static struct bp_coeffs bp_coeff_arr[BP_MAX_COEFS];

/*This initialization function will create the
band pass filter coefficients array, you have to specify:
fsfilt = Sampling Frequency
gb     = Gain at cut frequencies
Q      = Q factor, Higher Q gives narrower band
fstep  = Frequency step to increase center frequencies
in the array
fmin   = Minimum frequency for the range of center   frequencies
*/
void bp_iir_init(double fsfilt,double gb,double Q,short fstep, short fmin) {
      int i;
      double damp;
      double wo;
      
       damp = gb/sqrt(1 - pow(gb,2));

	for (i=0;i<BP_MAX_COEFS;i++) {
            wo = 2*PI*(fstep*i + fmin)/fsfilt;
		bp_coeff_arr[i].e = 1/(1 + damp*tan(wo/(Q*2)));
		bp_coeff_arr[i].p = cos(wo);
		bp_coeff_arr[i].d[0] = (1-bp_coeff_arr[i].e);
		bp_coeff_arr[i].d[1] = 2*bp_coeff_arr[i].e*bp_coeff_arr[i].p;
		bp_coeff_arr[i].d[2] = (2*bp_coeff_arr[i].e-1);
	}
}

/*This function loads a given set of band pass filter coefficients acording to a center frequency index
into a band pass filter object
H = filter object
ind = index of the array mapped to a center frequency
*/
void bp_iir_setup(struct bp_filter * H,int ind) {
	H->e = bp_coeff_arr[ind].e;
	H->p = bp_coeff_arr[ind].p;
	H->d[0] = bp_coeff_arr[ind].d[0];
	H->d[1] = bp_coeff_arr[ind].d[1];
	H->d[2] = bp_coeff_arr[ind].d[2];
}

/*This function loads a given set of band pass filter coefficients acording to a center frequency index
into a band pass filter object
H = filter object
ind = index of the array mapped to a center frequency
*/
double bp_iir_filter(double yin,struct bp_filter * H) {
	double yout;

	H->x[0] =  H->x[1]; 
	H->x[1] =  H->x[2]; 
	H->x[2] = yin;
	
	H->y[0] =  H->y[1]; 
	H->y[1] =  H->y[2]; 

	H->y[2] = H->d[0]* H->x[2] - H->d[0]* H->x[0] + (H->d[1]* H->y[1]) - H->d[2]* H->y[0];
	
	yout =  H->y[2];

	return yout;
}

/******************
bp_iir.h
**********************/

#ifndef __BP_IIR_H__
#define __BP_IIR_H__

struct bp_coeffs{
	double e;
	double p;
	double d[3];
};

struct bp_filter{
	double e;
	double p;
	double d[3];
	double x[3];
	double y[3];
};

extern void bp_iir_init(double fsfilt,double gb,double Q,short fstep, short fmin);
extern void bp_iir_setup(struct bp_filter * H,int index);
extern double bp_iir_filter(double yin,struct bp_filter * H);

#endif