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/* SpiralSynth
* Copyleft (C) 2000 David Griffiths <dave@pawfal.org>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
*/
/*
Resonant low pass filter source code.
This code was originally written by baltrax@hotmail.com
See <http://www.harmony-central.com/Computer/Programming/resonant-lp-filter.c>
for the full version with explanatory comments, and the maths!
*/
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#ifndef _IIR_FILTER
#define _IIR_FILTER
/* FILTER INFORMATION STRUCTURE FOR FILTER ROUTINES */
class FILTER{
public:
FILTER() {history=coef=NULL;}
unsigned int length; /* size of filter */
float *history; /* pointer to history in filter */
float *coef; /* pointer to coefficients of filter */
};
#define FILTER_SECTIONS 2 /* 2 filter sections for 24 db/oct filter */
typedef struct {
double a0, a1, a2; /* numerator coefficients */
double b0, b1, b2; /* denominator coefficients */
} BIQUAD;
extern BIQUAD ProtoCoef[FILTER_SECTIONS]; /* Filter prototype coefficients,
1 for each filter section */
float iir_filter(float input,FILTER *iir);
void szxform(
double *a0, double *a1, double *a2, /* numerator coefficients */
double *b0, double *b1, double *b2, /* denominator coefficients */
double fc, /* Filter cutoff frequency */
double fs, /* sampling rate */
double *k, /* overall gain factor */
float *coef); /* pointer to 4 iir coefficients */
void prewarp(double *a0, double *a1, double *a2, double fc, double fs);
void bilinear(
double a0, double a1, double a2, /* numerator coefficients */
double b0, double b1, double b2, /* denominator coefficients */
double *k, /* overall gain factor */
double fs, /* sampling rate */
float *coef);
#endif
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