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/*
* IIR filter
* Copyright (c) 2008 Konstantin Shishkov
*
* This file is part of FFmpeg.
*
* FFmpeg is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* FFmpeg 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
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with FFmpeg; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*/
/**
* @file libavcodec/iirfilter.c
* different IIR filters implementation
*/
#include "iirfilter.h"
#include <math.h>
/**
* IIR filter global parameters
*/
typedef struct FFIIRFilterCoeffs{
int order;
float gain;
int *cx;
float *cy;
}FFIIRFilterCoeffs;
/**
* IIR filter state
*/
typedef struct FFIIRFilterState{
float x[1];
}FFIIRFilterState;
/// maximum supported filter order
#define MAXORDER 30
av_cold struct FFIIRFilterCoeffs* ff_iir_filter_init_coeffs(enum IIRFilterType filt_type,
enum IIRFilterMode filt_mode,
int order, float cutoff_ratio,
float stopband, float ripple)
{
int i, j;
FFIIRFilterCoeffs *c;
double wa;
double p[MAXORDER + 1][2];
if(filt_type != FF_FILTER_TYPE_BUTTERWORTH || filt_mode != FF_FILTER_MODE_LOWPASS)
return NULL;
if(order <= 1 || (order & 1) || order > MAXORDER || cutoff_ratio >= 1.0)
return NULL;
c = av_malloc(sizeof(FFIIRFilterCoeffs));
c->cx = av_malloc(sizeof(c->cx[0]) * ((order >> 1) + 1));
c->cy = av_malloc(sizeof(c->cy[0]) * order);
c->order = order;
wa = 2 * tan(M_PI * 0.5 * cutoff_ratio);
c->cx[0] = 1;
for(i = 1; i < (order >> 1) + 1; i++)
c->cx[i] = c->cx[i - 1] * (order - i + 1LL) / i;
p[0][0] = 1.0;
p[0][1] = 0.0;
for(i = 1; i <= order; i++)
p[i][0] = p[i][1] = 0.0;
for(i = 0; i < order; i++){
double zp[2];
double th = (i + (order >> 1) + 0.5) * M_PI / order;
double a_re, a_im, c_re, c_im;
zp[0] = cos(th) * wa;
zp[1] = sin(th) * wa;
a_re = zp[0] + 2.0;
c_re = zp[0] - 2.0;
a_im =
c_im = zp[1];
zp[0] = (a_re * c_re + a_im * c_im) / (c_re * c_re + c_im * c_im);
zp[1] = (a_im * c_re - a_re * c_im) / (c_re * c_re + c_im * c_im);
for(j = order; j >= 1; j--)
{
a_re = p[j][0];
a_im = p[j][1];
p[j][0] = a_re*zp[0] - a_im*zp[1] + p[j-1][0];
p[j][1] = a_re*zp[1] + a_im*zp[0] + p[j-1][1];
}
a_re = p[0][0]*zp[0] - p[0][1]*zp[1];
p[0][1] = p[0][0]*zp[1] + p[0][1]*zp[0];
p[0][0] = a_re;
}
c->gain = p[order][0];
for(i = 0; i < order; i++){
c->gain += p[i][0];
c->cy[i] = (-p[i][0] * p[order][0] + -p[i][1] * p[order][1]) /
(p[order][0] * p[order][0] + p[order][1] * p[order][1]);
}
c->gain /= 1 << order;
return c;
}
av_cold struct FFIIRFilterState* ff_iir_filter_init_state(int order)
{
FFIIRFilterState* s = av_mallocz(sizeof(FFIIRFilterState) + sizeof(s->x[0]) * (order - 1));
return s;
}
#define FILTER(i0, i1, i2, i3) \
in = *src * c->gain \
+ c->cy[0]*s->x[i0] + c->cy[1]*s->x[i1] \
+ c->cy[2]*s->x[i2] + c->cy[3]*s->x[i3]; \
res = (s->x[i0] + in )*1 \
+ (s->x[i1] + s->x[i3])*4 \
+ s->x[i2] *6; \
*dst = av_clip_int16(lrintf(res)); \
s->x[i0] = in; \
src += sstep; \
dst += dstep; \
void ff_iir_filter(const struct FFIIRFilterCoeffs *c, struct FFIIRFilterState *s, int size, const int16_t *src, int sstep, int16_t *dst, int dstep)
{
int i;
if(c->order == 4){
for(i = 0; i < size; i += 4){
float in, res;
FILTER(0, 1, 2, 3);
FILTER(1, 2, 3, 0);
FILTER(2, 3, 0, 1);
FILTER(3, 0, 1, 2);
}
}else{
for(i = 0; i < size; i++){
int j;
float in, res;
in = *src * c->gain;
for(j = 0; j < c->order; j++)
in += c->cy[j] * s->x[j];
res = s->x[0] + in + s->x[c->order >> 1] * c->cx[c->order >> 1];
for(j = 1; j < c->order >> 1; j++)
res += (s->x[j] + s->x[c->order - j]) * c->cx[j];
for(j = 0; j < c->order - 1; j++)
s->x[j] = s->x[j + 1];
*dst = av_clip_int16(lrintf(res));
s->x[c->order - 1] = in;
src += sstep;
dst += sstep;
}
}
}
av_cold void ff_iir_filter_free_state(struct FFIIRFilterState *state)
{
av_free(state);
}
av_cold void ff_iir_filter_free_coeffs(struct FFIIRFilterCoeffs *coeffs)
{
if(coeffs){
av_free(coeffs->cx);
av_free(coeffs->cy);
}
av_free(coeffs);
}
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