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/*
* Lowpass 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 lowpass.c
* lowpass filter implementation
*/
#include "lowpass.h"
/**********************
* TODO:
* support filters with order != 4
* calculate coefficients for filter instead of taking approximate ones from the table
*********************/
/** filter order */
#define LOWPASS_FILTER_ORDER 4
/**
* IIR filter global parameters
*/
typedef struct FFLPFilterCoeffs{
float gain;
float c[LOWPASS_FILTER_ORDER];
}FFLPFilterCoeffs;
/**
* filter data for 4th order IIR lowpass Butterworth filter
*/
static const FFLPFilterCoeffs lp_filter_coeffs[] = {
{ 9.398085e-01, { -0.0176648009, 0.0000000000, -0.4860288221, 0.0000000000 } },
{ 6.816645e-01, { -0.4646665999, -2.2127207402, -3.9912017501, -3.2380429984 } },
{ 4.998150e-01, { -0.2498216698, -1.3392807613, -2.7693097862, -2.6386277439 } },
{ 3.103469e-01, { -0.0965076902, -0.5977763360, -1.4972580903, -1.7740085241 } },
{ 2.346995e-01, { -0.0557639007, -0.3623690447, -1.0304538354, -1.3066051440 } },
{ 1.528432e-01, { -0.0261686639, -0.1473794606, -0.6204721225, -0.6514716536 } },
{ 6.917529e-02, { -0.0202414073, 0.0780167640, -0.5277442247, 0.3631641670 } },
{ 6.178391e-02, { -0.0223681543, 0.1069446609, -0.5615167033, 0.4883976841 } },
{ 5.298685e-02, { -0.0261686639, 0.1473794606, -0.6204721225, 0.6514716536 } },
{ 2.229030e-02, { -0.0647354087, 0.4172275190, -1.1412129810, 1.4320761385 } },
{ 1.693903e-02, { -0.0823177861, 0.5192354923, -1.3444768251, 1.6365345642 } },
{ 7.374053e-03, { -0.1481421788, 0.8650973862, -1.9894244796, 2.1544844308 } },
{ 5.541768e-03, { -0.1742301048, 0.9921936565, -2.2090801108, 2.3024482658 } },
};
/** cutoff ratios for lp_filter_data[] */
static const float lp_cutoff_ratios[] = {
0.5000000000, 0.4535147392, 0.4166666667, 0.3628117914,
0.3333333333, 0.2916666667, 0.2267573696, 0.2187500000,
0.2083333333, 0.1587301587, 0.1458333333, 0.1133786848,
0.1041666667,
};
/**
* IIR filter state
*/
typedef struct FFLPFilterState{
float x[LOWPASS_FILTER_ORDER];
}FFLPFilterState;
const struct FFLPFilterCoeffs* ff_lowpass_filter_init_coeffs(int order, float cutoff_ratio)
{
int i, size;
//we can create only order-4 filters with cutoff ratio <= 0.5 for now
if(order != LOWPASS_FILTER_ORDER) return NULL;
size = sizeof(lp_cutoff_ratios) / sizeof(lp_cutoff_ratios[0]);
if(cutoff_ratio > lp_cutoff_ratios[0])
return NULL;
for(i = 0; i < size; i++){
if(cutoff_ratio >= lp_cutoff_ratios[i])
break;
}
if(i == size)
i = size - 1;
return &lp_filter_coeffs[i];
}
struct FFLPFilterState* ff_lowpass_filter_init_state(int order)
{
if(order != LOWPASS_FILTER_ORDER) return NULL;
return av_mallocz(sizeof(FFLPFilterState));
}
#define FILTER(i0, i1, i2, i3) \
in = *src * c->gain \
+ c->c[0]*s->x[i0] + c->c[1]*s->x[i1] \
+ c->c[2]*s->x[i2] + c->c[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_lowpass_filter(const struct FFLPFilterCoeffs *c, struct FFLPFilterState *s, int size, int16_t *src, int sstep, int16_t *dst, int dstep)
{
int i;
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);
}
}
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