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https://github.com/FFmpeg/FFmpeg.git
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130 lines
4.4 KiB
C
130 lines
4.4 KiB
C
/*
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* Lowpass IIR filter
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* Copyright (c) 2008 Konstantin Shishkov
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*
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* This file is part of FFmpeg.
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*
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* FFmpeg is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Lesser General Public
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* License as published by the Free Software Foundation; either
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* version 2.1 of the License, or (at your option) any later version.
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*
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* FFmpeg is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with FFmpeg; if not, write to the Free Software
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* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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*/
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/**
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* @file lowpass.c
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* lowpass filter implementation
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*/
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#include "lowpass.h"
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/**********************
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* TODO:
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* support filters with order != 4
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* calculate coefficients for filter instead of taking approximate ones from the table
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*********************/
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/** filter order */
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#define LOWPASS_FILTER_ORDER 4
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/**
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* IIR filter global parameters
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*/
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typedef struct FFLPFilterCoeffs{
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float gain;
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float c[LOWPASS_FILTER_ORDER];
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}FFLPFilterCoeffs;
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/**
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* filter data for 4th order IIR lowpass Butterworth filter
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*/
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static const FFLPFilterCoeffs lp_filter_coeffs[] = {
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{ 9.398085e-01, { -0.0176648009, 0.0000000000, -0.4860288221, 0.0000000000 } },
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{ 6.816645e-01, { -0.4646665999, -2.2127207402, -3.9912017501, -3.2380429984 } },
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{ 4.998150e-01, { -0.2498216698, -1.3392807613, -2.7693097862, -2.6386277439 } },
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{ 3.103469e-01, { -0.0965076902, -0.5977763360, -1.4972580903, -1.7740085241 } },
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{ 2.346995e-01, { -0.0557639007, -0.3623690447, -1.0304538354, -1.3066051440 } },
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{ 1.528432e-01, { -0.0261686639, -0.1473794606, -0.6204721225, -0.6514716536 } },
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{ 6.917529e-02, { -0.0202414073, 0.0780167640, -0.5277442247, 0.3631641670 } },
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{ 6.178391e-02, { -0.0223681543, 0.1069446609, -0.5615167033, 0.4883976841 } },
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{ 5.298685e-02, { -0.0261686639, 0.1473794606, -0.6204721225, 0.6514716536 } },
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{ 2.229030e-02, { -0.0647354087, 0.4172275190, -1.1412129810, 1.4320761385 } },
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{ 1.693903e-02, { -0.0823177861, 0.5192354923, -1.3444768251, 1.6365345642 } },
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{ 7.374053e-03, { -0.1481421788, 0.8650973862, -1.9894244796, 2.1544844308 } },
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{ 5.541768e-03, { -0.1742301048, 0.9921936565, -2.2090801108, 2.3024482658 } },
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};
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/** cutoff ratios for lp_filter_data[] */
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static const float lp_cutoff_ratios[] = {
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0.5000000000, 0.4535147392, 0.4166666667, 0.3628117914,
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0.3333333333, 0.2916666667, 0.2267573696, 0.2187500000,
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0.2083333333, 0.1587301587, 0.1458333333, 0.1133786848,
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0.1041666667,
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};
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/**
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* IIR filter state
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*/
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typedef struct FFLPFilterState{
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float x[LOWPASS_FILTER_ORDER];
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}FFLPFilterState;
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const struct FFLPFilterCoeffs* ff_lowpass_filter_init_coeffs(int order, float cutoff_ratio)
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{
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int i, size;
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//we can create only order-4 filters with cutoff ratio <= 0.5 for now
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if(order != LOWPASS_FILTER_ORDER) return NULL;
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size = sizeof(lp_cutoff_ratios) / sizeof(lp_cutoff_ratios[0]);
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if(cutoff_ratio > lp_cutoff_ratios[0])
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return NULL;
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for(i = 0; i < size; i++){
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if(cutoff_ratio >= lp_cutoff_ratios[i])
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break;
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}
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if(i == size)
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i = size - 1;
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return &lp_filter_coeffs[i];
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}
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struct FFLPFilterState* ff_lowpass_filter_init_state(int order)
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{
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if(order != LOWPASS_FILTER_ORDER) return NULL;
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return av_mallocz(sizeof(FFLPFilterState));
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}
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#define FILTER(i0, i1, i2, i3) \
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in = *src * c->gain \
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+ c->c[0]*s->x[i0] + c->c[1]*s->x[i1] \
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+ c->c[2]*s->x[i2] + c->c[3]*s->x[i3]; \
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res = (s->x[i0] + in )*1 \
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+ (s->x[i1] + s->x[i3])*4 \
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+ s->x[i2] *6; \
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*dst = av_clip_int16(lrintf(res)); \
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s->x[i0] = in; \
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src += sstep; \
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dst += dstep; \
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void ff_lowpass_filter(const struct FFLPFilterCoeffs *c, struct FFLPFilterState *s, int size, int16_t *src, int sstep, int16_t *dst, int dstep)
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{
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int i;
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for(i = 0; i < size; i += 4){
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float in, res;
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FILTER(0, 1, 2, 3);
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FILTER(1, 2, 3, 0);
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FILTER(2, 3, 0, 1);
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FILTER(3, 0, 1, 2);
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}
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}
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