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103 lines
2.8 KiB
C
103 lines
2.8 KiB
C
/*
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* QCELP decoder
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* Copyright (c) 2007 Reynaldo H. Verdejo Pinochet
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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 qcelp_lsp.c
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* QCELP decoder
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* @author Reynaldo H. Verdejo Pinochet
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* @remark FFmpeg merging spearheaded by Kenan Gillet
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* @remark Development mentored by Benjamin Larson
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*/
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#include "libavutil/mathematics.h"
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/**
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* initial coefficient to perform bandwidth expansion on LPC
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*
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* @note: 0.9883 looks like an approximation of 253/256.
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*
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* TIA/EIA/IS-733 2.4.3.3.6 6
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*/
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#define QCELP_BANDWITH_EXPANSION_COEFF 0.9883
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/**
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* Computes the Pa / (1 + z(-1)) or Qa / (1 - z(-1)) coefficients
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* needed for LSP to LPC conversion.
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* We only need to calculate the 6 first elements of the polynomial.
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*
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* @param lspf line spectral pair frequencies
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* @param f [out] polynomial input/output as a vector
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*
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* TIA/EIA/IS-733 2.4.3.3.5-1/2
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*/
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static void lsp2polyf(const float *lspf, double *f, int lp_half_order)
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{
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int i, j;
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f[0] = 1.0;
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f[1] = -2 * cos(M_PI * lspf[0]);
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lspf -= 2;
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for(i=2; i<=lp_half_order; i++)
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{
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double val = -2 * cos(M_PI * lspf[2*i]);
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f[i] = val * f[i-1] + 2*f[i-2];
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for(j=i-1; j>1; j--)
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f[j] += f[j-1] * val + f[j-2];
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f[1] += val;
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}
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}
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/**
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* Reconstructs LPC coefficients from the line spectral pair frequencies
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* and performs bandwidth expansion.
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*
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* @param lspf line spectral pair frequencies
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* @param lpc linear predictive coding coefficients
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*
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* @note: bandwith_expansion_coeff could be precalculated into a table
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* but it seems to be slower on x86
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*
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* TIA/EIA/IS-733 2.4.3.3.5
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*/
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void ff_qcelp_lspf2lpc(const float *lspf, float *lpc)
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{
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double pa[6], qa[6];
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int i;
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double bandwith_expansion_coeff = -QCELP_BANDWITH_EXPANSION_COEFF * 0.5;
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lsp2polyf(lspf, pa, 5);
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lsp2polyf(lspf + 1, qa, 5);
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for (i=4; i>=0; i--)
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{
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double paf = pa[i+1] + pa[i];
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double qaf = qa[i+1] - qa[i];
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lpc[i ] = paf + qaf;
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lpc[9-i] = paf - qaf;
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}
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for (i=0; i<10; i++)
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{
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lpc[i] *= bandwith_expansion_coeff;
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bandwith_expansion_coeff *= QCELP_BANDWITH_EXPANSION_COEFF;
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}
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}
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