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