libavcodec/qcelp_lsp.c

   1 /*
   2  * QCELP decoder
   3  * Copyright (c) 2007 Reynaldo H. Verdejo Pinochet
   4  *
   5  * This file is part of FFmpeg.
   6  *
   7  * FFmpeg is free software; you can redistribute it and/or
   8  * modify it under the terms of the GNU Lesser General Public
   9  * License as published by the Free Software Foundation; either
  10  * version 2.1 of the License, or (at your option) any later version.
  11  *
  12  * FFmpeg is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15  * Lesser General Public License for more details.
  16  *
  17  * You should have received a copy of the GNU Lesser General Public
  18  * License along with FFmpeg; if not, write to the Free Software
  19  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  20  */
  21
  22 /**
  23  * @file libavcodec/qcelp_lsp.c
  24  * QCELP decoder
  25  * @author Reynaldo H. Verdejo Pinochet
  26  * @remark FFmpeg merging spearheaded by Kenan Gillet
  27  * @remark Development mentored by Benjamin Larson
  28  */
  29
  30 #include "libavutil/mathematics.h"
  31
  32 /**
  33  * Computes the Pa / (1 + z(-1)) or Qa / (1 - z(-1)) coefficients
  34  * needed for LSP to LPC conversion.
  35  * We only need to calculate the 6 first elements of the polynomial.
  36  *
  37  * @param lspf line spectral pair frequencies
  38  * @param f [out] polynomial input/output as a vector
  39  *
  40  * TIA/EIA/IS-733 2.4.3.3.5-1/2
  41  */
  42 static void lsp2polyf(const double *lspf, double *f, int lp_half_order)
  43 {
  44     int i, j;
  45
  46     f[0] = 1.0;
  47     f[1] = -2 * lspf[0];
  48     lspf -= 2;
  49     for(i=2; i<=lp_half_order; i++)
  50     {
  51         double val = -2 * lspf[2*i];
  52         f[i] = val * f[i-1] + 2*f[i-2];
  53         for(j=i-1; j>1; j--)
  54             f[j] += f[j-1] * val + f[j-2];
  55         f[1] += val;
  56     }
  57 }
  58
  59 /**
  60  * Reconstructs LPC coefficients from the line spectral pair frequencies.
  61  *
  62  * @param lspf line spectral pair frequencies
  63  * @param lpc linear predictive coding coefficients
  64  */
  65 void ff_celp_lspf2lpc(const double *lspf, float *lpc)
  66 {
  67     double pa[6], qa[6];
  68     int   i;
  69
  70     lsp2polyf(lspf,     pa, 5);
  71     lsp2polyf(lspf + 1, qa, 5);
  72
  73     for (i=4; i>=0; i--)
  74     {
  75         double paf = pa[i+1] + pa[i];
  76         double qaf = qa[i+1] - qa[i];
  77
  78         lpc[i  ] = 0.5*(paf+qaf);
  79         lpc[9-i] = 0.5*(paf-qaf);
  80     }
  81 }