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 * initial coefficient to perform bandwidth expansion on LPC
34 *
35 * @note: 0.9883 looks like an approximation of 253/256.
36 *
37 * TIA/EIA/IS-733 2.4.3.3.6 6
38 */
39#define QCELP_BANDWITH_EXPANSION_COEFF 0.9883
40
41/**
42 * Computes the Pa / (1 + z(-1)) or Qa / (1 - z(-1)) coefficients
43 * needed for LSP to LPC conversion.
44 * We only need to calculate the 6 first elements of the polynomial.
45 *
46 * @param lspf line spectral pair frequencies
47 * @param f [out] polynomial input/output as a vector
48 *
49 * TIA/EIA/IS-733 2.4.3.3.5-1/2
50 */
51static void lsp2polyf(const float *lspf, double *f, int lp_half_order)
52{
53    int i, j;
54
55    f[0] = 1.0;
56    f[1] = -2 * cos(M_PI * lspf[0]);
57    lspf -= 2;
58    for(i=2; i<=lp_half_order; i++)
59    {
60        double val = -2 * cos(M_PI * lspf[2*i]);
61        f[i] = val * f[i-1] + 2*f[i-2];
62        for(j=i-1; j>1; j--)
63            f[j] += f[j-1] * val + f[j-2];
64        f[1] += val;
65    }
66}
67
68/**
69 * Reconstructs LPC coefficients from the line spectral pair frequencies
70 * and performs bandwidth expansion.
71 *
72 * @param lspf line spectral pair frequencies
73 * @param lpc linear predictive coding coefficients
74 *
75 * @note: bandwith_expansion_coeff could be precalculated into a table
76 *        but it seems to be slower on x86
77 *
78 * TIA/EIA/IS-733 2.4.3.3.5
79 */
80void ff_qcelp_lspf2lpc(const float *lspf, float *lpc)
81{
82    double pa[6], qa[6];
83    int   i;
84    double bandwith_expansion_coeff = QCELP_BANDWITH_EXPANSION_COEFF * 0.5;
85
86    lsp2polyf(lspf,     pa, 5);
87    lsp2polyf(lspf + 1, qa, 5);
88
89    for (i=4; i>=0; i--)
90    {
91        double paf = pa[i+1] + pa[i];
92        double qaf = qa[i+1] - qa[i];
93
94        lpc[i  ] = paf + qaf;
95        lpc[9-i] = paf - qaf;
96    }
97    for (i=0; i<10; i++)
98    {
99        lpc[i] *= bandwith_expansion_coeff;
100        bandwith_expansion_coeff *= QCELP_BANDWITH_EXPANSION_COEFF;
101    }
102}
103