adx: calculate correct LPC coeffs
[libav.git] / libavcodec / adx.c
1 /*
2 * Copyright (c) 2011 Justin Ruggles
3 *
4 * This file is part of Libav.
5 *
6 * Libav is free software; you can redistribute it and/or
7 * modify it under the terms of the GNU Lesser General Public
8 * License as published by the Free Software Foundation; either
9 * version 2.1 of the License, or (at your option) any later version.
10 *
11 * Libav is distributed in the hope that it will be useful,
12 * but WITHOUT ANY WARRANTY; without even the implied warranty of
13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 * Lesser General Public License for more details.
15 *
16 * You should have received a copy of the GNU Lesser General Public
17 * License along with Libav; if not, write to the Free Software
18 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
19 */
20
21 #include "libavutil/mathematics.h"
22 #include "adx.h"
23
24 void ff_adx_calculate_coeffs(int cutoff, int sample_rate, int bits, int *coeff)
25 {
26 double a, b, c;
27
28 a = M_SQRT2 - cos(2.0 * M_PI * cutoff / sample_rate);
29 b = M_SQRT2 - 1.0;
30 c = (a - sqrt((a + b) * (a - b))) / b;
31
32 coeff[0] = lrintf(c * 2.0 * (1 << bits));
33 coeff[1] = lrintf(-(c * c) * (1 << bits));
34 }