mirror of
https://github.com/cookiengineer/audacity
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103 lines
3.8 KiB
C
103 lines
3.8 KiB
C
/*
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* TwoLAME: an optimized MPEG Audio Layer Two encoder
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*
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* Copyright (C) 2001-2004 Michael Cheng
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* Copyright (C) 2004-2006 The TwoLAME Project
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*
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* This library 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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* This library 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 this library; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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*
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* $Id$
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*
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*/
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#include <stdio.h>
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#include <math.h>
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#include "twolame.h"
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#include "common.h"
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#include "ath.h"
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/* freq in hz */
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FLOAT ath_db(FLOAT f, FLOAT value)
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{
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/* from Painter & Spanias modified by Gabriel Bouvigne to better fit the reality ath = 3.640 *
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pow(f,-0.8) - 6.800 * exp(-0.6*pow(f-3.4,2.0)) + 6.000 * exp(-0.15*pow(f-8.7,2.0)) + 0.6*
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0.001 * pow(f,4.0);
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In the past LAME was using the Painter &Spanias formula. But we had some recurrent problems
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with HF content. We measured real ATH values, and found the older formula to be inacurate in
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the higher part. So we made this new formula and this solved most of HF problematic
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testcases. The tradeoff is that in VBR mode it increases a lot the bitrate. */
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/* this curve can be udjusted according to the VBR scale: it adjusts from something close to
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Painter & Spanias on V9 up to Bouvigne's formula for V0. This way the VBR bitrate is more
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balanced according to the -V value. */
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FLOAT ath;
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FLOAT valueold = 0.0;
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if (f < -.3)
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f = 3410;
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f /= 1000; // convert to khz
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f = MAX(0.01, f);
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f = MIN(18.0, f);
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ath = 3.640 * pow(f, -0.8)
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- 6.800 * exp(-0.6 * pow(f - 3.4, 2.0))
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+ 6.000 * exp(-0.15 * pow(f - 8.7, 2.0))
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+ (0.6 + 0.04 * valueold) * 0.001 * pow(f, 4.0);
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/* MFC Feb 2003 I've changed the fudge technique on the code. The "-l [float]" value
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raises/lowers the ATH by this many dB */
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return (ath + value);
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}
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/* Convert ATH values from dB into energy values as required by the psycho model */
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FLOAT ath_energy(FLOAT freq, FLOAT value)
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{
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FLOAT db;
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db = ath_db(freq, 0) + value; // Originally: ath_db(freq,value)
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/* The values in the standard, and from the ATH formula are in dB. In the psycho model we are
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working in the energy domain. Hence the values that are in the absthr_X tables are not in
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dB. This function converts from dB into the energy domain. As noted on the LAME mailing list
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from years ago (MFC FIX find the reference), the absolute threhsold of hearing values in the
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tables in the standard are dodgy - the ATH in the tables do not correspond to any previously
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known values of the ATH. From ISO 11172 Tables D.4.x "A value of 0dB represents a level in
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the absolute threshold calculation of 96dB below the energy of a sine wave of amplitude
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32760." But I still don't know why the factor of 41.837375 is the value that it is. MFC Feb
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2003 */
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return (pow(10.0, (db + 41.837375) * 0.1));
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}
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/* Convert a frequency (in Hz) to a bark value
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Taken from LAME. MFC Feb 2003
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see for example "Zwicker: Psychoakustik, 1982; ISBN 3-540-11401-7 */
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FLOAT ath_freq2bark(FLOAT freq)
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{
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if (freq < 0)
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freq = 0;
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freq = freq * 0.001;
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return 13.0 * atan(.76 * freq) + 3.5 * atan(freq * freq / (7.5 * 7.5));
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}
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// vim:ts=4:sw=4:nowrap:
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