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/* the Music Player Daemon (MPD)
* Copyright (C) 2008 Max Kellermann <max@duempel.org>
* This project's homepage is: http://www.musicpd.org
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program 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 General Public License for more details.
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*/
#include "pcm_dither.h"
#include "pcm_prng.h"
static int16_t
pcm_dither_sample_24_to_16(int32_t sample, struct pcm_dither *dither)
{
int32_t output, rnd;
enum {
from_bits = 24,
to_bits = 16,
scale_bits = from_bits - to_bits,
round = 1 << (scale_bits - 1),
mask = (1 << scale_bits) - 1,
ONE = 1 << (from_bits - 1),
MIN = -ONE,
MAX = ONE - 1
};
sample += dither->error[0] - dither->error[1] + dither->error[2];
dither->error[2] = dither->error[1];
dither->error[1] = dither->error[0] / 2;
/* round */
output = sample + round;
rnd = pcm_prng(dither->random);
output += (rnd & mask) - (dither->random & mask);
dither->random = rnd;
/* clip */
if (output > MAX) {
output = MAX;
if (sample > MAX)
sample = MAX;
} else if (output < MIN) {
output = MIN;
if (sample < MIN)
sample = MIN;
}
output &= ~mask;
dither->error[0] = sample - output;
return (int16_t)(output >> scale_bits);
}
void
pcm_dither_24_to_16(struct pcm_dither *dither,
int16_t *dest, const int32_t *src,
unsigned num_samples)
{
while (num_samples-- > 0)
*dest++ = pcm_dither_sample_24_to_16(*src++, dither);
}
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