src/pcm_dither.c

   1 /*
   2  * Copyright (C) 2003-2010 The Music Player Daemon Project
   3  * http://www.musicpd.org
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of the GNU General Public License as published by
   7  * the Free Software Foundation; either version 2 of the License, or
   8  * (at your option) any later version.
   9  *
  10  * This program is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13  * GNU General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU General Public License along
  16  * with this program; if not, write to the Free Software Foundation, Inc.,
  17  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  18  */
  19
  20 #include "config.h"
  21 #include "pcm_dither.h"
  22 #include "pcm_prng.h"
  23
  24 static int16_t
  25 pcm_dither_sample_24_to_16(int32_t sample, struct pcm_dither *dither)
  26 {
  27         int32_t output, rnd;
  28
  29         enum {
  30                 from_bits = 24,
  31                 to_bits = 16,
  32                 scale_bits = from_bits - to_bits,
  33                 round = 1 << (scale_bits - 1),
  34                 mask = (1 << scale_bits) - 1,
  35                 ONE = 1 << (from_bits - 1),
  36                 MIN = -ONE,
  37                 MAX = ONE - 1
  38         };
  39
  40         sample += dither->error[0] - dither->error[1] + dither->error[2];
  41
  42         dither->error[2] = dither->error[1];
  43         dither->error[1] = dither->error[0] / 2;
  44
  45         /* round */
  46         output = sample + round;
  47
  48         rnd = pcm_prng(dither->random);
  49         output += (rnd & mask) - (dither->random & mask);
  50
  51         dither->random = rnd;
  52
  53         /* clip */
  54         if (output > MAX) {
  55                 output = MAX;
  56
  57                 if (sample > MAX)
  58                         sample = MAX;
  59         } else if (output < MIN) {
  60                 output = MIN;
  61
  62                 if (sample < MIN)
  63                         sample = MIN;
  64         }
  65
  66         output &= ~mask;
  67
  68         dither->error[0] = sample - output;
  69
  70         return (int16_t)(output >> scale_bits);
  71 }
  72
  73 void
  74 pcm_dither_24_to_16(struct pcm_dither *dither,
  75                     int16_t *dest, const int32_t *src,
  76                     unsigned num_samples)
  77 {
  78         while (num_samples-- > 0)
  79                 *dest++ = pcm_dither_sample_24_to_16(*src++, dither);
  80 }
  81
  82 static int16_t
  83 pcm_dither_sample_32_to_16(int32_t sample, struct pcm_dither *dither)
  84 {
  85         return pcm_dither_sample_24_to_16(sample >> 8, dither);
  86 }
  87
  88 void
  89 pcm_dither_32_to_16(struct pcm_dither *dither,
  90                     int16_t *dest, const int32_t *src,
  91                     unsigned num_samples)
  92 {
  93         while (num_samples-- > 0)
  94                 *dest++ = pcm_dither_sample_32_to_16(*src++, dither);
  95 }