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-rw-r--r--src/pcm_dither.c82
1 files changed, 82 insertions, 0 deletions
diff --git a/src/pcm_dither.c b/src/pcm_dither.c
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+++ b/src/pcm_dither.c
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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"
+
+static unsigned long prng(unsigned long state)
+{
+ return (state * 0x0019660dL + 0x3c6ef35fL) & 0xffffffffL;
+}
+
+static int16_t
+pcm_dither_sample_24_to_16(int32_t sample, struct pcm_dither_24 *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 = 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_24 *dither,
+ int16_t *dest, const int32_t *src,
+ unsigned num_samples)
+{
+ while (num_samples-- > 0)
+ *dest++ = pcm_dither_sample_24_to_16(*src++, dither);
+}