pcm_dither: added generic 24 to 16 bit dithering

Copied and adapted code from the mp3 decoder plugin.  This library now
replaces the old and low-quality function pcm_convert_24_to_16().

1 files changed, 82 insertions, 0 deletions

diff --git a/src/pcm_dither.c b/src/pcm_dither.c
new file mode 100644
index 00000000..1296d092
--- /dev/null
+++ b/src/pcm_dither.c
@@ -0,0 +1,82 @@
+/* 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);
+}