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().

src/Makefile.am

@@ -66,6 +66,7 @@ mpd_headers = \
 	path.h \
 	mapper.h \
 	pcm_utils.h \
+	pcm_dither.h \
 	permission.h \
 	player_thread.h \
 	player_control.h \
@@ -145,6 +146,7 @@ mpd_SOURCES = \
 	path.c \
 	mapper.c \
 	pcm_utils.c \
+	pcm_dither.c \
 	permission.c \
 	player_thread.c \
 	player_control.c \

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);
+}

src/pcm_dither.h

@@ -0,0 +1,41 @@
+/* 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
+ */
+
+#ifndef PCM_DITHER_H
+#define PCM_DITHER_H
+
+#include <stdint.h>
+
+struct pcm_dither_24 {
+	int32_t error[3];
+	int32_t random;
+};
+
+static inline void
+pcm_dither_24_init(struct pcm_dither_24 *dither)
+{
+	dither->error[0] = dither->error[1] = dither->error[2] = 0;
+	dither->random = 0;
+}
+
+void
+pcm_dither_24_to_16(struct pcm_dither_24 *dither,
+		    int16_t *dest, const int32_t *src,
+		    unsigned num_samples);
+
+#endif

src/pcm_utils.c

@@ -208,6 +208,8 @@ void pcm_mix(char *buffer1, const char *buffer2, size_t size,
 void pcm_convert_init(struct pcm_convert_state *state)
 {
 	memset(state, 0, sizeof(*state));
+
+	pcm_dither_24_init(&state->dither);
 }
 
 #ifdef HAVE_LIBSAMPLERATE
@@ -457,17 +459,16 @@ pcm_convert_8_to_16(int16_t *out, const int8_t *in,
 }
 
 static void
-pcm_convert_24_to_16(int16_t *out, const int32_t *in,
+pcm_convert_24_to_16(struct pcm_dither_24 *dither,
+		     int16_t *out, const int32_t *in,
 		     unsigned num_samples)
 {
-	while (num_samples > 0) {
-		*out++ = *in++ >> 8;
-		--num_samples;
-	}
+	pcm_dither_24_to_16(dither, out, in, num_samples);
 }
 
 static const int16_t *
-pcm_convert_to_16(uint8_t bits, const void *src,
+pcm_convert_to_16(struct pcm_convert_state *convert,
+		  uint8_t bits, const void *src,
 		  size_t src_size, size_t *dest_size_r)
 {
 	static int16_t *buf;
@@ -500,7 +501,8 @@ pcm_convert_to_16(uint8_t bits, const void *src,
 			buf = xrealloc(buf, len);
 		}
 
-		pcm_convert_24_to_16((int16_t *)buf,
+		pcm_convert_24_to_16(&convert->dither,
+				     (int16_t *)buf,
 				     (const int32_t *)src,
 				     num_samples);
 		return buf;
@@ -524,7 +526,7 @@ size_t pcm_convert(const struct audio_format *inFormat,
 	assert(outFormat->bits == 16);
 
 	/* everything else supports 16 bit only, so convert to that first */
-	buf = pcm_convert_to_16(inFormat->bits, src, src_size, &len);
+	buf = pcm_convert_to_16(convState, inFormat->bits, src, src_size, &len);
 	if (!buf)
 		exit(EXIT_FAILURE);
 

src/pcm_utils.h

@@ -20,6 +20,7 @@
 #define PCM_UTILS_H
 
 #include "../config.h"
+#include "pcm_dither.h"
 
 #include <stdint.h>
 #include <stddef.h>
@@ -40,6 +41,9 @@ struct pcm_convert_state {
 	uint32_t lastInSampleRate;
 	uint32_t lastOutSampleRate;
 #endif
+
+	struct pcm_dither_24 dither;
+
 	/* Strict C99 doesn't allow empty structs */
 	int error;
 };