pcm_dsd: implement DSD to 24 bit USB conversion

Implements the dCS suggested standard:

 http://www.dcsltd.co.uk/page/assets/DSDoverUSB.pdf

Makefile.am

@@ -403,6 +403,7 @@ libpcm_a_SOURCES = \
 	src/pcm_convert.c src/pcm_convert.h \
 	src/dsd2pcm/dsd2pcm.c src/dsd2pcm/dsd2pcm.h \
 	src/pcm_dsd.c src/pcm_dsd.h \
+	src/pcm_dsd_usb.c src/pcm_dsd_usb.h \
 	src/pcm_volume.c src/pcm_volume.h \
 	src/pcm_mix.c src/pcm_mix.h \
 	src/pcm_channels.c src/pcm_channels.h \

src/pcm_convert.c

@@ -22,6 +22,7 @@
 #include "pcm_channels.h"
 #include "pcm_format.h"
 #include "pcm_pack.h"
+#include "pcm_dsd_usb.h"
 #include "audio_format.h"
 #include "glib_compat.h"
 
@@ -325,6 +326,40 @@ pcm_convert(struct pcm_convert_state *state,
 	    size_t *dest_size_r,
 	    GError **error_r)
 {
+	struct audio_format usb_format;
+
+	if (src_format->format == SAMPLE_FORMAT_DSD &&
+	    dest_format->format == SAMPLE_FORMAT_DSD_OVER_USB) {
+		size_t u_size;
+		const uint32_t *u = pcm_dsd_to_usb(&state->dsd.buffer,
+						   src_format->channels,
+						   src, src_size,
+						   &u_size);
+		if (u == NULL) {
+			g_set_error_literal(error_r, pcm_convert_quark(), 0,
+					    "DSD to USB conversion failed");
+			return NULL;
+		}
+
+		usb_format = *src_format;
+		usb_format.format = SAMPLE_FORMAT_DSD_OVER_USB;
+
+		/* each DSD-over-USB sample contains 2 DSD bytes (16
+		   DSD bits), which means the sample rate must be
+		   halved; this is not the real 1 bit sample rate, but
+		   MPD's point of view */
+		usb_format.sample_rate = usb_format.sample_rate / 2;
+
+		if (audio_format_equals(&usb_format, dest_format)) {
+			*dest_size_r = u_size;
+			return u;
+		}
+
+		src_format = &usb_format;
+		src = u;
+		src_size = u_size;
+	}
+
 	struct audio_format float_format;
 	if (src_format->format == SAMPLE_FORMAT_DSD) {
 		size_t f_size;

src/pcm_dsd_usb.c

@@ -0,0 +1,74 @@
+/*
+ * Copyright (C) 2003-2012 The Music Player Daemon Project
+ * 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.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "config.h"
+#include "pcm_dsd_usb.h"
+#include "pcm_buffer.h"
+#include "audio_format.h"
+
+G_GNUC_CONST
+static inline uint32_t
+pcm_two_dsd_to_usb(uint8_t a, uint8_t b)
+{
+	return 0xffaa0000 | (a << 8) | b;
+}
+
+const uint32_t *
+pcm_dsd_to_usb(struct pcm_buffer *buffer, unsigned channels,
+	       const uint8_t *src, size_t src_size,
+	       size_t *dest_size_r)
+{
+	assert(buffer != NULL);
+	assert(audio_valid_channel_count(channels));
+	assert(src != NULL);
+	assert(src_size > 0);
+	assert(src_size % channels == 0);
+
+	const unsigned num_src_samples = src_size;
+	const unsigned num_src_frames = num_src_samples / channels;
+
+	/* this rounds down and discards the last odd frame; not
+	   elegant, but good enough for now */
+	const unsigned num_frames = num_src_frames / 2;
+	const unsigned num_samples = num_frames * channels;
+
+	const size_t dest_size = num_samples * 4;
+	*dest_size_r = dest_size;
+	uint32_t *const dest0 = pcm_buffer_get(buffer, dest_size),
+		*dest = dest0;
+
+	for (unsigned i = num_frames; i > 0; --i) {
+		for (unsigned c = channels; c > 0; --c) {
+			/* each 24 bit sample has 16 DSD sample bits
+			   plus the magic 0xaa marker */
+
+			*dest++ = pcm_two_dsd_to_usb(src[0], src[channels]);
+
+			/* seek the source pointer to the next
+			   channel */
+			++src;
+		}
+
+		/* skip the second byte of each channel, because we
+		   have already copied it */
+		src += channels;
+	}
+
+	return dest0;
+}

src/pcm_dsd_usb.h

@@ -0,0 +1,41 @@
+/*
+ * Copyright (C) 2003-2012 The Music Player Daemon Project
+ * 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.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_PCM_DSD_USB_H
+#define MPD_PCM_DSD_USB_H
+
+#include "check.h"
+
+#include <stdbool.h>
+#include <stdint.h>
+#include <stddef.h>
+
+struct pcm_buffer;
+
+/**
+ * Pack DSD 1 bit samples into (padded) 24 bit PCM samples for
+ * playback over USB, according to the dCS suggested standard:
+ * http://www.dcsltd.co.uk/page/assets/DSDoverUSB.pdf
+ */
+const uint32_t *
+pcm_dsd_to_usb(struct pcm_buffer *buffer, unsigned channels,
+	       const uint8_t *src, size_t src_size,
+	       size_t *dest_size_r);
+
+#endif