pcm_dsd: implement DSD to 24 bit USB conversion

Implements the dCS suggested standard:

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

1 files changed, 74 insertions, 0 deletions

diff --git a/src/pcm_dsd_usb.c b/src/pcm_dsd_usb.c
new file mode 100644
index 00000000..976420c0
--- /dev/null
+++ b/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;
+}