pcm/Convert: add option to enable the integer-only dsd2pcm implementation

NEWS

@@ -22,6 +22,7 @@ ver 0.22 (not yet released)
   - ffmpeg: new plugin based on FFmpeg's libavfilter library
   - hdcd: new plugin based on FFmpeg's "af_hdcd" for HDCD playback
   - volume: convert S16 to S24 to preserve quality and reduce dithering noise
+  - dsd: add integer-only DSD to PCM converter
 * output
   - jack: add option "auto_destination_ports"
   - jack: report error details

doc/user.rst

@@ -568,6 +568,26 @@ Sometimes, music needs to be resampled before it can be played; for example, CDs
 Check the :ref:`resampler_plugins` reference for a list of resamplers
 and how to configure them.
 
+DSD
+^^^
+
+There are three ways to play back DSD (Direct Stream Digital):
+
+- natively if your output supports it (ALSA only and requires a sound
+  card with native DSD support)
+- `DSD over PCM <http://dsd-guide.com/dop-open-standard>`_: wrapped
+  inside fake 24 bit PCM samples (ALSA and OSX only and requires an
+  external DAC with DoP support)
+- for everything else, MPD automatically converts DSD to PCM
+
+For the last option, the setting ``dsd2pcm_converter`` selects a
+conversion method.  The default is ``float`` which converts to
+floating point samples.  For CPUs with a weak floating point unit
+(e.g. some ARMs like the one in the Raspberry Pi), it may be faster to
+use ``integer``, which is an integer-only implementation of the
+conversion algorithm.
+
+
 Client Connections
 ------------------
 

src/config/Option.hxx

@@ -79,6 +79,7 @@ enum class ConfigOption {
 	DESPOTIFY_USER,
 	DESPOTIFY_PASSWORD,
 	DESPOTIFY_HIGH_BITRATE,
+	DSD2PCM_CONVERTER,
 	MAX
 };
 

src/config/Templates.cxx

@@ -75,6 +75,7 @@ const ConfigTemplate config_param_templates[] = {
 	{ "despotify_user", false, true },
 	{ "despotify_password", false, true },
 	{ "despotify_high_bitrate", false, true },
+	{ "dsd2pcm_converter" },
 };
 
 static constexpr unsigned n_config_param_templates =

src/pcm/Convert.cxx

@@ -19,16 +19,35 @@
 
 #include "Convert.hxx"
 #include "ConfiguredResampler.hxx"
+#include "config/Data.hxx"
 #include "util/ConstBuffer.hxx"
+#include "util/StringAPI.hxx"
 
 #include <stdexcept>
 
 #include <assert.h>
 
+#ifdef ENABLE_DSD
+static bool dsd2pcm_integer = false;
+#endif
+
 void
 pcm_convert_global_init(const ConfigData &config)
 {
 	pcm_resampler_global_init(config);
+
+#ifdef ENABLE_DSD
+	dsd2pcm_integer = config.With(ConfigOption::DSD2PCM_CONVERTER, [](const char *s){
+		if (s == nullptr)
+			return false;
+		else if (StringIsEqual(s, "float"))
+			return false;
+		else if (StringIsEqual(s, "integer"))
+			return true;
+		else
+			throw std::runtime_error("Unrecognized value");
+	});
+#endif
 }
 
 PcmConvert::PcmConvert(const AudioFormat _src_format,