output/osx channel_map feature

doc/mpd.conf.5

@@ -251,6 +251,32 @@ errors on bandwidth-limited devices.  Some users have reported good results
 with this set to 50000, but not all devices support values this high.  Most
 users do not need to change this.  The default is 256000000 / sample_rate(kHz),
 or 5804 microseconds for CD-quality audio.
+.SH OPTIONAL OS X OUTPUT PARAMETERS
+.TP
+.B device <dev>
+This specifies the device to use for audio output. The default is
+"default". Use the names listed in the "Audio Devices" window of
+"Audio MIDI Setup".
+.TP
+.B channel_map <input,input,input...>
+Specifies a channel map. If your audio device has more than two
+outputs this allows you to route audio to auxillary outputs. For
+predictable results you should also specify a "format" with a fixed
+number of channels, e.g. "*:*:2". The number of items in the channel
+map must match the number of output channels of your output device.
+Each list entry specifies the source for that output channel; use "-1"
+to silence an output. For example, if you have a four-channel output
+device and you wish to send stereo sound (format "*:*:2") to outputs 3
+and 4 while leaving outputs 1 and 2 silent then set the channel map to
+"-1,-1,0,1". In this example '0' and '1' denote the left and right
+channel respectively.
+
+The channel map may not refer to outputs that do not exist according
+to the format. If the format is "*:*:1" (mono) and you have a
+four-channel sound card then "-1,-1,0,0" (dual mono output on the
+second pair of sound card outputs) is a valid channel map but
+"-1,-1,0,1" is not because the second channel ('1') does not exist
+when the output is mono.
 .SH FILES
 .TP
 .BI ~/.mpdconf

doc/mpdconf.example

@@ -313,6 +313,15 @@ input {
 #	mixer_type	"software"
 #}
 #
+# An example of an OS X output:
+#
+#audio_output {
+#	type		"osx"
+#	name		"My OS X Device"
+##	device		"Built-in Output"	# optional
+##	channel_map      "-1,-1,0,1"	# optional
+#}
+#
 ## Example "pipe" output:
 #
 #audio_output {