Add upmix and vdownmix plugins

Added PCM upmix and vdownmix plugins.

The upmix plugin is for upmixing to 4.0 or 5.1 surrounds
by simple copying (and delay line for rear).

The vdownmix plugin is a kind of "virtual surround", which
downmixes 4.0 or 5.1 input to 2.0 output with some effect.

1 files changed, 23 insertions, 0 deletions

diff --git a/doc/vdownmix.txt b/doc/vdownmix.txt
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+++ b/doc/vdownmix.txt
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+VDOWNMIX PLUGIN
+===============
+
+The vdownmix plugin is a downmixer from 4-6 channels to 2-channel
+stereo headphone output.  This plugin processes the input signals with
+a simple spacialization, so the output sounds like a kind of "virtual
+surround".
+
+For example, define the below:
+
+	pcm.!surround51 {
+		type vdownmix
+		slave.pcm "default"
+	}
+	pcm.!surround40 {
+		type vdownmix
+		slave.pcm "default"
+	}
+
+and the outputs from video player to these PCMs are converted to the
+default 2.0 output with a proper downmix.
+
+The accepted format is currently only S16.