Surround Sound handling by folding 5 channels down to stereo.

media/audio/audio_util.h

Index: media/audio/audio_util.h
===================================================================
--- media/audio/audio_util.h	(revision 21354)
+++ media/audio/audio_util.h	(working copy)
@@ -9,6 +9,11 @@
 
 namespace media {
 
+// For all audio functions 3 audio formats are supported:
+// 8 bits unsigned 0 to 255.
+// 16 bit signed (little endian).
+// 32 bit signed (little endian)
+
 // AdjustVolume() does a software volume adjustment of a sample buffer.
 // The samples are multiplied by the volume, which should range from
 // 0.0 (mute) to 1.0 (full volume).
@@ -25,6 +30,25 @@
                   int bytes_per_sample,
                   float volume);
 
+// FoldChannels() does a software multichannel folding down to stereo.
+// Channel order is assumed to be 5.1 Dolby standard which is
+// front left, front right, center, surround left, surround right.
+// The subwoofer is ignored.
+// 6.1 adds a rear center speaker, and 7.1 has 2 rear speakers.  These
+// channels are rare and ignored.
+// After summing the channels, volume is adjusted and the samples are
+// clipped to the maximum value.
+// Volume should normally range from 0.0 (mute) to 1.0 (full volume), but
+// since clamping is performed a value of more than 1 is allowed to increase
+// volume.
+// The buffer is modified in-place to avoid memory management, as this
+// function may be called in performance critical code.
+bool FoldChannels(void* buf,
+                  size_t buflen,
+                  int channels,
+                  int bytes_per_sample,
+                  float volume);
+
 }  // namespace media
 
 #endif  // MEDIA_AUDIO_AUDIO_UTIL_H_