Surround Sound handling by folding 5 channels down to stereo.

media/audio/audio_util.h

 // Copyright (c) 2009 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MEDIA_AUDIO_AUDIO_UTIL_H_
 #define MEDIA_AUDIO_AUDIO_UTIL_H_
 
 #include "base/basictypes.h"
 
 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).
 // Using software allows each audio and video to have its own volume without
 // affecting the master volume.
 // In the future the function may be used to adjust the sample format to
 // simplify hardware requirements and to support a wider variety of input
 // formats.
 // The buffer is modified in-place to avoid memory management, as this
 // function may be called in performance critical code.
 bool AdjustVolume(void* buf,
                   size_t buflen,
                   int channels,
                   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_