Add software audio mixing to the audio utils.

media/audio/audio_util.h

 // Copyright (c) 2012 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 <string>
 #include <vector>
 
@@ skipping 22 matching lines @@
 // 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.
 MEDIA_EXPORT bool AdjustVolume(void* buf,
                                size_t buflen,
                                int channels,
                                int bytes_per_sample,
                                float volume);
 
+// MixStreams() mixes 2 audio streams, adjusting volume on one of them.
+// Dst += Src * volume.

[vrk (LEFT CHROMIUM), 2012/03/12 18:40:24]

This function assumes |dst| and |src| have the same sample rate, right? Add
comment?

[enal1, 2012/03/12 21:20:53]

Done.

+MEDIA_EXPORT void MixStreams(void* dst,
+                             void* src,
+                             size_t buflen,
+                             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
@@ skipping 64 matching lines @@
 
 // Does Windows support WASAPI? We are checking in lot of places, and
 // sometimes check was written incorrectly, so move into separate function.
 MEDIA_EXPORT bool IsWASAPISupported();
 
 #endif  // defined(OS_WIN)
 
 }  // namespace media
 
 #endif  // MEDIA_AUDIO_AUDIO_UTIL_H_