Adds Analog Gain Control (AGC) to the WebRTC client.

media/audio/mac/audio_low_latency_input_mac.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.
 //
 // Implementation of AudioInputStream for Mac OS X using the special AUHAL
 // input Audio Unit present in OS 10.4 and later.
 // The AUHAL input Audio Unit is for low-latency audio I/O.
 //
 // Overview of operation:
 //
@@ skipping 20 matching lines @@
 //   1) Hardware latency, which includes Audio Unit latency, audio device
 //      latency;
 //   2) The delay between the actual recording instant and the time when the
 //      data packet is provided as a callback.
 //
 #ifndef MEDIA_AUDIO_MAC_AUDIO_LOW_LATENCY_INPUT_MAC_H_
 #define MEDIA_AUDIO_MAC_AUDIO_LOW_LATENCY_INPUT_MAC_H_
 
 #include <AudioUnit/AudioUnit.h>
 
+#include "base/atomicops.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/synchronization/lock.h"
+#include "base/time.h"
 #include "media/audio/audio_io.h"
 #include "media/audio/audio_parameters.h"
 
 class AudioManagerMac;
 
 class AUAudioInputStream : public AudioInputStream {
  public:
   // The ctor takes all the usual parameters, plus |manager| which is the
   // the audio manager who is creating this object.
   AUAudioInputStream(AudioManagerMac* manager,
                      const AudioParameters& params,
                      AudioDeviceID audio_device_id);
   // The dtor is typically called by the AudioManager only and it is usually
   // triggered by calling AudioInputStream::Close().
   virtual ~AUAudioInputStream();
 
   // Implementation of AudioInputStream.
   virtual bool Open() OVERRIDE;
   virtual void Start(AudioInputCallback* callback) OVERRIDE;
   virtual void Stop() OVERRIDE;
   virtual void Close() OVERRIDE;
   virtual double GetMaxVolume() OVERRIDE;
   virtual void SetVolume(double volume) OVERRIDE;
   virtual double GetVolume() OVERRIDE;
+  virtual void SetAutomaticGainControl(bool enabled) OVERRIDE;
+  virtual bool GetAutomaticGainControl() OVERRIDE;
 
   // Returns the current hardware sample rate for the default input device.
   static double HardwareSampleRate();
 
   bool started() const { return started_; }
   AudioUnit audio_unit() { return audio_unit_; }
   AudioBufferList* audio_buffer_list() { return &audio_buffer_list_; }
 
  private:
   // AudioOutputUnit callback.
@@ skipping 56 matching lines @@
   // True after successfull Start(), false after successful Stop().
   bool started_;
 
   // Fixed capture hardware latency in frames.
   double hardware_latency_frames_;
 
   // The number of channels in each frame of audio data, which is used
   // when querying the volume of each channel.
   int number_of_channels_in_frame_;
 
+  // Keeps track of last time a new volume level was fed to the client in an
+  // OnData() callback.
+  base::Time last_volume_update_time_;
+
+  // Contains last result of internal call to GetVolume(). We save resources
+  // by not quering the capture volume for each callback.
+  double volume_;
+
+  // True when Automatic Gain Control is enabled, false otherwise.
+  // This member is modified on the audio thread and read on the internal
+  // Audio Unit thread in each capture callback.
+  base::subtle::Atomic32 agc_is_enabled_;

[scherkus (not reviewing), 2012/03/21 09:21:21]

please avoid using base::subtle -- it's intended to be used to build
synchronization primitives (i.e., Lock)

there's two ways around this:
  1) disallow changing AGC on the fly (i.e., must be set prior to recording)
  2) use a lock -- there will be no contention for the lock seeing as this value
is set once

if I had to pick I'd go with (1) assuming that WebRTC knows whether it wants AGC
or not prior to recording and does not need to change it on the fly

[henrika (OOO until Aug 14), 2012/03/21 10:16:04]

Good idea. I'll come back with a new version and will go for #1 as you propose.

+
   DISALLOW_COPY_AND_ASSIGN(AUAudioInputStream);
 };
 
 #endif  // MEDIA_AUDIO_MAC_AUDIO_LOW_LATENCY_INPUT_MAC_H_