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diff --git a/media/audio/audio_input_stream_impl.h b/media/audio/audio_input_stream_impl.h
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+++ b/media/audio/audio_input_stream_impl.h
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+// 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_INPUT_STREAM_IMPL_H_
+#define MEDIA_AUDIO_AUDIO_INPUT_STREAM_IMPL_H_
+
+#include "base/compiler_specific.h"
+#include "base/synchronization/lock.h"
+#include "base/time.h"
+#include "media/audio/audio_io.h"
+
+// AudioInputStreamImpl implements platform-independent parts of the
+// AudioInputStream interface. Each platform dependent implementation
+// should derive from this class.
+// TODO(henrika): we can probably break out more parts from our current
+// AudioInputStream implementation and move out to this class.
+class MEDIA_EXPORT AudioInputStreamImpl : public AudioInputStream {
+ public:
+  AudioInputStreamImpl();
+  virtual ~AudioInputStreamImpl();
+
+  // Sets the automatic gain control (AGC) to on or off. When AGC is enabled,
+  // the microphone volume is queried periodically and the volume level is
+  // provided in each AudioInputCallback::OnData() callback and fed to the
+  // render-side AGC.
+  virtual void SetAutomaticGainControl(bool enabled) OVERRIDE;
+
+  // Gets the current automatic gain control state.
+  virtual bool GetAutomaticGainControl() OVERRIDE;
+
+ protected:
+  // Stores a new volume level by asking the audio hardware.
+  // This method only has an effect if AGC is enabled.
+  void UpdateAgcVolume();
+
+  // Gets the latest stored volume level if AGC is enabled and if
+  // more than one second has passed since the volume was updated the last time.
+  void QueryAgcVolume(double* normalized_volume);
+
+ private:
+  // Takes a volume sample and stores it in |normalized_volume_|.
+  void GetNormalizedVolume();
+
+  // True when automatic gain control is enabled, false otherwise.
+  // Guarded by |lock_|.
+  bool agc_is_enabled_;
+
+  // Stores the maximum volume which is used for normalization to a volume
+  // range of [0.0, 1.0].
+  double max_volume_;
+
+  // Contains last result of internal call to GetVolume(). We save resources
+  // but not querying the capture volume for each callback. Guarded by |lock_|.
+  // The range is normalized to [0.0, 1.0].
+  double normalized_volume_;
+
+  // Protects |agc_is_enabled_| and |volume_| .
+  base::Lock lock_;
+
+  // Keeps track of the last time the microphone volume level was queried.
+  base::Time last_volume_update_time_;
+
+  DISALLOW_COPY_AND_ASSIGN(AudioInputStreamImpl);
+};
+
+#endif  // MEDIA_AUDIO_AUDIO_INPUT_STREAM_IMPL_H_