Improved AGC update scheme for the audio backend in Chrome

media/audio/audio_input_stream_impl.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_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"
-
-namespace media {
-
-// 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);
-};
-
-}  // namespace media
-
-#endif  // MEDIA_AUDIO_AUDIO_INPUT_STREAM_IMPL_H_