Replace AudioSilenceDetector with an AudioPowerMonitor.

media/audio/audio_power_monitor.h

Index: media/audio/audio_power_monitor.h
diff --git a/media/audio/audio_power_monitor.h b/media/audio/audio_power_monitor.h
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+// Copyright (c) 2013 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_POWER_MONITOR_H_
+#define MEDIA_AUDIO_AUDIO_POWER_MONITOR_H_
+
+#include <limits>
+
+#include "base/callback.h"
+#include "media/base/media_export.h"
+
+// An audio signal power monitor.  It is periodically provided an AudioBus by
+// the native audio thread, and the audio samples in each channel are analyzed
+// to determine the average power of the signal over a time period.  Here
+// "average power" is a running average calculated by using a first-order
+// low-pass filter over the square of the samples scanned.  Whenever reporting
+// the power level, this running average is converted to dBFS (decibels relative
+// to full-scale) units.
+//
+// Note that extreme care has been taken to make the AudioPowerMonitor::Scan()
+// method safe to be called on the native audio thread.  The code acquires no
+// locks, nor engages in any operation that could result in an
+// undetermined/unbounded amount of run-time.
+
+namespace base {
+class MessageLoop;
+class TimeDelta;
+}
+
+namespace media {
+
+class AudioBus;
+
+class MEDIA_EXPORT AudioPowerMonitor {
+ public:
+  // Reports power level in terms of dBFS (see zero_power() and max_power()
+  // below).  |clipped| is true if any *one* sample exceeded maximum amplitude
+  // since the last invocation.
+  typedef base::Callback<void(float power_dbfs, bool clipped)>
+      PowerMeasurementCallback;
+
+  // |sample_rate| is the audio signal sample rate (Hz).  |time_constant|
+  // characterizes how samples are averaged over time to determine the power
+  // level; and is the amount of time it takes a zero power level to increase to
+  // ~63.2% of maximum given a step input signal.  |measurement_period| is the
+  // time length of signal to analyze before invoking the callback to report the
+  // current power level.  |message_loop| is where the |callback| task will be
+  // posted.
+  AudioPowerMonitor(int sample_rate,
+                    const base::TimeDelta& time_constant,
+                    const base::TimeDelta& measurement_period,
+                    base::MessageLoop* message_loop,
+                    const PowerMeasurementCallback& callback);
+
+  ~AudioPowerMonitor();
+
+  // Scan more |frames| of audio data from |buffer|.  It is safe to call this
+  // from a real-time priority thread.
+  void Scan(const AudioBus& buffer, int frames);
+
+  // dBFS value corresponding to zero power in the audio signal.
+  static float zero_power() { return -std::numeric_limits<float>::infinity(); }
+
+  // dBFS value corresponding to maximum power in the audio signal.
+  static float max_power() { return 0.0f; }
+
+ private:
+  // The weight applied when averaging-in each sample.  Computed from the
+  // |sample_rate| and |time_constant|.
+  const float sample_weight_;
+
+  // Number of audio frames to be scanned before reporting the current power
+  // level via callback, as computed from |sample_rate| and
+  // |measurement_period|.
+  const int num_frames_per_callback_;
+
+  // MessageLoop and callback used to notify of the current power level.
+  base::MessageLoop* const message_loop_;
+  const PowerMeasurementCallback power_level_callback_;
+
+  // Accumulated results over one or more calls to Scan().
+  float average_power_;
+  bool clipped_since_last_notification_;
+  int frames_since_last_notification_;
+
+  // Keep track of last reported results to forgo making redundant callbacks.
+  float last_reported_power_;
+  bool last_reported_clipped_;
+
+  DISALLOW_COPY_AND_ASSIGN(AudioPowerMonitor);
+};
+
+}  // namespace media
+
+#endif  // MEDIA_AUDIO_AUDIO_POWER_MONITOR_H_