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// Copyright 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.
#include <limits>
#include <utility>
#include "base/callback.h"
#include "base/synchronization/lock.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 TimeDelta;
namespace media {
class AudioBus;
class MEDIA_EXPORT AudioPowerMonitor {
// |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.
AudioPowerMonitor(int sample_rate, const base::TimeDelta& time_constant);
// Reset power monitor to initial state (zero power level). This should not
// be called while another thread is scanning.
void Reset();
// 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);
// Returns the current power level in dBFS and clip status. Clip status is
// true whenever any *one* sample scanned exceeded maximum amplitude since
// this method's last invocation. It is safe to call this method from any
// thread.
std::pair<float, bool> ReadCurrentPowerAndClip();
// 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; }
// The weight applied when averaging-in each sample. Computed from the
// |sample_rate| and |time_constant|.
const float sample_weight_;
// Accumulated results over one or more calls to Scan(). These should only be
// touched by the thread invoking Scan().
float average_power_;
bool has_clipped_;
// Copies of power and clip status, used to deliver results synchronously
// across threads.
base::Lock reading_lock_;
float power_reading_;
bool clipped_reading_;
} // namespace media