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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_SILENCE_DETECTOR_H_
#define MEDIA_AUDIO_AUDIO_SILENCE_DETECTOR_H_
#include "base/atomic_ref_count.h"
#include "base/callback.h"
#include "base/memory/scoped_ptr.h"
#include "base/threading/thread_checker.h"
#include "base/timer.h"
#include "media/base/media_export.h"
// An audio silence detector. It is periodically provided an AudioBus by the
// native audio thread, where simple logic determines whether the audio samples
// in each channel in the buffer represent silence. If a long-enough period of
// contiguous silence is observed in all channels, a notification callback is
// run on the thread that constructed AudioSilenceDetector.
//
// Note that extreme care has been taken to make the
// AudioSilenceDetector::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. Comments in
// audio_silence_detector.cc elaborate further on the silence detection
// algorithm.
namespace base {
class TimeDelta;
}
namespace media {
class AudioBus;
class MEDIA_EXPORT AudioSilenceDetector {
public:
typedef base::Callback<void(bool)> AudibleCallback;
// Tunable parameters: |questionable_silence_period| is the amount of time
// where audio must remain silent before triggerring a callback.
// |indistinguishable_silence_threshold| is the value range below which audio
// is considered silent, in full-scale units.
//
// TODO(miu): |indistinguishable_silence_threshold| should be specified in
// dbFS units rather than full-scale. We need a dbFS data type for
// media/audio first.
AudioSilenceDetector(int sample_rate,
const base::TimeDelta& questionable_silence_period,
float indistinguishable_silence_threshold);
~AudioSilenceDetector();
// Start detecting silence, notifying via the given callback.
void Start(const AudibleCallback& notify_is_audible);
// Stop detecting silence. If |notify_ending_in_silence| is true, a final
// notify_is_audible(false) call will be made here.
void Stop(bool notify_ending_in_silence);
// Scan more |frames| of audio data from |buffer|. This is usually called
// within the native audio thread's "more data" callback.
void Scan(const AudioBus* buffer, int frames);
private:
// Called by |poll_timer_| at regular intervals to determine whether to invoke
// the callback due to a silence state change.
void MaybeInvokeAudibleCallback();
// Returns true if the first |num_frames| frames in all channels in |buffer|
// probably contain silence. A simple heuristic is used to quickly examine a
// subset of the samples in each channel, hence the name of this method.
// "Silence" means that the range of the sample values examined does not
// exceed |silence_threshold_|.
bool ProbablyContainsSilence(const AudioBus* buffer, int num_frames);
// Time between polls for changes in state.
const base::TimeDelta polling_period_;
// Number of frames of contiguous silence to be observed before setting
// |observing_silence_| to false.
const int frames_before_observing_silence_;
// Threshold below which audio should be considered indistinguishably silent.
const float silence_threshold_;
// Number of frames of contiguous silence observed thus far on the native
// audio thread.
int frames_silent_so_far_;
// Boolean state (0 or 1) set by the native audio thread. This is polled
// regularly by the thread that invokes the callback.
base::AtomicRefCount observing_silence_;
// Callback for notifying of a detected transition to silence or non-silence.
AudibleCallback notify_is_audible_;
// Last reported audible state, used for de-duping callback invocations.
bool was_audible_;
// Fires regularly, calling MaybeInvokeAudibleCallback().
base::RepeatingTimer<AudioSilenceDetector> poll_timer_;
// Constructor, destructor and most methods must be called on the same thread.
base::ThreadChecker thread_checker_;
DISALLOW_COPY_AND_ASSIGN(AudioSilenceDetector);
};
} // namespace media
#endif // MEDIA_AUDIO_AUDIO_SILENCE_DETECTOR_H_
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