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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.
#include "base/logging.h"
#include "base/memory/scoped_ptr.h"
#include "base/synchronization/lock.h"
#include "base/threading/thread_checker.h"
#include "base/timer/timer.h"
#include "media/audio/audio_io.h"
// The template based AgcAudioStream implements platform-independent parts
// of the AudioInterface interface. Supported interfaces to pass as
// AudioInterface are AudioIntputStream and AudioOutputStream. Each platform-
// dependent implementation should derive from this class.
// Usage example (on Windows):
// class WASAPIAudioInputStream : public AgcAudioStream<AudioInputStream> {
// public:
// WASAPIAudioInputStream();
// ...
// };
// Call flow example:
// 1) User creates AgcAudioStream<AudioInputStream>
// 2) User calls AudioInputStream::SetAutomaticGainControl(true) =>
// AGC usage is now initialized but not yet started.
// 3) User calls AudioInputStream::Start() => implementation calls
// AgcAudioStream<AudioInputStream>::StartAgc() which detects that AGC
// is enabled and then starts the periodic AGC timer.
// 4) Microphone volume samples are now taken and included in all
// AudioInputCallback::OnData() callbacks.
// 5) User calls AudioInputStream::Stop() => implementation calls
// AgcAudioStream<AudioInputStream>::StopAgc() which stops the timer.
// Note that, calling AudioInputStream::SetAutomaticGainControl(false) while
// AGC measurements are active will not have an effect until StopAgc(),
// StartAgc() are called again since SetAutomaticGainControl() only sets a
// a state.
// Calling SetAutomaticGainControl(true) enables the AGC and StartAgc() starts
// a periodic timer which calls QueryAndStoreNewMicrophoneVolume()
// approximately once every second. QueryAndStoreNewMicrophoneVolume() asks
// the actual microphone about its current volume level. This value is
// normalized and stored so it can be read by GetAgcVolume() when the real-time
// audio thread needs the value. The main idea behind this scheme is to avoid
// accessing the audio hardware from the real-time audio thread and to ensure
// that we don't take new microphone-level samples too often (~1 Hz is a
// suitable compromise). The timer will be active until StopAgc() is called.
// This class should be created and destroyed on the audio manager thread and
// a thread checker is added to ensure that this is the case (uses DCHECK).
// All methods except GetAgcVolume() should be called on the creating thread
// as well to ensure that thread safety is maintained. It will also guarantee
// that the periodic timer runs on the audio manager thread.
// |normalized_volume_|, which is updated by QueryAndStoreNewMicrophoneVolume()
// and read in GetAgcVolume(), is protected by a lock to ensure that it can
// be accessed from any real-time audio thread that needs it to update the its
// AGC volume.
namespace media {
template <typename AudioInterface>
class MEDIA_EXPORT AgcAudioStream : public AudioInterface {
// Time between two successive timer events.
static const int kIntervalBetweenVolumeUpdatesMs = 1000;
: agc_is_enabled_(false), max_volume_(0.0), normalized_volume_(0.0) {
virtual ~AgcAudioStream() {
// Starts the periodic timer which periodically checks and updates the
// current microphone volume level.
// The timer is only started if AGC mode is first enabled using the
// SetAutomaticGainControl() method.
void StartAgc() {
DVLOG(1) << "StartAgc()";
if (!agc_is_enabled_ || timer_.IsRunning())
// Query and cache the volume to avoid sending 0 as volume to AGC at the
// beginning of the audio stream, otherwise AGC will try to raise the
// volume from 0.
this, &AgcAudioStream::QueryAndStoreNewMicrophoneVolume);
// Stops the periodic timer which periodically checks and updates the
// current microphone volume level.
void StopAgc() {
DVLOG(1) << "StopAgc()";
if (timer_.IsRunning())
// Stores a new microphone volume level by checking the audio input device.
// Called on the audio manager thread.
void UpdateAgcVolume() {
if (!timer_.IsRunning())
// We take new volume samples once every second when the AGC is enabled.
// To ensure that a new setting has an immediate effect, the new volume
// setting is cached here. It will ensure that the next OnData() callback
// will contain a new valid volume level. If this approach was not taken,
// we could report invalid volume levels to the client for a time period
// of up to one second.
// Gets the latest stored volume level if AGC is enabled.
// Called at each capture callback on a real-time capture thread (platform
// dependent).
void GetAgcVolume(double* normalized_volume) {
base::AutoLock lock(lock_);
*normalized_volume = normalized_volume_;
// Sets the automatic gain control (AGC) to on or off. When AGC is enabled,
// the microphone volume is queried periodically and the volume level can
// be read in each AudioInputCallback::OnData() callback and fed to the
// render-side AGC. User must call StartAgc() as well to start measuring
// the microphone level.
virtual void SetAutomaticGainControl(bool enabled) OVERRIDE {
DVLOG(1) << "SetAutomaticGainControl(enabled=" << enabled << ")";
agc_is_enabled_ = enabled;
// Gets the current automatic gain control state.
virtual bool GetAutomaticGainControl() OVERRIDE {
return agc_is_enabled_;
// Takes a new microphone volume sample and stores it in |normalized_volume_|.
// Range is normalized to [0.0,1.0] or [0.0, 1.5] on Linux.
// This method is called periodically when AGC is enabled and always on the
// audio manager thread. We use it to read the current microphone level and
// to store it so it can be read by the main capture thread. By using this
// approach, we can avoid accessing audio hardware from a real-time audio
// thread and it leads to a more stable capture performance.
void QueryAndStoreNewMicrophoneVolume() {
// Cach the maximum volume if this is the first time we ask for it.
if (max_volume_ == 0.0)
max_volume_ = static_cast<AudioInterface*>(this)->GetMaxVolume();
// Retrieve the current volume level by asking the audio hardware.
// Range is normalized to [0.0,1.0] or [0.0, 1.5] on Linux.
if (max_volume_ != 0.0) {
double normalized_volume =
static_cast<AudioInterface*>(this)->GetVolume() / max_volume_;
base::AutoLock auto_lock(lock_);
normalized_volume_ = normalized_volume;
// Ensures that this class is created and destroyed on the same thread.
base::ThreadChecker thread_checker_;
// Repeating timer which cancels itself when it goes out of scope.
// Used to check the microphone volume periodically.
base::RepeatingTimer<AgcAudioStream<AudioInterface> > timer_;
// True when automatic gain control is enabled, false otherwise.
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
// by not querying the capture volume for each callback. Guarded by |lock_|.
// The range is normalized to [0.0, 1.0].
double normalized_volume_;
// Protects |normalized_volume_| .
base::Lock lock_;
} // namespace media