Adds Analog Gain Control (AGC) to the WebRTC client.

media/audio/mac/audio_low_latency_input_mac.cc

Index: media/audio/mac/audio_low_latency_input_mac.cc
diff --git a/media/audio/mac/audio_low_latency_input_mac.cc b/media/audio/mac/audio_low_latency_input_mac.cc
index 22b6df5ce169f78a70de6cbec04ae1071e30c54d..66423067d378691e1632a8115f5b83c4a12f9e23 100644
--- a/media/audio/mac/audio_low_latency_input_mac.cc
+++ b/media/audio/mac/audio_low_latency_input_mac.cc
@@ -12,6 +12,8 @@
 #include "media/audio/audio_util.h"
 #include "media/audio/mac/audio_manager_mac.h"
 
+static const int kMinIntervalBetweenVolumeUpdatesMs = 1000;
+
 static std::ostream& operator<<(std::ostream& os,
                                 const AudioStreamBasicDescription& format) {
   os << "sample rate       : " << format.mSampleRate << std::endl
@@ -38,7 +40,9 @@ AUAudioInputStream::AUAudioInputStream(
       input_device_id_(audio_device_id),
       started_(false),
       hardware_latency_frames_(0),
-      number_of_channels_in_frame_(0) {
+      number_of_channels_in_frame_(0),
+      volume_(0.0),
+      agc_is_enabled_(0) {
   DCHECK(manager_);
 
   // Set up the desired (output) format specified by the client.
@@ -289,7 +293,8 @@ double AUAudioInputStream::GetMaxVolume() {
 }
 
 void AUAudioInputStream::SetVolume(double volume) {
-  DCHECK(volume <= 1.0 && volume >= 0.0);
+  DCHECK_GE(volume, 0.0);
+  DCHECK_LE(volume, 1.0);
 
   // Verify that we have a valid device.
   if (input_device_id_ == kAudioObjectUnknown) {
@@ -336,6 +341,16 @@ void AUAudioInputStream::SetVolume(double volume) {
 
   DLOG_IF(WARNING, successful_channels == 0)
       << "Failed to set volume to " << volume_float32;
+
+  // 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.
+  if (GetAutomaticGainControl()) {
+    volume_ = GetVolume();
+  }
 }
 
 double AUAudioInputStream::GetVolume() {
@@ -396,6 +411,14 @@ double AUAudioInputStream::GetVolume() {
   return 0.0;
 }
 
+void AUAudioInputStream::SetAutomaticGainControl(bool enabled) {
+  base::subtle::Release_Store(&agc_is_enabled_, enabled);
+}
+
+bool AUAudioInputStream::GetAutomaticGainControl() {
+  return (base::subtle::Acquire_Load(&agc_is_enabled_) == 1);
+}
+
 // AUHAL AudioDeviceOutput unit callback
 OSStatus AUAudioInputStream::InputProc(void* user_data,
                                        AudioUnitRenderActionFlags* flags,
@@ -441,7 +464,24 @@ OSStatus AUAudioInputStream::Provide(UInt32 number_of_frames,
   if (!audio_data)
     return kAudioUnitErr_InvalidElement;
 
-  sink_->OnData(this, audio_data, buffer.mDataByteSize, capture_delay_bytes);
+  // Query a new volume level if AGC is enabled and if it is time to update
+  // the old value.
+  if (GetAutomaticGainControl()) {
+    base::Time now = base::Time::Now();
+    if ((now - last_volume_update_time_).InMilliseconds() >
+        kMinIntervalBetweenVolumeUpdatesMs) {
+      // Retrieve the current volume level. Range is [0.0,1.0].
+      volume_ = static_cast<double>(GetVolume());
+      last_volume_update_time_ = now;
+    }
+  }
+
+  // Deliver data packet, delay estimation and volume level to the user.
+  sink_->OnData(this,
+                audio_data,
+                buffer.mDataByteSize,
+                capture_delay_bytes,
+                volume_);
 
   return noErr;
 }