Adds volume level measurements to the AudioInputController for low-latency clients

media/audio/audio_input_controller.h

Index: media/audio/audio_input_controller.h
diff --git a/media/audio/audio_input_controller.h b/media/audio/audio_input_controller.h
index 3facfb511d18670a4a6d2ce0e0f19071099fe0e8..2d266e083f12a931e70fd0554fc29954cb02e4b1 100644
--- a/media/audio/audio_input_controller.h
+++ b/media/audio/audio_input_controller.h
@@ -16,6 +16,9 @@
 #include "base/timer/timer.h"
 #include "media/audio/audio_io.h"
 #include "media/audio/audio_manager_base.h"
+#include "media/audio/audio_parameters.h"
+#include "media/audio/audio_power_monitor.h"
+#include "media/base/audio_bus.h"
 
 // An AudioInputController controls an AudioInputStream and records data
 // from this input stream. The two main methods are Record() and Close() and
@@ -309,6 +312,13 @@ class MEDIA_EXPORT AudioInputController
 
   UserInputMonitor* user_input_monitor_;
 
+  // Scans audio samples from OnData() as input to compute audio levels.
+  scoped_ptr<AudioPowerMonitor> audio_level_;

[tommi (sloooow) - chröme, 2014/05/17 13:45:00]

Is computing the average energy levels vs the per packet energy levels the right
approach?
Will this for example report 0 (or the dbfs equivalent of no signal), for some
non-0 frames in a data packet?

I think this is probably the right way to measure audio levels, but I'm
wondering if we get false positives with this approach when we want to detect a
hardware muted mic or no signal at all.

[henrika (OOO until Aug 14), 2014/05/19 11:58:51]

I selected the existing AudioPowerMonitor since it is used in Chrome already and
seems to be well optimized for most platforms.

It takes some time for the monitor to generate the min value but we are only
talking about milliseconds and I can modify the time constant to use a smoothing
factor of almost 1 which is the same as not taking old samples into account.
E.g. y(n)=alpha*x(n) + (1-alpha)*y(n-1). Today alpha is about 0.02 (I think).

In any case, the tests I've done shows that the output results in "0" very
quickly when I mute a mic. 

+
+  scoped_ptr<AudioBus> audio_bus_;
+  base::TimeTicks last_audio_level_log_time_;
+  media::AudioParameters audio_params_;
+
   size_t prev_key_down_count_;
 
   DISALLOW_COPY_AND_ASSIGN(AudioInputController);