AGC2 dummy module: fixed gain param, APM integration, audioproc_f adaptation

webrtc/modules/audio_processing/include/audio_processing.h

Index: webrtc/modules/audio_processing/include/audio_processing.h
diff --git a/webrtc/modules/audio_processing/include/audio_processing.h b/webrtc/modules/audio_processing/include/audio_processing.h
index 4c4bc2f6d95db5b827f549ca6da4b1c5bc56778e..f5e42ae3184e858a47ec7cd0949a1979c81e5dc2 100644
--- a/webrtc/modules/audio_processing/include/audio_processing.h
+++ b/webrtc/modules/audio_processing/include/audio_processing.h
@@ -276,6 +276,8 @@ class AudioProcessing : public rtc::RefCountInterface {
     // does not yet have the desired behavior.
     struct GainController2 {
       bool enabled = false;
+      // Fixed digital gain params.
+      float fixed_gain_db = 5.f;

[peah-webrtc, 2017/08/18 04:51:06]

I think the default gain should be 0 (dB).

[aleloi, 2017/08/18 08:28:31]

Idea: make the default gain 0 (dB), but also refuse creating a GainController2
if the gain is 0 by changing the constructor or the Validate method. This is to
make it harder to use the API incorrectly: if you leave GainController2 enabled,
there is no point configuring it in a way that doesn't modify the signal. In the
spirit of making interfaces hard to use incorrectly and simple to use correctly.

The correct way to use AGC2 is to *both* enable it *and* set the constants to
something nonzero. I don't think we want users to enable the AGC without
explicitly choosing a parameter combination.

[peah-webrtc, 2017/08/18 09:12:46]

This is (or at least is intended to be) implemented such that the Validate
method should return false, but that the ApplyConfig should ensure that no
incorrect value is used by modifying incorrect config values.
I like that, as it is safe in the sense that the user ends up in a usable state.

The only way to refuse creating the GainController2 object would be for Create
to return null (or the constructor to crash). I don't think either is nice as
both will happen during runtime and complicates things for the user.

     } gain_controller2;
   };