Pass time constant to bwe smoothing filter.

webrtc/voice_engine/channel.cc

Index: webrtc/voice_engine/channel.cc
diff --git a/webrtc/voice_engine/channel.cc b/webrtc/voice_engine/channel.cc
index f3021fef6fec31f8e727fa3698158628dc1d89ee..93b129eeba79c478d6893d44ba0c559f1cfc0080 100644
--- a/webrtc/voice_engine/channel.cc
+++ b/webrtc/voice_engine/channel.cc
@@ -1304,7 +1304,7 @@ int32_t Channel::SetSendCodec(const CodecInst& codec) {
   return 0;
 }
 
-void Channel::SetBitRate(int bitrate_bps) {
+void Channel::SetBitRate(int bitrate_bps, int probing_interval_ms) {
   WEBRTC_TRACE(kTraceInfo, kTraceVoice, VoEId(_instanceId, _channelId),
                "Channel::SetBitRate(bitrate_bps=%d)", bitrate_bps);
   audio_coding_->ModifyEncoder([&](std::unique_ptr<AudioEncoder>* encoder) {
@@ -1315,10 +1315,9 @@ void Channel::SetBitRate(int bitrate_bps) {
 
   // We give smoothed bitrate allocation to audio network adaptor as
   // the uplink bandwidth.
-  // TODO(michaelt) : Remove kDefaultBitrateSmoothingTimeConstantMs as soon as
-  // we pass the probing interval to this function.
-  constexpr int64_t kDefaultBitrateSmoothingTimeConstantMs = 20000;
-  bitrate_smoother_.SetTimeConstantMs(kDefaultBitrateSmoothingTimeConstantMs);
+  // The probing spikes should not affect the bitrate smoother more than 25%.
+  // 0.22 = 1 - e^(-t/probing_interval * 4) | t = probing_interval.

[stefan-webrtc, 2016/11/22 16:39:05]

This comment needs to be expanded, as I don't understand it... :)

Is it describing the implementation of the smoother?

[michaelt, 2016/11/23 12:28:29]

Yes. I tried to to show how the filter would react to a step response. 
Since its a exponential filter, the formula shows which value we would reach on
a sept response of the length of the probing interval. 

Will try to make this more clear in the comment.

[minyue-webrtc, 2016/11/28 12:38:20]

It may be clearer to say something like the following.

The step response of an exponential filter is
u(t) = 1 - e^(-t / time_constant).
In order to limit the affect of BWE spikes within 25% before the next probing,
we would choose a time constant that fufills
1 - e^(-probing_interval / time_constant) < 0.25
Then 4 * probing_interval is a good choice.

[michaelt, 2016/11/28 13:03:37]

Done according to offline discussion. 

+  bitrate_smoother_.SetTimeConstantMs(probing_interval_ms * 4);
   bitrate_smoother_.AddSample(bitrate_bps);
   audio_coding_->ModifyEncoder([&](std::unique_ptr<AudioEncoder>* encoder) {
     if (*encoder) {