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..81f4c13077be630d7d15022dad7130bd4655bde7 100644
--- a/webrtc/voice_engine/channel.cc
+++ b/webrtc/voice_engine/channel.cc
@@ -1304,7 +1304,8 @@ int32_t Channel::SetSendCodec(const CodecInst& codec) {
   return 0;
 }
 
-void Channel::SetBitRate(int bitrate_bps) {
+void Channel::SetBitRate(int bitrate_bps,
+                         rtc::Optional<int64_t> 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) {
@@ -1315,17 +1316,22 @@ 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);
-  bitrate_smoother_.AddSample(bitrate_bps);
-  audio_coding_->ModifyEncoder([&](std::unique_ptr<AudioEncoder>* encoder) {
-    if (*encoder) {
-      (*encoder)->OnReceivedUplinkBandwidth(
-          static_cast<int>(*bitrate_smoother_.GetAverage()));
-    }
-  });
+  // The probing spikes should not affect the bitrate smoother more than 25%.
+  // To simplify the calculations we use a step response as input signal.
+  // The calculation show a step response for an
+  // exponential filter (bitrate_smoother use an exponential filter) with the
+  // length of probing_interval and an amplitude of 1.
+  // 0.22 = 1 - e^(-t/probing_interval * 4) | t = probing_interval.
+  if (probing_interval_ms) {
+    bitrate_smoother_.SetTimeConstantMs(*probing_interval_ms * 4);
+    bitrate_smoother_.AddSample(bitrate_bps);
+    audio_coding_->ModifyEncoder([&](std::unique_ptr<AudioEncoder>* encoder) {
+      if (*encoder) {
+        (*encoder)->OnReceivedUplinkBandwidth(
+            static_cast<int>(*bitrate_smoother_.GetAverage()));
+      }
+    });
+  }
 }
 
 void Channel::OnIncomingFractionLoss(int fraction_lost) {