audio_processing/aec: make delay estimator aware of starving farend buffer

webrtc/modules/audio_processing/aec/aec_core.c

Index: webrtc/modules/audio_processing/aec/aec_core.c
diff --git a/webrtc/modules/audio_processing/aec/aec_core.c b/webrtc/modules/audio_processing/aec/aec_core.c
index 16e3389cefc1662f0012b0d1a9d4c8934d3a9862..5dc5da8feec399a5824479081de4a5203191920e 100644
--- a/webrtc/modules/audio_processing/aec/aec_core.c
+++ b/webrtc/modules/audio_processing/aec/aec_core.c
@@ -857,6 +857,14 @@ static void TimeToFrequency(float time_data[PART_LEN2],
   }
 }
 
+static int MoveFarReadPtrWithoutSystemDelayUpdate(AecCore* self, int elements) {
+  WebRtc_MoveReadPtr(self->far_buf_windowed, elements);
+#ifdef WEBRTC_AEC_DEBUG_DUMP
+  WebRtc_MoveReadPtr(self->far_time_buf, elements);
+#endif
+  return WebRtc_MoveReadPtr(self->far_buf, elements);
+}
+
 static int SignalBasedDelayCorrection(AecCore* self) {
   int delay_correction = 0;
   int last_delay = -2;
@@ -897,9 +905,13 @@ static int SignalBasedDelayCorrection(AecCore* self) {
     const int do_correction = delay <= lower_bound || delay > upper_bound;
     if (do_correction == 1) {
       int available_read = (int)WebRtc_available_read(self->far_buf);
-      // Adjust w.r.t. a |shift_offset| to account for not as reliable estimates
-      // in the beginning, hence we are more conservative.
-      delay_correction = -(delay - self->shift_offset);
+      // With |shift_offset| we gradually rely on the delay estimates.  For
+      // positive delays we reduce the correction by |shift_offset| to lower the
+      // risk of pushing the AEC into a non causal state.  For negative delays
+      // we rely on the values up to a rounding error, hence compensate by 1
+      // element to make sure to push the delay into the causal region.
+      delay_correction = -delay;
+      delay_correction += delay > self->shift_offset ? self->shift_offset : 1;
       self->shift_offset--;
       self->shift_offset = (self->shift_offset <= 1 ? 1 : self->shift_offset);
       if (delay_correction > available_read - self->mult - 1) {
@@ -1715,11 +1727,7 @@ void WebRtcAec_BufferFarendPartition(AecCore* aec, const float* farend) {
 }
 
 int WebRtcAec_MoveFarReadPtr(AecCore* aec, int elements) {
-  int elements_moved = WebRtc_MoveReadPtr(aec->far_buf_windowed, elements);
-  WebRtc_MoveReadPtr(aec->far_buf, elements);
-#ifdef WEBRTC_AEC_DEBUG_DUMP
-  WebRtc_MoveReadPtr(aec->far_time_buf, elements);
-#endif
+  int elements_moved = MoveFarReadPtrWithoutSystemDelayUpdate(aec, elements);
   aec->system_delay -= elements_moved * PART_LEN;
   return elements_moved;
 }
@@ -1792,42 +1800,27 @@ void WebRtcAec_ProcessFrames(AecCore* aec,
       // which should be investigated. Maybe, allow for a non-symmetric
       // rounding, like -16.
       int move_elements = (aec->knownDelay - knownDelay - 32) / PART_LEN;
-      int moved_elements = WebRtc_MoveReadPtr(aec->far_buf, move_elements);
-      WebRtc_MoveReadPtr(aec->far_buf_windowed, move_elements);
+      int moved_elements =
+          MoveFarReadPtrWithoutSystemDelayUpdate(aec, move_elements);
       aec->knownDelay -= moved_elements * PART_LEN;
-  #ifdef WEBRTC_AEC_DEBUG_DUMP
-      WebRtc_MoveReadPtr(aec->far_time_buf, move_elements);
-  #endif
     } else {
       // 2 b) Apply signal based delay correction.
       int move_elements = SignalBasedDelayCorrection(aec);
-      int moved_elements = WebRtc_MoveReadPtr(aec->far_buf, move_elements);
-      WebRtc_MoveReadPtr(aec->far_buf_windowed, move_elements);
-  #ifdef WEBRTC_AEC_DEBUG_DUMP
-      WebRtc_MoveReadPtr(aec->far_time_buf, move_elements);
-  #endif
+      int moved_elements =
+          MoveFarReadPtrWithoutSystemDelayUpdate(aec, move_elements);
+      int far_near_buffer_diff = WebRtc_available_read(aec->far_buf) -
+          WebRtc_available_read(aec->nearFrBuf) / PART_LEN;
       WebRtc_SoftResetDelayEstimator(aec->delay_estimator, moved_elements);
       WebRtc_SoftResetDelayEstimatorFarend(aec->delay_estimator_farend,
                                            moved_elements);
       aec->signal_delay_correction += moved_elements;
-      // TODO(bjornv): Investigate if this is reasonable.  I had to add this
-      // guard when the signal based delay correction replaces the system based
-      // one. Otherwise there was a buffer underrun in the "qa-new/01/"
-      // recording when adding 44 ms extra delay.  This was not seen if we kept
-      // both delay correction algorithms running in parallel.
-      // A first investigation showed that we have a drift in this case that
-      // causes the buffer underrun.  Compared to when delay correction was
-      // turned off, we get buffer underrun as well which was triggered in 1)
-      // above.  In addition there was a shift in |knownDelay| later increasing
-      // the buffer.  When running in parallel, this if statement was not
-      // triggered.  This suggests two alternatives; (a) use both algorithms, or
-      // (b) allow for smaller delay corrections when we operate close to the
-      // buffer limit.  At the time of testing we required a change of 6 blocks,
-      // but could change it to, e.g., 2 blocks. It requires some testing
-      // though.
-      if ((int)WebRtc_available_read(aec->far_buf) < (aec->mult + 1)) {
-        // We don't have enough data so we stuff the far-end buffers.
-        WebRtcAec_MoveFarReadPtr(aec, -(aec->mult + 1));
+      // If we rely on reported system delay values only, a buffer underrun here
+      // can never occur since we've taken care of that in 1) above.  Here, we
+      // apply signal based delay correction and can therefore end up with
+      // buffer underruns since the delay estimation can be wrong.  We therefore
+      // stuff the buffer with enough elements if needed.
+      if (far_near_buffer_diff < 0) {
+        WebRtcAec_MoveFarReadPtr(aec, far_near_buffer_diff);
       }
     }