Bug 1667728 - Keep track of the output time by counting frames rather than accumulating packets durations in microseconds. r=bryce

There should be no accumulating rounding error here since packet durations are
exactly 200ms which does not lead to a rounding error when converting to
microseconds.

But this is for sanity, since the behavior prior to this patch is exactly how
you get an accumulating rounding error.

Differential Revision: https://phabricator.services.mozilla.com/D91955

dom/media/encoder/OpusTrackEncoder.cpp

@@ -130,7 +130,8 @@ OpusTrackEncoder::OpusTrackEncoder(TrackRate aTrackRate)
                                                           : kOpusSamplingRate),
       mEncoder(nullptr),
       mLookahead(0),
-      mResampler(nullptr) {}
+      mResampler(nullptr),
+      mNumOutputFrames(0) {}
 
 OpusTrackEncoder::~OpusTrackEncoder() {
   if (mEncoder) {
@@ -190,9 +191,6 @@ nsresult OpusTrackEncoder::Init(int aChannels) {
     return NS_ERROR_FAILURE;
   }
 
-  // Calculate offset in microseconds
-  mCodecDelay = FramesToTimeUnit(mLookahead, mOutputSampleRate);
-
   SetInitialized();
 
   return NS_OK;
@@ -442,12 +440,13 @@ nsresult OpusTrackEncoder::GetEncodedTrack(
 
     // timestamp should be the time of the first sample
     aData.AppendElement(MakeRefPtr<EncodedFrame>(
-        mOutputTimeStamp + mCodecDelay, duration, kOpusSamplingRate,
-        EncodedFrame::OPUS_AUDIO_FRAME, std::move(frameData)));
+        FramesToTimeUnit(mNumOutputFrames + mLookahead, mOutputSampleRate),
+        duration, kOpusSamplingRate, EncodedFrame::OPUS_AUDIO_FRAME,
+        std::move(frameData)));
 
-    mOutputTimeStamp +=
-        FramesToTimeUnit(NumOutputFramesPerPacket(), kOpusSamplingRate);
-    LOG("[Opus] mOutputTimeStamp %.3f.", mOutputTimeStamp.ToSeconds());
+    mNumOutputFrames += NumOutputFramesPerPacket();
+    LOG("[Opus] mOutputTimeStamp %.3f.",
+        FramesToTimeUnit(mNumOutputFrames, mOutputSampleRate).ToSeconds());
   }
 
   return result >= 0 ? NS_OK : NS_ERROR_FAILURE;

dom/media/encoder/OpusTrackEncoder.h

@@ -107,7 +107,10 @@ class OpusTrackEncoder : public AudioTrackEncoder {
    */
   nsTArray<AudioDataValue> mResampledLeftover;
 
-  media::TimeUnit mOutputTimeStamp;
+  /**
+   * Number of audio frames encoded, in kOpusSamplingRate.
+   */
+  uint64_t mNumOutputFrames;
 };
 
 }  // namespace mozilla