Adding an option to disable audio PTS adjustment on sender reports (#721)

API consumers should use this option when the media processing pipeline needs stable - monotonically increasing PTS sequence - small adjustments coming from RTCP sender reports could cause gaps in the audio. Media processing pipeline could opt out of auto PTS adjustments and handle the gap by e.g modifying tempo to compensate instead

Author: milos-lk

pkg/synchronizer/synchronizer.go

@@ -29,8 +29,9 @@ type SynchronizerOption func(*SynchronizerConfig)
 
 // SynchronizerConfig holds configuration for the Synchronizer
 type SynchronizerConfig struct {
-	MaxTsDiff time.Duration
-	OnStarted func()
+	MaxTsDiff                  time.Duration
+	OnStarted                  func()
+	AudioPTSAdjustmentDisabled bool
 }
 
 // WithMaxTsDiff sets the maximum acceptable difference between RTP packets
@@ -48,6 +49,17 @@ func WithOnStarted(onStarted func()) SynchronizerOption {
 	}
 }
 
+// WithAudioPTSAdjustmentDisabled - disables auto PTS adjustments after sender reports
+// Use case: when media processing pipeline needs stable - monotonically increasing
+// PTS sequence - small adjustments coming from RTCP sender reports could cause gaps in the audio
+// Media processing pipeline could opt out of auto PTS adjustments and handle the gap
+// by e.g modifying tempo to compensate instead
+func WithAudioPTSAdjustmentDisabled() SynchronizerOption {
+	return func(config *SynchronizerConfig) {
+		config.AudioPTSAdjustmentDisabled = true
+	}
+}
+
 // a single Synchronizer is shared between all audio and video writers
 type Synchronizer struct {
 	sync.RWMutex

pkg/synchronizer/track.go

@@ -49,7 +49,8 @@ type TrackSynchronizer struct {
 	*rtpConverter
 
 	// config
-	maxTsDiff time.Duration // maximum acceptable difference between RTP packets
+	maxTsDiff                   time.Duration // maximum acceptable difference between RTP packets
+	audioPTSAdjustmentsDisabled bool          // disable audio packets PTS adjustments on SRs
 
 	// timing info
 	startTime       time.Time     // time first packet was pushed
@@ -71,11 +72,12 @@ type TrackSynchronizer struct {
 
 func newTrackSynchronizer(s *Synchronizer, track TrackRemote) *TrackSynchronizer {
 	t := &TrackSynchronizer{
-		sync:         s,
-		track:        track,
-		logger:       logger.GetLogger().WithValues("trackID", track.ID(), "codec", track.Codec().MimeType),
-		rtpConverter: newRTPConverter(int64(track.Codec().ClockRate)),
-		maxTsDiff:    s.config.MaxTsDiff,
+		sync:                        s,
+		track:                       track,
+		logger:                      logger.GetLogger().WithValues("trackID", track.ID(), "codec", track.Codec().MimeType),
+		rtpConverter:                newRTPConverter(int64(track.Codec().ClockRate)),
+		maxTsDiff:                   s.config.MaxTsDiff,
+		audioPTSAdjustmentsDisabled: s.config.AudioPTSAdjustmentDisabled,
 	}
 
 	return t
@@ -128,11 +130,14 @@ func (t *TrackSynchronizer) GetPTS(pkt *rtp.Packet) (time.Duration, error) {
 		t.startRTP = ts - t.toRTP(pts)
 	}
 
-	if t.currentPTSOffset > t.desiredPTSOffset {
-		t.currentPTSOffset = max(t.currentPTSOffset-maxAdjustment, t.desiredPTSOffset)
-	} else if t.currentPTSOffset < t.desiredPTSOffset {
-		t.currentPTSOffset = min(t.currentPTSOffset+maxAdjustment, t.desiredPTSOffset)
+	if t.shouldAdjustPTS() {
+		if t.currentPTSOffset > t.desiredPTSOffset {
+			t.currentPTSOffset = max(t.currentPTSOffset-maxAdjustment, t.desiredPTSOffset)
+		} else if t.currentPTSOffset < t.desiredPTSOffset {
+			t.currentPTSOffset = min(t.currentPTSOffset+maxAdjustment, t.desiredPTSOffset)
+		}
 	}
+
 	adjusted := pts + t.currentPTSOffset
 
 	// if past end time, return EOF
@@ -184,6 +189,14 @@ func (t *TrackSynchronizer) acceptable(d time.Duration) bool {
 	return d > -t.maxTsDiff && d < t.maxTsDiff
 }
 
+func (t *TrackSynchronizer) shouldAdjustPTS() bool {
+	adjustmentEnabled := true
+	if t.track.Kind() == webrtc.RTPCodecTypeAudio {
+		adjustmentEnabled = !t.audioPTSAdjustmentsDisabled
+	}
+	return adjustmentEnabled && (t.currentPTSOffset != t.desiredPTSOffset)
+}
+
 type rtpConverter struct {
 	ts  uint64
 	rtp uint64