Skip RTP padding packets instead of concealing them

Padding/probe packets use a sequence number but carry no audio. They were
treated as lost, so each one played an invented 20 ms frame (a glitch) and
pushed the buffer a frame deeper for drift correction to remove again.
Signature in the first live test: lost ~= drift-drops (74 vs 72 per minute).
They are now skipped and counted as 'padding'.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

call.go

@@ -266,8 +266,8 @@ func (c *Call) Close() {
 		c.jbMu.Unlock()
 		if jb != nil {
 			k, _, t := jb.snapshot()
-			log.Printf("call totals: lost %d (fec %d) concealed %d late %d underruns %d drift-drops %d inserted %d, final target %d ms",
-				k.Lost, k.FEC, k.PLC, k.Late, k.Underruns, k.Dropped, k.Inserted, t)
+			log.Printf("call totals: lost %d (fec %d) concealed %d late %d underruns %d drift-drops %d inserted %d padding %d, final target %d ms",
+				k.Lost, k.FEC, k.PLC, k.Late, k.Underruns, k.Dropped, k.Inserted, k.Padding, t)
 		}
 	})
 }
@@ -285,8 +285,8 @@ func (c *Call) logRx(jb *jitterBuffer) {
 		case <-t.C:
 		}
 		k, d, tg := jb.snapshot()
-		log.Printf("rx 1m: buffer %d/%d ms, lost %d (fec %d), concealed %d, late %d, underruns %d, drift-drops %d, inserted %d",
-			d, tg, k.Lost-prev.Lost, k.FEC-prev.FEC, k.PLC-prev.PLC, k.Late-prev.Late, k.Underruns-prev.Underruns, k.Dropped-prev.Dropped, k.Inserted-prev.Inserted)
+		log.Printf("rx 1m: buffer %d/%d ms, lost %d (fec %d), concealed %d, late %d, underruns %d, drift-drops %d, inserted %d, padding %d",
+			d, tg, k.Lost-prev.Lost, k.FEC-prev.FEC, k.PLC-prev.PLC, k.Late-prev.Late, k.Underruns-prev.Underruns, k.Dropped-prev.Dropped, k.Inserted-prev.Inserted, k.Padding-prev.Padding)
 		prev = k
 	}
 }
@@ -411,7 +411,7 @@ func (c *Call) startPlay(t *webrtc.TrackRemote) {
 		log.Printf("decoder: %v", err)
 		return
 	}
-	jb := newJitterBuffer(dec)
+	jb := newJitterBuffer(dec, uint8(t.PayloadType()))
 	c.jbMu.Lock()
 	c.jb = jb
 	c.jbMu.Unlock()
@@ -529,6 +529,8 @@ type jitterBuffer struct {
 	mu        sync.Mutex
 	dec       *opus.Decoder
 	pkts      map[uint16][]byte
+	skip      map[uint16]bool // sequence numbers that carry no audio (padding/probes)
+	pt        uint8           // the negotiated Opus payload type
 	nextSeq   uint16
 	haveSeq   bool // nextSeq is meaningful
 	playing   bool // false = filling up to target
@@ -545,6 +547,7 @@ type jitterBuffer struct {
 
 type jbCounters struct {
 	Lost, FEC, PLC, Dropped, Inserted, Underruns, Late int
+	Padding                                            int // packets with no audio (skipped, not concealed)
 }
 
 const (
@@ -567,14 +570,15 @@ func (j *jitterBuffer) isQuiet(pcm []int16) bool {
 	return r < lim
 }
 
-func newJitterBuffer(dec *opus.Decoder) *jitterBuffer {
-	return &jitterBuffer{dec: dec, pkts: map[uint16][]byte{}, target: jbFloor, pcm: make([]int16, 5760*channels), lastBad: time.Now(), floor: quietRMS}
+func newJitterBuffer(dec *opus.Decoder, pt uint8) *jitterBuffer {
+	return &jitterBuffer{dec: dec, pt: pt, pkts: map[uint16][]byte{}, skip: map[uint16]bool{}, target: jbFloor, pcm: make([]int16, 5760*channels), lastBad: time.Now(), floor: quietRMS}
 }
 
 func seqLess(a, b uint16) bool { return int16(a-b) < 0 }
 
 func (j *jitterBuffer) reset() {
 	j.pkts = map[uint16][]byte{}
+	j.skip = map[uint16]bool{}
 	j.haveSeq, j.playing, j.lateRun, j.insertNow = false, false, 0, false
 }
 
@@ -591,7 +595,17 @@ func (j *jitterBuffer) bump() {
 func (j *jitterBuffer) push(p *rtp.Packet) {
 	j.mu.Lock()
 	defer j.mu.Unlock()
-	if len(p.Payload) == 0 {
+	if len(p.Payload) == 0 || (j.pt != 0 && p.PayloadType != j.pt) {
+		// Padding / bandwidth probe: it uses a sequence number but carries no audio and no
+		// time. v1/v2a ignored it, which left a hole that was then "concealed" - an
+		// invented 20 ms frame that the drift correction later had to remove again.
+		j.c.Padding++
+		if !j.haveSeq || !seqLess(p.SequenceNumber, j.nextSeq) {
+			if len(j.skip) > 200 {
+				j.skip = map[uint16]bool{}
+			}
+			j.skip[p.SequenceNumber] = true
+		}
 		return
 	}
 	if j.haveSeq {
@@ -695,6 +709,11 @@ func (j *jitterBuffer) next() []int16 {
 	}
 
 	for {
+		if j.skip[j.nextSeq] {
+			delete(j.skip, j.nextSeq)
+			j.nextSeq++
+			continue
+		}
 		if p, ok := j.pkts[j.nextSeq]; ok {
 			delete(j.pkts, j.nextSeq)
 			j.nextSeq++
@@ -744,6 +763,6 @@ func (j *jitterBuffer) stats() map[string]any {
 	return map[string]any{
 		"depthMs": d, "targetMs": t,
 		"lost": c.Lost, "fec": c.FEC, "plc": c.PLC, "dropped": c.Dropped, "inserted": c.Inserted,
-		"underruns": c.Underruns, "late": c.Late,
+		"underruns": c.Underruns, "late": c.Late, "padding": c.Padding,
 	}
 }