Fix backchannel audio for xiaomi chuangmi.camera.72ac1

pkg/opus/opus.go

@@ -85,3 +85,34 @@ func parseFrames(c byte) byte {
 	}
 	return 0xFF
 }
+
+func JoinFrames(b1, b2 []byte) []byte {
+	// can't join
+	if b1[0]&0b11 != 0 || b2[0]&0b11 != 0 {
+		return append(b1, b2...)
+	}
+
+	size1, size2 := len(b1)-1, len(b2)-1
+
+	// join same sizes
+	if size1 == size2 {
+		b := make([]byte, 1+size1+size2)
+		copy(b, b1)
+		copy(b[1+size1:], b2[1:])
+		b[0] |= 0b01
+		return b
+	}
+
+	b := make([]byte, 1, 3+size1+size2)
+	b[0] = b1[0] | 0b10
+	if size1 >= 252 {
+		b0 := 252 + byte(size1)&0b11
+		b = append(b, b0, byte(size1/4)-b0)
+	} else {
+		b = append(b, byte(size1))
+	}
+
+	b = append(b, b1[1:]...)
+	b = append(b, b2[1:]...)
+	return b
+}

pkg/xiaomi/backchannel.go

@@ -4,6 +4,7 @@ import (
 	"time"
 
 	"github.com/AlexxIT/go2rtc/pkg/core"
+	"github.com/AlexxIT/go2rtc/pkg/opus"
 	"github.com/AlexxIT/go2rtc/pkg/pcm"
 	"github.com/AlexxIT/go2rtc/pkg/xiaomi/miss"
 	"github.com/pion/rtp"
@@ -45,8 +46,22 @@ func (p *Producer) AddTrack(media *core.Media, _ *core.Codec, track *core.Receiv
 			}
 		}
 	case core.CodecOpus:
-		sender.Handler = func(pkt *rtp.Packet) {
-			_ = p.client.WriteAudio(miss.CodecOPUS, pkt.Payload)
+		if p.model == "chuangmi.camera.72ac1" {
+			var buf []byte
+			sender.Handler = func(pkt *rtp.Packet) {
+				if buf == nil {
+					buf = pkt.Payload
+				} else {
+					// convert two 20ms to one 40ms
+					buf = opus.JoinFrames(buf, pkt.Payload)
+					_ = p.client.WriteAudio(miss.CodecOPUS, buf)
+					buf = nil
+				}
+			}
+		} else {
+			sender.Handler = func(pkt *rtp.Packet) {
+				_ = p.client.WriteAudio(miss.CodecOPUS, pkt.Payload)
+			}
 		}
 	}