webrtc: fix reading Opus stereo tracks with Chrome (#2043)

internal/core/webrtc_manager.go

@@ -88,7 +88,7 @@ var audioCodecs = []webrtc.RTPCodecParameters{
 			MimeType:    webrtc.MimeTypeOpus,
 			ClockRate:   48000,
 			Channels:    2,
-			SDPFmtpLine: "minptime=10;useinbandfec=1",
+			SDPFmtpLine: "minptime=10;useinbandfec=1;stereo=1;sprop-stereo=1",
 		},
 		PayloadType: 111,
 	},

internal/core/webrtc_publish_index.html

@@ -276,7 +276,7 @@
 };
 
 const editAnswer = (answer, videoCodec, audioCodec, videoBitrate, audioBitrate, audioVoice) => {
-    const sections = answer.split('m=');
+    const sections = answer.sdp.split('m=');
 
     for (let i = 0; i < sections.length; i++) {
         const section = sections[i];
@@ -287,7 +287,7 @@
         }
     }
 
-    return sections.join('m=');
+    answer.sdp = sections.join('m=');
 };
 
 class Transmitter {
@@ -377,19 +377,16 @@
 			return;
 		}
 
-        answer = new RTCSessionDescription({
-            type: 'answer',
-            sdp: editAnswer(
-                answer.sdp,
-                document.getElementById('video_codec').value,
-                document.getElementById('audio_codec').value,
-                document.getElementById('video_bitrate').value,
-                document.getElementById('audio_bitrate').value,
-                document.getElementById('audio_voice').value,
-            ),
-        });
+        editAnswer(
+            answer,
+            document.getElementById('video_codec').value,
+            document.getElementById('audio_codec').value,
+            document.getElementById('video_bitrate').value,
+            document.getElementById('audio_bitrate').value,
+            document.getElementById('audio_voice').value,
+        );
 
-        this.pc.setRemoteDescription(new RTCSessionDescription(answer));
+        this.pc.setRemoteDescription(answer);
 
         if (this.queuedCandidates.length !== 0) {
             this.sendLocalCandidates(this.queuedCandidates);

internal/core/webrtc_read_index.html

@@ -64,6 +64,48 @@
     return ret;
 };
 
+const enableStereoOpus = (section) => {
+    let opusPayloadFormat = '';
+    let lines = section.split('\r\n');
+
+    for (let i = 0; i < lines.length; i++) {
+        if (lines[i].startsWith('a=rtpmap:') && lines[i].toLowerCase().includes('opus/')) {
+            opusPayloadFormat = lines[i].slice('a=rtpmap:'.length).split(' ')[0];
+            break;
+        }
+    }
+
+    if (opusPayloadFormat === '') {
+        return section;
+    }
+
+    for (let i = 0; i < lines.length; i++) {
+        if (lines[i].startsWith('a=fmtp:' + opusPayloadFormat + ' ')) {
+            if (!lines[i].includes('stereo')) {
+                lines[i] += ';stereo=1';
+            }
+            if (!lines[i].includes('sprop-stereo')) {
+                lines[i] += ';sprop-stereo=1';
+            }
+        }
+    }
+
+    return lines.join('\r\n');
+};
+
+const editOffer = (offer) => {
+    const sections = offer.sdp.split('m=');
+
+    for (let i = 0; i < sections.length; i++) {
+        const section = sections[i];
+        if (section.startsWith('audio')) {
+            sections[i] = enableStereoOpus(section);
+        }
+    }
+
+    offer.sdp = sections.join('m=');
+};
+
 const generateSdpFragment = (offerData, candidates) => {
     const candidatesByMedia = {};
     for (const candidate of candidates) {
@@ -139,6 +181,8 @@
     }
 
     onLocalOffer(offer) {
+        editOffer(offer);
+
         this.offerData = parseOffer(offer.sdp);
         this.pc.setLocalDescription(offer);
 
@@ -186,7 +230,7 @@
             return;
         }
 
-		this.pc.setRemoteDescription(new RTCSessionDescription(answer));
+		this.pc.setRemoteDescription(answer);
 
         if (this.queuedCandidates.length !== 0) {
             this.sendLocalCandidates(this.queuedCandidates);