From e3340a776b1af7818f9392e514adeba94a368e75 Mon Sep 17 00:00:00 2001
From: "russell@unturf.com" <russell@unturf.com>
Date: Thu, 28 May 2026 10:51:57 -0400
Subject: [PATCH] zebra-report: force G.711/G.722 codec (fix amplitude modem
 over WebRTC)

---
 zebra-report/index.html | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/zebra-report/index.html b/zebra-report/index.html
index b564d93..e142f3c 100644
--- a/zebra-report/index.html
+++ b/zebra-report/index.html
@@ -1150,6 +1150,28 @@ function ensurePC() {
   if (!carrierOn) throw new Error('start carrier first');
   pc = new RTCPeerConnection(RTC_CONFIG);
   for (const tr of outboundStream.getTracks()) pc.addTrack(tr, outboundStream);
+  /* Force a memoryless codec (G.711 PCMU/PCMA, else G.722) ahead of Opus.
+   * Opus is a perceptual codec that re-quantizes in 20ms frames and smears the
+   * MARK/SPACE bit-edges this modem rides on; G.711 companding is per-sample,
+   * preserving the carrier envelope like the PulseAudio path. */
+  try {
+    const caps = (RTCRtpSender.getCapabilities && RTCRtpSender.getCapabilities('audio')) || null;
+    if (caps && caps.codecs) {
+      const rank = (mime) => {
+        const m = (mime || '').toLowerCase();
+        if (m.endsWith('/pcmu')) return 0;
+        if (m.endsWith('/pcma')) return 1;
+        if (m.endsWith('/g722')) return 2;
+        if (m.includes('opus'))  return 99;
+        return 50;
+      };
+      const ordered = caps.codecs.slice().sort((a, b) => rank(a.mimeType) - rank(b.mimeType));
+      for (const t of pc.getTransceivers()) {
+        if (t.setCodecPreferences) { try { t.setCodecPreferences(ordered); } catch (_) {} }
+      }
+      logLine('sys', 'codec preference: G.711/G.722 ahead of Opus (clean amplitude)');
+    }
+  } catch (_) {}
   pc.ontrack = (ev) => {
     const stream = ev.streams[0] || new MediaStream([ev.track]);
     attachInboundTrack(stream, ev.track.id);