From 838c3426ba8e6e403f734bcf08ea1eb929aa8aed Mon Sep 17 00:00:00 2001
From: Russell Ballestrini <russell.ballestrini@gmail.com>
Date: Thu, 28 May 2026 10:51:55 -0400
Subject: [PATCH] chat.html: force G.711/G.722 codec over WebRTC so the modem
 survives
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WebRTC defaulted to Opus, a perceptual codec that re-quantizes audio in 20ms
frames and smears the MARK/SPACE bit-edges the amplitude modem rides on — which
is why the PulseAudio C path does ~800 baud but the WebRTC path couldn't carry
frames at all. Pin the audio transceiver to memoryless G.711 (PCMU/PCMA), then
G.722, ahead of Opus via setCodecPreferences, restoring a clean amplitude
channel. Decoder resolution is now bounded by the AudioWorklet quantum (~375Hz),
not the codec.
---
 web/chat.html | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/web/chat.html b/web/chat.html
index b564d93..e142f3c 100644
--- a/web/chat.html
+++ b/web/chat.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);