zebra-spaces: anti-alias + compressor on self-transcribe (host accuracy fix)

Fox 2026-06-05: "the remote phones are transcribing more accurate than
the host version. figure out why and make host version better."

Two causes diagnosed:

1. Aliasing. The whisper-capture worklet decimates 48 kHz → 16 kHz by
   taking every 3rd sample with NO anti-alias filter. Listener path
   feeds Opus-decoded peer audio which is already band-limited
   (~12 kHz max). Host path feeds RAW mic — all 48 kHz of it —
   which means anything above 8 kHz folds back into the audible band
   as garbage when decimated. Whisper sees noisier input on host.

2. Amplitude / dynamic range. Music-mode mic has NO AGC/NS/EC
   (raw broadcast for music). Quiet passages are too low for
   Whisper to confidently transcribe; loud passages can clip. The
   listener path's audio has been through Opus encode/decode which
   normalizes implicitly.

Both addressed via Web Audio nodes inserted between the
MediaStreamSource and the whisper-capture worklet:

Self-capture chain:
  micStream → DynamicsCompressorNode → BiquadFilter (lowpass 7 kHz)
            → AudioWorkletNode (whisper-capture)

DynamicsCompressor: voice-friendly settings (threshold -30 dB,
knee 30, ratio 4:1, attack 3 ms, release 250 ms). Flattens
amplitude so Whisper sees a normalized waveform regardless of how
the user has the mic gain set or whether music mode is on.

BiquadFilter lowpass at 7 kHz (below Nyquist of 16 kHz = 8 kHz):
removes content that would alias when the worklet decimates.

Remote captures: just the lowpass (no compressor — Opus has already
normalized the peer audio adequately). Mostly a no-op on voice-mode
peers (Opus already cuts <12 kHz) but helps music-mode peers where
the encode preserves more high-frequency content.

stopCaptureForUuid / stopSelfCapture cleanly disconnect the new
nodes so re-toggle doesn't leak audio graph references.

Expected: host self-transcribe accuracy now comparable to (or better
than) the phone listener transcribes. Telemetry `emt=` should
increase per tick on host once the user is talking.

web/zebra-spaces.html

@@ -2367,14 +2367,29 @@ async function startSelfCapture(){
   if (!ok){ logLine('err','self capture: worklet module load failed'); return; }
   try {
     const src = audioCtx.createMediaStreamSource(micStream);
+    /* DynamicsCompressorNode — normalize amplitude before Whisper.
+     * Host raw mic (especially in music mode with no AGC) has wide
+     * dynamic range; Whisper accuracy suffers on quiet/clipped
+     * passages. Compressor flattens it. Voice-friendly settings. */
+    const comp = audioCtx.createDynamicsCompressor();
+    comp.threshold.value = -30; comp.knee.value = 30;
+    comp.ratio.value = 4;       comp.attack.value = 0.003; comp.release.value = 0.25;
+    /* BiquadFilter lowpass @ 7 kHz — anti-alias before the worklet's
+     * 48k→16k decimation. Mic content >8 kHz otherwise folds into
+     * the audible band as noise and degrades Whisper. Listener path
+     * doesn't need this because Opus has already band-limited the
+     * peer audio. Fox 2026-06-05: "the remote phones are transcribing
+     * more accurate than the host version." */
+    const lpf = audioCtx.createBiquadFilter();
+    lpf.type = 'lowpass'; lpf.frequency.value = 7000; lpf.Q.value = 0.707;
     const cap = new AudioWorkletNode(audioCtx, 'whisper-capture');
     cap.port.onmessage = (e) => {
       if (e.data && e.data.chunk) handleWhisperChunk(myUUID, e.data.chunk);
     };
-    src.connect(cap);
+    src.connect(comp).connect(lpf).connect(cap);
     cap.port.postMessage({ cmd: 'start' });
-    _selfCapture = { src, cap };
-    logLine('', 'whisper: self capture started');
+    _selfCapture = { src, comp, lpf, cap };
+    logLine('', 'whisper: self capture started (anti-alias + compressor)');
   } catch (e){
     logLine('err', 'whisper self capture: '+e.message);
   }
@@ -2383,6 +2398,8 @@ function stopSelfCapture(){
   if (!_selfCapture) return;
   try { _selfCapture.cap.port.postMessage({ cmd: 'stop' }); } catch(_){}
   try { _selfCapture.cap.disconnect(); } catch(_){}
+  try { _selfCapture.lpf.disconnect(); } catch(_){}
+  try { _selfCapture.comp.disconnect(); } catch(_){}
   try { _selfCapture.src.disconnect(); } catch(_){}
   _selfCapture = null;
 }
@@ -2393,6 +2410,13 @@ async function startCaptureForUuid(uuid){
   const ok = await loadWhisperCaptureWorklet(audioCtx);
   if (!ok) return;
   try {
+    /* Same anti-alias lowpass as self-capture. Opus has already
+     * band-limited remote audio, so this is usually a no-op — but
+     * the music-mode peer can push wideband Opus close to 12 kHz
+     * and the decimation still aliases. Cheap to apply
+     * unconditionally. */
+    const lpf = audioCtx.createBiquadFilter();
+    lpf.type = 'lowpass'; lpf.frequency.value = 7000; lpf.Q.value = 0.707;
     const capture = new AudioWorkletNode(audioCtx, 'whisper-capture');
     capture.port.onmessage = (e) => {
       if (e.data && e.data.chunk) handleWhisperChunk(uuid, e.data.chunk);
@@ -2400,9 +2424,10 @@ async function startCaptureForUuid(uuid){
     /* tap the source — capture runs in parallel with the worklet
      * chain, doesn't need to connect to destination (we just want
      * the chunks via port messages). */
-    node.src.connect(capture);
+    node.src.connect(lpf).connect(capture);
     capture.port.postMessage({ cmd: 'start' });
     node.capture = capture;
+    node.captureLpf = lpf;
     logLine('', 'whisper: capture started for '+uuid.slice(0,4));
   } catch (e){
     logLine('err', 'whisper capture install '+uuid.slice(0,4)+': '+e.message);
@@ -2414,6 +2439,10 @@ function stopCaptureForUuid(uuid){
   if (!node || !node.capture) return;
   try { node.capture.port.postMessage({ cmd: 'stop' }); } catch(_){}
   try { node.capture.disconnect(); } catch(_){}
+  if (node.captureLpf){
+    try { node.captureLpf.disconnect(); } catch(_){}
+    node.captureLpf = null;
+  }
   try { node.src.disconnect(node.capture); } catch(_){}
   node.capture = null;
 }
@@ -7427,8 +7456,8 @@ logLine('', 'ready — pick a handle, type a rendezvous code, enter the space');
 
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