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zebra-spaces: chromium decoder anchor — muted <audio> alongside MediaStreamSource

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Chromium will not run its WebRTC audio decoder for a remote track
that is only consumed by a MediaStreamAudioSourceNode. The track
must also be attached to an HTMLMediaElement to "kick" the decoder
into running. Without the anchor the worklet's queue fills with
zeros — the 'started' event fires, ctxState=running, RTP packets
arrive at 50 pps, yet PulseAudio Playback shows the chrome stream
present with the level meter dead at 0. Fox 2026-06-07 fedora
chrome silent for days; firefox unaffected (its decoder runs
unconditionally for MediaStreamSource consumers).

Each attach now creates a hidden muted <audio> bound to the same
MediaStream. The element makes no audible output (muted=true); its
sole purpose is to keep chromium's decoder running so the worklet's
MediaStreamSource sees real samples. setWorkletStream swaps the
anchor's srcObject alongside the source node so an in-place stream
swap doesn't strand the anchor on a dead track. detachListenerStream
tears the anchor down with the rest of the chain.

Tests pin the contract: anchor created on attach, anchor follows the
stream on setWorkletStream, anchor torn down on detach.
This commit is contained in:
Russell Ballestrini 2026-06-07 20:06:05 -04:00
parent 1984bb1d41
commit dad697d141
No known key found for this signature in database
2 changed files with 92 additions and 4 deletions
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41
web/zebra-spaces.html
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@ -3365,7 +3365,31 @@ function attachAudioStreamViaWorklet(uuid, stream, targetSeconds){
gain.gain.value = 1.0;
src.connect(gain);
gain.connect(audioCtx.destination);
const node = { src, gain, stream, targetSeconds };
/* Chrome decoder anchor: chromium's WebRTC decoder will not run for
* a remote audio track that is only consumed by a MediaStreamAudioSourceNode.
* The track must also be attached to an HTMLMediaElement consumer to
* "kick" the decoder; otherwise the MediaStreamSource produces silence
* even though packets arrive and the worklet 'started' event fires
* (queue fills with zeros). The element is created muted so it makes
* no sound on its own — its only job is to keep chrome's decoder
* running so the worklet sees real samples. Firefox doesn't have
* this limitation but the same anchor is harmless there.
* Fox 2026-06-07 blanka-chrome: PulseAudio Playback showed the chrome
* stream present with zero level meter movement until this anchor
* landed. */
let anchor = null;
try {
anchor = document.createElement('audio');
anchor.muted = true;
anchor.autoplay = true;
anchor.playsInline = true;
anchor.srcObject = stream;
anchor.style.display = 'none';
document.body.appendChild(anchor);
const p = anchor.play();
if (p && p.catch) p.catch(()=>{}); /* muted autoplay should never reject, but swallow defensively */
} catch(e){ logLine('err','decoder anchor '+uuid.slice(0,4)+': '+e.message); }
const node = { src, gain, stream, targetSeconds, anchor };
listenerAudioNodes.set(uuid, node);
/* fire-and-forget worklet load on first use; once ready, every
* existing stream is swapped through the buffer (see loadJitterWorklet) */
@ -3421,6 +3445,9 @@ function detachListenerStream(uuid){
try { if (node.jbuf) node.jbuf.disconnect(); } catch(_){}
try { if (node.capture) node.capture.disconnect(); } catch(_){}
try { node.gain.disconnect(); } catch(_){}
if (node.anchor){
try { node.anchor.srcObject = null; node.anchor.pause(); node.anchor.remove(); } catch(_){}
}
listenerAudioNodes.delete(uuid);
}
@ -3451,6 +3478,12 @@ function setWorkletStream(uuid, newStream){
else newSrc.connect(node.gain);
node.src = newSrc;
node.stream = newStream;
/* swap the decoder anchor's srcObject so chrome's decoder keeps
* running for the new track (see attachAudioStreamViaWorklet for
* the anchor's role). */
if (node.anchor){
try { node.anchor.srcObject = newStream; } catch(_){}
}
logLine('', 'worklet source swapped for '+uuid.slice(0,4));
return true;
}
@ -8091,9 +8124,9 @@ logLine('', 'ready — pick a handle, type a rendezvous code, enter the space');
</script>
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