The capture path carried three latencies that had nothing to do with the
network: a 2048-frame tap request (42.7 ms bursts before the first
sample could even be sliced), 20 ms Opus framing, and a mono signal
duplicated into both stereo channels because the encoder was configured
like the downlink. CoreAudio's Opus encoder takes mFramesPerPacket=480
and mChannelsPerFrame=1 just fine — probed empirically: the converter
truly emits 10 ms mono CELT packets (TOC config 30), one per 480-frame
chunk, and the stereo-shaped decoder upmixes them with the tone intact —
so the uplink now asks for 10 ms tap buffers and encodes 48 kbps mono
10 ms packets, with 960 kept only as an init-time fallback. The host
decodes any Opus frame ≤120 ms, so nothing changes on the wire's far
end.
The tap thread also stops burning cycles per callback: the mono fold and
resampler scratch buffers are allocated once (regrown only if a larger
device quantum ever arrives), and the chunk slicer walks a head index
instead of removeFirst — which memmoved the entire backlog for every
packet on a render-adjacent thread. iOS additionally asks the session
for 5 ms IO quanta at 48 kHz when the mic is on (best-effort; the
hardware decides).
Verified: swift build (macOS arm64), swift test OpusCodecTests +
AudioChannelFoldTests, and an iOS arm64 cross-build; the 480/mono
behavior confirmed by TOC inspection on macOS 15.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Both directions of the audio plane, on CoreAudio's built-in Opus codec
(kAudioFormatOpus — no bundled libopus; OpusCodec.swift, round trip unit-tested):
- Playback: a drain thread pulls nextAudio() packets, decodes, and writes a priming
jitter ring feeding an AVAudioSourceNode (~20 ms prefill, adaptive to the device's
render quantum so large-buffer devices don't oscillate prime/dropout; a high-water
clamp sheds stall backlog so one network hiccup can't permanently lag audio behind
video; underrun re-primes — one dip, not sustained crackle).
- Mic: a second engine taps the input device, resamples to 48 kHz stereo, Opus-encodes
20 ms chunks and sendMic()s them into the host's virtual PipeWire source. Permission
via AVCaptureDevice (NSMicrophoneUsageDescription added to the Xcode target).
- Settings: Speaker + Microphone pickers (CoreAudio HAL enumeration, persisted by
device UID — "System default" leaves the engine unpinned so it follows macOS device
changes) and a "Send microphone" toggle (default on). Applies from the next session.
- Audio starts with streaming, never during the trust prompt (no host sound — and no
mic uplink — before the user trusted the host); teardown stops audio before close().
Adversarial-review fixes baked in: stop() and the dangling mic-permission callback
share one lock+flag protocol (no hot mic with no owner), the connect-success handler
bails when the attempt was abandoned mid-handshake (no session/mic for a dead window),
SessionAudio gets a deinit backstop (a dropped instance can't pin the connection via
its drain thread), and the render scratch buffer is block-owned (was leaked per
session).
Verified live against the box: remote test decodes 100 host Opus packets to PCM and
the host opens its virtual mic on the first uplinked frame ("punktfunk/1 virtual mic
ready"); on-glass session runs with both engines up.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
enricobuehlerandClaude Fable 5