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>
enricobuehlerandClaude Fable 5
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