The iOS mic was unusable — a consistent ~40-60ms flutter + volume fade
('slow fan') on every preset. The core sends each captured frame
synchronously (no send pacer), so packet cadence == capture cadence, and
the receiver's playout keeps near-zero buffering by design (its jitter
estimate keys off the regular sender timestamp, so it's blind to arrival
jitter). That's smooth only for a steady sender (desktop miniaudio =
steady 20ms); the iOS AVAudioEngine tap delivers ~2 frames per ~40ms
callback -> bursty -> receiver underruns -> PLC fade.
Fix (iOS-only): the mic tap writes converted 48kHz int16 to an SPSC ring;
a 20ms feed pump drains it and calls feedPcm at a steady cadence. The pump
primes a small prebuffer cushion (3 frames ~60ms, self-healing up to
~120ms on underrun) before releasing, so the tap's bursts can't drain it
to empty. Never reads a partial frame (read consumes what it returns ->
partials were the crackle), and rebuilds with the current channel count
each rebuild() (a frozen count fed mono-as-stereo = octave-up on a
Stereo->Voice Chat switch).
Trade-off: ~60-120ms added mic-send latency, unavoidable when de-bursting
for a near-zero-buffer receiver. PROGRESS.md notes the proper follow-up:
make the jitter buffer measure real RFC-3550 arrival jitter so the
receiver absorbs bursts itself.
Verified: xcodebuild Debug BUILD SUCCEEDED (iOS Simulator, arm64).
Talon
19c2fb6ec9