User confirmed at +72 the symptom is 'voice + constant clicks +
choppy fragments'. The diagnostic data conclusively pointed to
iOS VPIO render-callback timing as the cause:
* frames_changes=60+ per 100 callbacks at cb>=1800
iOS keeps switching num_frames between 960 and 1104
on roughly 60% of callbacks
* peak_out_i16 is sensible (2500-16870, never clipping)
when AudioHandler returns content
* input_was_zero=true on most callbacks during active speech
AudioHandler keeps entering buffering_samples state
The cause: previous render callback called fill_buffer
synchronously every iOS audio thread invocation. With iOS
calling at irregular rates with irregular sizes, AudioHandler's
jitter buffer (sized around 20 ms Opus frames) cannot satisfy
arbitrary-sized requests and falls back to returning silence
(&[] empty slice) on misaligned reads. The silent gaps in the
middle of the output buffer create discontinuities = audible
clicks; the missing-tail content produces choppy fragments.
Fix (architectural): decouple the AudioHandler decoder from the
VPIO render callback via a lock-free SPSC ring buffer.
Producer (tokio task, 50 Hz):
every 20 ms:
fill_buffer(scratch_stereo_f32, 1920 = 20 ms stereo)
downmix L+R -> mono i16 (960 samples)
ring_buffer.push_slice(mono_i16)
Consumer (VPIO render callback, iOS audio thread):
every callback:
pop num_frames samples from ring buffer into out
zero-fill tail on underrun
Why it works:
* Producer always asks AudioHandler for a stable 20 ms chunk
(perfectly aligned with internal Opus frame size). No more
buffering_samples false-triggers.
* Consumer pulls whatever iOS asks for whenever iOS schedules
it; ring buffer's 200 ms depth absorbs the callback jitter.
* This is the standard pattern every production VoIP audio
engine uses (WebRTC, Discord, FaceTime) to bridge bursty
Opus decoders to bursty platform audio callbacks.
Implementation:
* New dep: rtrb 0.3.4 (RustAudio realtime-safe SPSC ring
buffer, 6.8M downloads, lock-free push/pop with no
allocation on the audio thread).
* RING_BUFFER_SAMPLES = 9600 (200 ms mono i16 at 48 kHz).
Sized for 10x producer ticks of headroom.
* PRODUCER_TICK_MS = 20 (matches Opus 50 Hz packet rate).
set_missed_tick_behavior(Skip) to avoid burst catch-up on
runtime stalls.
* Producer task spawned in IosVoiceUnit::start, shutdown
via tokio::oneshot when IosVoiceUnit drops.
* Render callback is now just: pop into out, zero-fill tail,
apply mute then gain.
* Gain applied CONSUMER-side so user volume changes take
effect within one callback (<= 200 ms latency).
* Underrun diagnostics: count underrun callbacks + total
zero-filled samples, log every 100 callbacks.
Threading + safety:
* rtrb is lock-free SPSC. Audio thread never blocks.
* Producer can block briefly on Arc<Mutex<AudioHandler>>
contention with the inbound forwarder (handle_packet), but
not with the audio thread.
* Producer task is owned by tokio runtime; explicit shutdown
channel ensures it exits when the engine stops.
Build verify:
* Linux host: cargo check clean in 4.06s (downloads rtrb 0.3.4).
* iOS Mac: cargo check clean in 2.14s.
Build counter 72 -> 73.
EdisonJwa
99584fbc1a