User reported persistent crackling/popping from peer audio on Linux even
after fixing the 48k->device-rate resampler boundary discontinuities,
clamping pre-Opus-encode peaks, and pre-allocating the playback scratch
buffer. Logs confirmed cpal opened raw ALSA at 44.1k native, no callback
budget violations, no underrun warnings -- yet the audio was still poor.
Root cause: cpal on Linux opens raw ALSA's 'default' PCM. On modern
PipeWire / pipewire-alsa boxes that virtual device routes through ALSA's
dmix + plug layers, whose default resampler is nearest-neighbour. cpal
also picks a small default period size (~256 frames / 5.8 ms) leaving no
headroom for kernel scheduler jitter. Both effects compound into the
crackling the user heard.
Upstream tsclientlib's own audio example
(tsclientlib/examples/audio_utils/ts_to_audio.rs) and the official Qint
client both use SDL2 with AudioSpecDesired { freq: 48000, channels: 2,
samples: 960 }. SDL2 on the same systems routes through PipeWire's PA
bridge (or PulseAudio directly), both carrying high-quality resamplers.
Fix:
* Add sdl2 = '0.37' as a target_os=linux dependency. Links libSDL2-2.0
.so (Arch sdl2-compat over SDL3, Debian libsdl2-2.0-0, Fedora SDL2).
* New module crates/chanora_audio/src/sdl_output.rs implementing
SdlOutput: opens a 48 kHz stereo 960-frame callback that zeroes the
buffer and calls AudioHandler::fill_buffer directly (no user-side
resampler). Master gain + hard-mute atomics wired in identically to
the cpal callback so set_output_gain / set_output_muted keep working.
* engine.rs cfg-gated: target_os='linux' builds SdlOutput; everywhere
else continues with the cpal output path (including the device-native-
rate negotiation and resampler-continuity fixes shipped earlier --
those remain correct on Windows/macOS where cpal targets WASAPI /
CoreAudio cleanly).
* The cpal output helpers (build_output_stream, PlaybackResampleState,
FromF32) are now cfg(not(target_os='linux'))-gated so the Linux
build doesn't emit dead-code warnings.
Capture path still cpal on every platform -- outbound audio was not
reported as bad. Resampler-continuity fix on the capture side stays:
microphone -> Opus encoder still goes through the linear interpolator
with the last-sample anchor.
Tests: 32 / 0 / 0 (chanora_audio), workspace 78 / 0 / 1 unchanged.
EdisonJwa
d9c330c23b