The 0xC9 audio datagrams ride the lossy plane with no FEC, and no client ever
consulted the per-packet sequence: a lost 5 ms Opus packet played out as a hard
gap in the ring — an audible click/pop on every drop, i.e. constantly on the
Wi-Fi links where video loss is already being FEC-absorbed.
Now a shared `AudioGapTracker` (punktfunk-core::audio — pure data, wrap-safe,
unit-tested incl. u32 wraparound / reorder / duplicate cases) tells the decoder
how many packets went missing immediately before each received one, and both
native clients (pf-client-core PipeWire path, Android AAudio path) synthesize
that many frames of libopus packet-loss concealment first: `decode` with empty
input (the opus crate maps it to a NULL data pointer = PLC), sized by the last
real frame's sample count. Interpolated fade instead of a click.
Bounds: a gap is capped at 10 packets (50 ms) — libopus PLC fades to silence
after a few frames anyway, so past the cap the rings' existing underrun/re-prime
path takes over. Reorders and duplicates conceal nothing (the plane has no
reorder buffer; playing a late packet where it lands is the existing behaviour).
In-band Opus FEC (LBRR) is deliberately NOT used: the host sends 5 ms frames
and LBRR needs ≥10 ms frames to carry anything.
The cap is a crate-private const so cbindgen keeps it out of the C ABI header.
Host cargo tests + clippy green; android crate verified via cargo ndk check.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
enricobuehler
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