Phase 2 of the DRED integration (docs/PRD-dred-integration.md). With
Phase 1 having enabled DRED on every Opus profile, the app-level RaptorQ
layer is now redundant overhead on those tiers: +20% bitrate, +40–100 ms
receive-side latency (block wait), +CPU for stats we never used. This
phase removes RaptorQ from the Opus encode and decode paths on both the
desktop (wzp-client/call.rs) and Android (wzp-android/engine.rs) sides.
Codec2 tiers keep RaptorQ with their current ratios unchanged — DRED is
libopus-only and Codec2 has no neural equivalent.
Encoder changes (the real bandwidth / CPU win):
- CallEncoder::encode_frame and engine.rs encode loop now gate the
RaptorQ path on !codec.is_opus():
- Opus source packets emit fec_block=0, fec_symbol=0,
fec_ratio_encoded=0 in the MediaHeader
- fec_enc.add_source_symbol is skipped on Opus
- generate_repair + repair packet emission is skipped on Opus
- block_id and frame_in_block counters stay frozen at 0 for Opus
- Codec2 path is byte-for-byte identical to pre-Phase-2 behavior.
Decoder changes (mostly cleanup, since both live decoder paths were
already reading audio directly from source packets and only using the
RaptorQ decoder output for stats):
- CallDecoder::ingest skips fec_dec.add_symbol on Opus packets. Source
packets still flow to the jitter buffer; Opus repair packets from old
senders are dropped cleanly (repair packets never hit the jitter
buffer either).
- engine.rs recv loop skips fec_dec.add_symbol, fec_dec.try_decode, and
fec_dec.expire_before on Opus packets. The `fec_recovered` stat
counter becomes Codec2-only (a separate DRED reconstruction counter
lands in Phase 4).
Wire-format backward compat verified at pre-flight:
- Old receiver + new sender: engine.rs pipeline.rs path gates on
non-zero fec_block/fec_symbol which now never fire for Opus, so the
RaptorQ decoder simply isn't fed. Audio flows normally. Desktop
CallDecoder's old path accumulated packets into the stale-eviction
HashMap, which cleans up after 2s — harmless.
- New receiver + old sender: new receiver skips RaptorQ on Opus so
old-sender repair packets are ignored entirely (no crash, no double-
decode). Loses the (previously vestigial) RaptorQ recovery benefit,
which was never actually active in the audio path. Source packets
still decode normally.
- No wire format version bump required. MediaHeader is unchanged; we
just zero the FEC fields on Opus packets.
Test changes:
- Removed `encoder_generates_repair_on_full_block` — asserted the old
(pre-Phase-2) RaptorQ-on-Opus behavior and is now incorrect. Replaced
with two symmetric tests:
- `opus_source_packets_have_zero_fec_header_fields` — verifies
Phase 2 invariants on Opus packets
- `opus_encoder_never_emits_repair_packets` — runs 20 frames of
non-silent sine wave through a GOOD-profile encoder, asserts
exactly 20 output packets, zero repair
- `codec2_encoder_generates_repair_on_full_block` — same shape as
the old test but on CATASTROPHIC profile (Codec2 1200, 8
frames/block, ratio 1.0) to verify Codec2 path still emits
repairs as before
Verification:
- cargo check --workspace: zero errors
- cargo test -p wzp-codec --lib: 61 passing (Phase 1 baseline held)
- cargo test -p wzp-client --lib: 32 passing (+3 new Phase 2 tests,
-1 old test removed)
- cargo check -p wzp-android --lib: zero errors (host link of
wzp-android tests fails on -llog per pre-existing Android-only
build.rs, unrelated to this work; integration build via
build-and-notify.sh will validate Android end-to-end)
- Pre-existing broken integration test in
crates/wzp-client/tests/handshake_integration.rs (SignalMessage
schema drift) is NOT caused by this commit — baseline had the same
3 compile errors before Phase 2. Flagged as a separate cleanup task.
Expected observable effects on a real call:
- Opus 24k outgoing bitrate drops from ~28.8 kbps (ratio 0.2 RaptorQ)
to ~25 kbps (base 24 kbps + DRED ~1–10 kbps signal-dependent)
- Opus receive-side latency drops ~40 ms on clean network (no more
block wait — jitter buffer emits as soon as a source packet arrives)
- Codec2 calls show no latency or bitrate change
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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