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Phase 1 of the DRED integration (docs/PRD-dred-integration.md). The Opus encoder now emits DRED (Deep REDundancy) bytes in every packet, carrying a neural-coded history of recent audio that the decoder can use to reconstruct loss bursts up to the configured window. Opus inband FEC (LBRR) is disabled because DRED does the same job better and running both wastes bitrate on overlapping protection. Tiered DRED duration policy per PRD: Studio (Opus 32k/48k/64k): 10 frames = 100 ms Normal (Opus 16k/24k): 20 frames = 200 ms Degraded (Opus 6k): 50 frames = 500 ms Each profile switch (via adaptive quality) updates the DRED duration to match the new tier. A 5% packet_loss floor is applied whenever DRED is active, because libopus 1.5 gates DRED emission on non-zero packet_loss. Real loss measurements from the quality adapter override upward. Escape hatch: AUDIO_USE_LEGACY_FEC=1 reverts the encoder to Phase 0 behavior (inband FEC Mode1, DRED off, no loss floor). Read once at OpusEncoder::new; call-scoped, not re-read mid-call. Trait-level set_inband_fec becomes a no-op in DRED mode to preserve the invariant even if external callers forget. Observations from the bitrate probe test (dred_mode_roundtrip_voice_pattern): DRED mode: 3649 bytes/sec (~29.2 kbps) on Opus 24k + 300 Hz sine Legacy mode: 2383 bytes/sec (~19.1 kbps) Delta: +10.1 kbps The delta is considerably larger than the "+1 kbps flat" figure I carried into the PRD from hazy memory of published DRED benchmarks. Likely because the input (300 Hz sine) is very compressible so the base Opus rate in legacy mode is well below the 24 kbps target, making the delta look disproportionate. Signal-dependent — real speech would probably show a different ratio. If production telemetry shows the overhead is excessive, we can cut DRED duration on the normal tier from 200 ms to 100 ms as a first tuning lever. Not blocking Phase 1 since the test still passes within the reasonable 2000–8000 bytes/sec bounds. Test changes (+8 tests, total wzp-codec: 61 passing): - dred_duration_for_studio_tiers_is_100ms (per-profile policy) - dred_duration_for_normal_tiers_is_200ms - dred_duration_for_degraded_tier_is_500ms - dred_duration_for_codec2_is_zero - default_mode_is_dred_not_legacy (sanity check on fresh construction) - dred_mode_roundtrip_voice_pattern (observes DRED bitrate, asserts bounds) - profile_switch_refreshes_dred_duration (verifies set_profile updates DRED) - set_inband_fec_noop_in_dred_mode (trait-level inband FEC no-op) Verification: - cargo check --workspace: zero errors, no new warnings - cargo test -p wzp-codec: 61/61 passing (53 pre-Phase-1 baseline + 8 new) - Empirical DRED bitrate observed via `rtk proxy cargo test dred_mode_roundtrip_voice_pattern -- --nocapture` Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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