feat(realtime): scale uplink maxBitrate with publish fps below 15 fps - #192
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eilon-decart wants to merge 1 commit into
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feat(realtime): scale uplink maxBitrate with publish fps below 15 fps#192eilon-decart wants to merge 1 commit into
eilon-decart wants to merge 1 commit into
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The encoder spends its per-second bitrate budget regardless of cadence, so a 10 fps publisher packs ~3x the bytes into every frame; the oversized frames' wire-arrival span inflates the server's ingest jitter buffer by ~35-100 ms. Scaling the cap to ~200 kbit/frame below 15 fps restores 30 fps-parity ingest latency at equal-or-better QP (measured: 10 fps ingest hold 129.7 -> 28.4 ms, e2e 660 -> 557 ms from us-east4; full causal A/B in the api repo, experiments/2026-07-28-livekit-size-vs-interval-jb). No-op for every registry model today (all declare 30 fps) and for >=15 fps publishers; react-native path unchanged (no publishFps -> natural rate).
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What
getDefaultVideoPublishOptionsderives the uplinkmaxBitratefrom the intended publish fps instead of using the static 3.5M cap, engaging only below 15 fps:The fps is threaded from
prepareBrowserConnection(which already receivesresolveFpsNumber(model.fps)for the mirror canvas) into the media-channel factory. React-native path unchanged (nopublishFps→ natural rate).Why
Chrome's encoder spends its per-second budget regardless of cadence, so a 10 fps publisher packs ~3× the bytes into every frame (44KB vs 14KB). Those frames' wire-arrival span inflates the server's ingest jitter buffer — measured +98ms at 10 fps on a clean path. Scaling to ~200kbit/frame restores full 30 fps-parity:
Full causal A/B (19 sessions, size vs interval decomposition, dose–response, both IL and us-east4 clients):
DecartAI/api→experiments/2026-07-28-livekit-size-vs-interval-jb/(brancheilon/feature/lk-input-latency).Guardrail evidence baked into the constants: at ≥15 fps a binding cap measurably hurts (rate-controller churn, +32ms), and very low caps hit the simulcast layer-starvation cliff — hence the 15 fps threshold and the 1M floor.
Tests
tests/publish-options.unit.test.ts— 6 new unit tests (threshold, scaling, floor, natural-rate ceiling, omitted-fps fallback), all green.pnpm typecheck/ suite: the pre-existingframe-metadata-diagnostics.tstypecheck error and the mswunit.test.tsenv failure exist identically onorigin/main— not introduced here (verified via stash A/B).Follow-up (separate PR)
This covers declared low-fps models. Users whose camera actually delivers <15 fps while the model declares 30 need the measured-fps adaptive loop (
sender.getStats()→ livesetParametersre-apply with hysteresis) — mechanism already validated by the probe's cap loop.Linear (Platform team): PLA-660
Note
Medium Risk
Touches realtime uplink encoding for all browser sessions where fps is passed, but the scaling gate limits behavior change to declared fps below 15 fps; default registry paths remain unchanged.
Overview
Low-fps models now get a dynamic uplink
maxBitrateinstead of the fixed 3.5M cap.getDefaultVideoPublishOptionsscales toward ~200 kbit/frame when declared publish fps is below 15, with a 1M floor and the existing codec ceiling; at ≥15 fps or when fps is omitted, behavior stays byte-identical to today.Browser setup passes the model’s intended
publishFpsfromprepareBrowserConnectionintocreateLiveKitMediaChannel, so LiveKit publish options reflect declared cadence. NewREALTIME_CONFIG.livekitknobs document the threshold and scaling constants.Tests:
publish-options.unit.test.tscovers threshold, scaling, floor, ceiling, and omitted-fps fallback.Reviewed by Cursor Bugbot for commit 87b18b6. Bugbot is set up for automated code reviews on this repo. Configure here.