34 lines
3.6 KiB
Markdown
34 lines
3.6 KiB
Markdown
# ECP-0117: Live Fragment Duration and Audio Unlock
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Status: Draft
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## Problem / context
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Hosted live playback can subscribe to a local HDHomeRun stream while the visible frame stays frozen and audio stays muted. Browser inspection showed `currentTime` advancing through keyframe-spaced buffered ranges, but each range was only one microsecond long. The web wrapper also left the `<moq-watch muted>` attribute in place, so a user gesture could be overwritten back to muted.
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## Decision
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Publish WebTransport fMP4 with keyframe fragments where every emitted video frame is a keyframe (`g=1`, `keyint_min=1`). For HDHomeRun-style live input, cap the default WebTransport transcode to 6 fps so the hosted watcher receives independently decodable video groups at a sustainable cadence. Expose publisher knobs for the ffmpeg video filter, GOP interval, and fMP4 movflags so runtime operators can tune without another code edit. Keep `wt-publish` / `nbc-wt-publish` on the non-passthrough CMAF sample path. On the hosted web player, render live video through the `<moq-watch>` canvas/WebCodecs path instead of the `<video>`/MSE path, remove the `muted` attribute, and reapply unmuted state to the watcher backend after a user gesture.
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## Consequences
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- Browser playback receives continuous media ranges without turning each GOP into a single playback jump.
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- Live playback and observation diffing receive independently decodable video groups at frame cadence.
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- WebTransport video publishing uses more bandwidth per frame, but the 6 fps cap keeps group churn lower than full-rate all-intra OTA publishing.
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- Operators can raise or lower `--video-filter`, `--gop-frames`, and `--movflags` from publisher configuration instead of rebuilding.
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- Hosted live rendering avoids the upstream MSE path that side-browser validation showed repeatedly skipping slow groups.
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- Relay subscribers receive `video0.m4s` and `audio0.m4s` media groups by default instead of catalog-only passthrough announcements.
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- Audio remains gesture-gated for autoplay policy, but the gesture now actually unmutes the player.
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## Alternatives considered
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- Raise web jitter again. Rejected because the buffered media ranges were effectively zero-length; more latency does not turn still ranges into playable media.
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- Keep passthrough mode as the default. Rejected because relay probes received only `catalog.json` while the non-passthrough sample path delivered video and audio media groups.
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- Use keyframe-duration fragments. Rejected after side-browser validation showed repeated `seeking forward/backward` corrections and GOP-sized visual jumps even though audio was healthy.
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- Keep 48-frame GOPs with every-frame fMP4 fragments. Rejected because relay archive proof still showed 48-frame video groups; `moq-mux` groups video by keyframe in the non-passthrough path.
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- Use full-rate or 12 fps all-intra every-frame fragments. Rejected because relay proof showed one-frame video groups, but side-browser validation produced heavy `skipping slow group` churn from too many tiny media groups.
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- Keep the `<video>` child renderer. Rejected because the hosted side browser showed the MSE renderer repeating frames and emitting slow-group skips while subscribed.
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## Rollout / teardown
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Rebuild/restart local and hosted publishers, deploy the updated web asset, and verify hosted playback by checking canvas frame hashes over time plus side-browser console seek corrections. Teardown is setting `--gop-frames 48`, setting `--video-filter` back to the prior source cadence, restoring passthrough defaults to true, and restoring the prior `<video>` child plus muted wrapper behavior.
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