GoKawiilMediabunny finds hidden AV-sync drift in WebCodecs audio pipeline
Mediabunny developer Vanilagy discovered that audio transcoded through WebCodecs can drift out of sync with video by small amounts invisible on a single pass, despite appearing perfectly synced. Repeating the transcode 64 times in a row made the accumulated delay obvious and audible, revealing a systemic issue in the pipeline rather than a one-off glitch.
GoKawiil's interpretation of the reporting above, not reported fact.
The finding suggests that relying on WebCodecs alone does not guarantee sample-accurate AV sync, which could affect any library or app built on the same assumption. It also illustrates how human perceptual thresholds (around 125ms for lagging audio) can mask small technical errors that compound over repeated processing, a risk for pipelines that chain multiple transcodes or edits.
- Audio lagging video becomes noticeable only after ~125ms, while leading audio is detectable at just 45ms.
- Mediabunny, built on WebCodecs, exhibited subtle AV sync drift not visible in a single transcode.
- Repeating the transcode 64 times revealed the cumulative desync, exposing a deeper pipeline issue.
Source: mediabunny.dev, 2026-11-04
Published there as: “Project Sync: The journey to sample-perfect audio/video sync with WebCodecs”
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