Project Sync: The journey to sample-perfect audio/video sync with WebCodecs | Mediabunny

Mediabunny reported that a decode-encode roundtrip through WebCodecs’ AAC AudioEncoder introduced cumulative audio delay, causing audio to lag video by more than two seconds after 64 successive transcodes of a five-second Big Buck Bunny clip, which also grew to nearly nine seconds. The library said its own pipeline only passes audio packets between WebCodecs decoders and encoders without altering timestamps, pointing to the encoder as the source of the drift. The issue matters because delays below roughly 125 milliseconds go unnoticed by viewers, while audio leading video becomes perceptible at about 45 milliseconds, so small per-pass errors can compound silently in repeated processing. Mediabunny said it tested the encoder with a 20 Hz to 100 Hz sine sweep sampled at 48 kHz on a Mac to isolate the behavior. Next, watch whether the company ships a timestamp or padding correction and whether other WebCodecs-based tools exhibit the same drift.