fix(editor): let the audio element free-run instead of re-seeking it every frame - #898
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…every frame The editor plays picture and sound from two media elements on the same file: a muted `<video>` and a separate `<audio>`. An rAF tick kept the audio on the video's clock, and wrote `currentTime` whenever the two were more than 25 ms apart. Measured in the shipped editor during ordinary playback, on a Snapdragon X Elite: 157 `seeking` events in 20 s against a single `seeked`, and 87 `currentTime` writes in 15 s — six a second. Each write stepped the element forward by about 0.1 s, which is exactly the time that had passed: it was being sent to where it was already heading. Every write flushes the audio pipeline, so the sound broke up several times a second. The 25 ms leash assumed the rAF tick runs close behind the video clock. It does not when the renderer main thread is loaded — measured around 44 % blocked during playback — because ticks then land 100 ms and more apart, by which time the element has free-run past 25 ms and gets yanked back. The yank stalls it, so it falls behind again, which is why the drift sat at a steady ~100 ms instead of decaying. The storm sustained itself. Two changes, both of them things the video path already knew. The leash while playing becomes 120 ms, under the ~125 ms at which audio behind picture starts reading as out of sync and well clear of the drift the storm was holding; a parked element still gets placed exactly, because nothing sustains its position and placing it costs nothing. And a write is never issued onto an element that is already seeking — a write there restarts the seek instead of finishing it, so the element never arrives and the drift that triggered it never closes. That guard is the lesson of issue getopenscreen#395, which the video path took and the audio path did not. The decision moves into `shouldResyncAudio`, so the imported-track path (issue getopenscreen#350) stops carrying its own copy of the rule. Its wider 300 ms leash is unchanged and now sits beside the primary one as a named constant: it syncs to `virtualTimeSec`, derived from the video clock and noisier still, and it carries BGM or voiceover rather than lip sync. Measured after, three runs of 20 s each: two with zero `seeking` events and the media clock within 3 and 20 ms of the wall clock across 200 samples; one with 12 seeks. That third run also logged 13 `waiting` events — the element was starved, not mis-synced. Decode starvation is a separate problem, most likely contention with the native compositor decoding the same 2560x1440 file, and this change does not address it. Not covered: the residual starvation above, and the renderer main-thread load that made the tight leash untenable in the first place (~44 % blocked during playback, dominated by React reconciliation at rAF cadence). Both are worth their own work.
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Included review availability: This review used your included allowance. Your plan provides up to 8 included reviews per hour; 3 remain after this review. 📝 WalkthroughWalkthroughVirtualPreview now steers primary and supplemental audio based on drift, playback state, and explicit picture moves. It avoids restarting in-flight seeks and adjusts audio playback rates to correct ordinary drift. Tests cover steering decisions and simulated playback synchronization. ChangesAudio synchronization
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~20 minutes Change: Bug fix Merge Risk: ⚪ Minimal · up to This change makes editor audio follow the picture smoothly and avoids repeated re-seeking. No merge-blocking issues were found. The author should still listen to playback on the affected hardware. 🚥 Pre-merge checks | ✅ 3 | ❌ 2❌ Failed checks (2 warnings)
✅ Passed checks (3 passed)
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Review comments at @src/components/ai-edition/VirtualPreview.tsx:
- Line 129: Update the drift handling around the leash comparison so explicit
video seeks or timeline jumps trigger audio alignment to the new target once
seeking ends, even when the resulting drift is below the playing threshold. Keep
ordinary clock-drift handling on the existing threshold path, and locate the
seek flow via seekToVirtualTime.
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Thanks for the measurements. The seek-storm diagnosis (157 1. The leash leaves the audio out of sync. It is ±120 ms and symmetric, and nothing pulls the offset back below it, so a 99 ms offset stays for the whole playback. Audio ahead of the picture is noticed from roughly 45 ms. Could the leash be asymmetric (tighter when the audio is ahead), or could the resync land on zero once it does trigger? 2. Small jumps are no longer caught. The tick loop is the only place the audio follows a video seek ( 3. The test does not cover the regression. Two nits: If you would rather not carry these, say so and I can push them to your branch. |
…nction The leash constants were inserted between the doc comment and the function it describes.
…er a seek A 120 ms symmetric leash ended the seek storm but left the audio out of sync: a seek lands it about 100 ms behind, both elements then run at the same rate, and nothing pulls it back. Audio is noticed from about 45 ms when it leads. A playing audio element that rides the video's clock is now nudged: past 20 ms of drift its playbackRate is trimmed by 5 % toward the picture until it is within 5 ms, in either direction. A hard seek is kept for drift above 300 ms only. The picture moving on purpose (scrub, skipped cut, clip junction) is no longer told apart from clock drift by size: applySourceTime marks the audio elements, and the tick seeks each one once it is not seeking, whatever the drift. The audio's rate now changes in the same call as the video's, not a tick later, which left it 100-150 ms off at the edge of a speed region. The tests drive the rAF loop through the playback harness against a small clock model, and fail on the PR commit and on its parent.
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Pushed two commits on top of yours, as a fast-forward: your commit and its authorship are untouched.
I could not listen to it, only simulate it. @christian-wr, would you review, and try it on your Snapdragon where you measured the storm? |
2 participants
The symptom
The sound breaks up during editor playback — a stutter several times a second. The picture is fine.
The cause
The editor plays picture and sound from two media elements on the same file: a muted
<video>and a separate<audio>. An rAF tick keeps the audio on the video's clock and writescurrentTimewhenever the two are more than 25 ms apart (VirtualPreview.tsx).Measured in the shipped editor during ordinary playback, on a Snapdragon X Elite:
seekingevents in 20 s against a singleseekedcurrentTimewrites in 15 s — six a secondThat last number is the tell: the element was being sent to where it was already heading. Every write flushes the audio pipeline, so each one is an audible break.
The 25 ms leash assumed the rAF tick runs close behind the video clock. It does not when the renderer main thread is loaded — measured around 44 % blocked during playback — because ticks then land 100 ms and more apart. By then the element has free-run past 25 ms and gets yanked back; the yank stalls it, so it falls behind again. That is why the drift sat at a steady ~100 ms instead of decaying: the storm sustained itself.
The fix
Two changes, both of them things the video path already knew:
The decision moves into
shouldResyncAudio, so the imported-track path (issue #350) stops carrying its own copy of the rule. Its wider 300 ms leash is unchanged and now sits beside the primary one as a named constant — it syncs tovirtualTimeSec, derived from the video clock and noisier still, and it carries BGM or voiceover rather than lip sync.Evidence
Three runs of 20 s each on the affected machine, after the change:
seekingeventsBefore: 157 in 20 s, every run.
Run 2 also logged 13
waitingevents — that element was starved, not mis-synced. See below.Not covered
waitingevents are a separate problem, most likely contention with the native compositor decoding the same 2560×1440 file at the same time. This change does not address it.currentTimeSecis written every frame and eight components subscribe). The frame path is not the cost —createImageBitmapis 213 ms of 5222 ms,drawImage0 ms. Worth its own work.Summary by CodeRabbit