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Give a frame that outruns its epoch's video a provisional offset

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The demuxer interleaves, so at a boundary the streams do not reset on the
same frame: audio for the next segment can reach the muxer before the video
frame that opens the epoch it belongs to. Only the primary video track may
open one, so those frames rode the just-ended offset and landed a whole
segment in the past. Downstream the strictly-monotonic block nudge then
crushed the entire run onto one instant a tick apart, which is audible.

Such a frame is recognised on its OWN raw PTS, which within an epoch only
advances, so a large backward step is unambiguous — a different signal from
the shared frontier, which is what the old false-positive ratchet keyed on.
It then rides a provisional offset private to its track, computed the same
way the video path computes a real one, and drops it the moment the video
retires an epoch, so the run rejoins with no seam.

The offset is deliberately private: it never writes offset_ns, never
advances the frontier and never retires an epoch, so it cannot move the
video timeline. Letting a passive track open a REAL epoch was tried first
and inflated a 476.776 s title to 656.216 s, because every track meets a
boundary at its own pace and the video path rebased again on top.

Measured on a real DVD title with 8 cell boundaries: audio frames stamped
inside one cadence 120 -> 0, outsized gaps 7 -> 0, subtitle 1 -> 0, and the
video span byte-identical at 476.484. On the HD-DVD title the remaining
boundary gap on its first audio track falls from 0.999 s to 0.275 s.
Acceptance: 76 pass, 0 fail, 0 skip.
This commit is contained in:
Matthew Jackson
2026-08-08 00:48:10 -07:00
parent 70e1807e61
commit 835e97ce71
2 changed files with 157 additions and 10 deletions
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src/mux/demux_sink.rs
+5 -5
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@@ -1543,14 +1543,14 @@ mod tests {
fn timeline_track0_drives_epoch_others_ride() {
let mut tl = TimelineContinuity::new();
// Clip 1: video 0..10s (track 0 drives the epoch).
assert_eq!(tl.adjust(0, true), 0);
assert_eq!(tl.adjust(0, false), 0); // audio rides the same offset
assert_eq!(tl.adjust(10_000_000_000, true), 10_000_000_000);
assert_eq!(tl.adjust(0, true, 0), 0);
assert_eq!(tl.adjust(0, false, 1), 0); // audio rides the same offset
assert_eq!(tl.adjust(10_000_000_000, true, 0), 10_000_000_000);
// Clip 2 seam: video PTS jumps back to ~0 (> 3s back) → new epoch.
let out = tl.adjust(0, true);
let out = tl.adjust(0, true, 0);
assert!(out >= 10_000_000_000, "epoch must advance past prev high");
// Audio in clip 2 (non-epoch) gets the SAME offset (A/V sync preserved).
let a = tl.adjust(0, false);
let a = tl.adjust(0, false, 1);
assert_eq!(a, out);
}