Mouth-motion against speech is a statistical proxy and it fails on ordinary talking clips: the
per-window estimates swing by 700 ms and the honest answer is "unmeasurable". A clap is not a
proxy — it has one unmistakable audio spike and one unmistakable visual spike, a fixed distance
apart, with no periodicity to lock onto wrongly.
So when the correlation is weak, the tool now detects sharp audio transients (99th percentile,
one event per 0.6 s), finds the biggest visual change within ±400 ms of each, and reports the
median of the cluster within 60 ms of it — at least three pairs or it still says unmeasurable.
The cluster matters: an arm entering the frame moves more than the clap itself and lands early
(two pairs on the validation clip came out at +248 and +283 ms against -285).
Validated on ty-20261112-1405-621-0000.mp4: the correlation path said +98 ms and the clap path
+116 ms (three tight pairs at +117, +115, +107), so the applied 350 ms should come down to about
240. Two methods, same answer, no listening required.
gramps
1abc86358a