Open benchmark: A/V sync (lipsync) across screen/HDMI recorders

[andratek]

Been digging into screen/HDMI recording internals for a while, and one thing that bugs me is how little anyone actually measures A/V sync drift instead of just eyeballing it.

So I built a small diagnostic tool that reads the MP4 structurally (mdhd/elst edit-list boxes, etc.) and reports the real audio/video offset in milliseconds — not "does it look synced," an actual number.

Ran it against Bandicam and EaseUS RecExperts on identical clips: residual offset landed anywhere from ~10ms to ~40ms, with no drift correction over time — the offset doesn't shrink, it just sits there (or slowly grows on longer files).

OBS is the interesting one. Officially it's a fine, capable recorder, but its own forums are full of threads reporting drift in the 0.5s–3s range on longer sessions — and this isn't some fringe capture-card edge case. Multiple threads describe it happening with plain USB webcams and USB mics, no exotic hardware involved. The root cause seems to be physics, not settings: your audio interface has its own internal clock, OBS timestamps frames off the system clock, and a typical ~50ppm clock mismatch alone adds up to roughly 0.18s of drift per hour — which is why short clips look perfect and 2-hour streams don't.

Haven't run numbers yet on Action! or ShadowPlay/ReLive — if anyone here uses those regularly, curious what your longer recordings look like.

Post your recorder + recording length and I'll run the file through the tool for a second opinion, or grab it yourself and check your own footage. Not claiming this is exhaustive — just tired of "trust me it's synced" being the default answer.

[andratek]

Been digging into screen/HDMI recording internals for a while, and one thing that bugs me is how little anyone actually measures A/V sync drift instead of just eyeballing it.

So I built a small diagnostic tool that reads the MP4 structurally (mdhd/elst edit-list boxes, etc.) and reports the real audio/video offset in milliseconds — not "does it look synced," an actual number.

Ran it against Bandicam and EaseUS RecExperts on identical clips: residual offset landed anywhere from ~10ms to ~40ms, with no drift correction over time — the offset doesn't shrink, it just sits there (or slowly grows on longer files).

OBS is the interesting one. Officially it's a fine, capable recorder, but its own forums are full of threads reporting drift in the 0.5s–3s range on longer sessions — and this isn't some fringe capture-card edge case. Multiple threads describe it happening with plain USB webcams and USB mics, no exotic hardware involved. The root cause seems to be physics, not settings: your audio interface has its own internal clock, OBS timestamps frames off the system clock, and a typical ~50ppm clock mismatch alone adds up to roughly 0.18s of drift per hour — which is why short clips look perfect and 2-hour streams don't.

Haven't run numbers yet on Action! or ShadowPlay/ReLive — if anyone here uses those regularly, curious what your longer recordings look like.

Post your recorder + recording length and I'll run the file through the tool for a second opinion, or grab it yourself and check your own footage. Not claiming this is exhaustive — just tired of "trust me it's synced" being the default answer.

AthenaRecorder — ~2h recording, A/V sync result

Recording length: 1h 58m (7082.57s)
Recorder: AthenaRecorder
Ran through the diagnostic tool afterward:

Video duration (mdhd): 7082.566s
Audio duration (mdhd): 7082.566s
Difference: -0.000s (-0.0000%)
Effective A/V difference: +0.000s
[OK] Difference within acceptable margin (<= 0.3s, elst discounted)
[OK] No structural problems detected. Container appears consistent.

Zero drift, even at ~2 hours — no elst compensation needed, the raw numbers already land at 0.000s. This is the kind of result I was talking about: no mixer board, no external audio interface, no expensive capture gear, just the recorder's own sync handling.

Screenshot from the tool attached.

Who's next? Post your recorder + length and I'll take a look, or run it yourself.