Audio and video sync test
When mouths move before the words arrive, something between the video and your ears is adding delay — Bluetooth headphones, a soundbar, a television doing picture processing, or the computer itself. This puts a flash and a click on the same beat and lets you move the click until the two land together. Where the slider ends up is your offset, in milliseconds, measured by the only instrument that matters for watching a video: you.
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A flash and a click, once a second. Move the slider until they land together — then lock it in. Give each position five or six beats before judging.
Flash and click together
- Measured offset—
- Verdict—
What did the offset come out at?
Running it
Start the beat. A white square flashes and a click sounds, once a second. Watch and listen for five or six beats before touching anything — the first impression is unreliable.
Then move the slider. Moving it left plays the click earlier, right plays it later. Find the position where the flash and the click feel like one event, give it another five beats to be sure, and lock it in. The result is the delay your system puts between picture and sound, with its sign: sound behind the picture, or ahead of it.
Use whatever you actually watch with. If the question is about Bluetooth headphones, wear them; if it is about the television, sit where you sit. The measurement includes everything between the computer and you, which is the point.
What the number includes
Everything. The display pipeline and the panel; the audio driver, its buffer, and whatever the sound travels through afterwards — a Bluetooth link, an HDMI hop through a receiver, an optical cable to a soundbar. A browser cannot see any of those, and no page that claims to measure lip sync by machine is seeing them either. This page does not try. It puts a known event on each path and lets you report where they meet.
That makes it a comparison tool as much as a measurement. Run it wired, then wireless. Run it with the television’s game mode off, then on. The difference between two runs is exact even though each run is a judgement.
Windows 11
- Windows has no global audio delay setting. Compensation is done in the player: VLC under Tools → Track Synchronization, Kodi and Plex in their playback settings.
- Bluetooth headphones show as two devices — Stereo and Hands-Free. Stereo is the high-quality, high-delay one; Hands-Free is lower delay and telephone quality. Windows switches to Hands-Free whenever an application opens the microphone, which is why the delay changes when a call starts.
- If the sound goes through a receiver or soundbar over HDMI, the receiver’s own lip-sync setting applies on top of everything here.
macOS
- Same story: no system setting, compensation per player. VLC and IINA both have an audio delay control; QuickTime does not.
- AirPods negotiate a lower-delay mode with Apple devices than with anything else, so the same headphones measure differently on a Mac and on a Windows machine, and both readings are correct.
- A display connected over HDMI may be identified as a television and treated to processing on its end; the measurement will show it, and the display’s own PC or game mode is the fix.
When it is the content
If the equipment measures in sync and a particular video is not, the offset is baked into the file or added by the player. Streaming services occasionally ship a stream with a fixed offset, and some players introduce one under load. Try another player, then another file, before changing a setting that will then be wrong for everything else.
Common questions
How does moving a click measure anything?
By cancellation. The page schedules the flash and the click for the same instant. If your system delivers sound 120 ms late, they will not look together — until you slide the click 120 ms earlier, at which point they do. The slider position is the delay your system added, with the sign flipped. It is the same method as the calibration screen in a rhythm game, and it works because your judgement of simultaneity is good to a few tens of milliseconds, which is the same precision the problem has.
Why does the page not just measure it?
Because nothing in a browser can see when the light left the screen or when the sound left the speaker. It can see when it asked for each, and between asking and happening sit the display pipeline, the audio driver, the Bluetooth link, the soundbar. Those are exactly the things you want measured, and they are invisible to code. Your eye and ear are the only sensors on the far side of them. The audio latency test measures a different thing — the round trip through your microphone — and can be done by machine.
My headphones are Bluetooth. What should I expect?
Somewhere between 100 and 250 ms of sound arriving late, depending on the codec, and it is not a fault: Bluetooth audio is buffered to survive dropouts. Some headphones and phones negotiate a low-latency codec that brings it down to around 40–80 ms; some do not. The only way to remove it is a cable. Video players that know about it — most phone players and streaming apps — delay the picture to match automatically, which is why films are usually fine and games and live video are not.
Sound is early, not late. How is that possible?
The picture is late. A television or projector doing motion smoothing, noise reduction or upscaling holds each frame for a while before showing it, and if the sound goes a different route — a soundbar via optical, headphones from the computer — the sound gets there first. Game mode or PC mode on the display turns that processing off and is usually the entire fix.
What number is acceptable?
Under about 45 ms is not noticed by most people; up to about 125 ms is noticed and tolerated; beyond that it reads as wrong. Sound arriving late is caught sooner than sound arriving early, because light reaching you before sound is the order nature always produces and the other way round never happens. These thresholds come from the broadcast standards work, and they match everyday experience well.
Where do I enter the number I measured?
In the player or the display, whichever has an audio delay control. VLC: Tools, Track Synchronization. Kodi and Plex: audio offset in playback settings. Most televisions: Sound settings, audio delay or lip sync. Enter the measured offset — positive to delay sound if it was arriving early, or delay the picture if the sound was late and the setting allows it. Windows and macOS have no system-wide setting; it is done per player or per device.