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AV Sync Test — Lip-Sync Calibration

Calibrate lip-sync with flash-and-click cadence across 5 offsets and dial in TV or soundbar delay. Find video vs audio lag. Free in your browser, no download.

How it works

Visual-acoustic coincidence calibration with a 5-step rhythmic cadence (4 prep ticks + 1 primary sync flash/click) and continuous -300 ms to +300 ms relative offset adjustment adhering to ITU-R BT.1359-1 standards.

Audio-video synchronization (lip sync) is the precise temporal alignment between the image displayed on your screen and the sound waves reaching your ears.

The Physics of AV Synchronization

In the natural acoustic environment, sound travels through air at approximately 343 meters per second (1,125 ft/s, or roughly 3 milliseconds per meter). When you watch a musician on stage from 20 meters away, their sound naturally reaches your ears 58 milliseconds after you see their fingers strike the strings.

Because the human visual and auditory processing cortex evolved in this physical world:

  1. Audio Lagging Video (Sound After Picture) is psychologically natural up to ~125 ms.
  2. Audio Leading Video (Sound Before Picture) violates natural causality and is immediately detected at just +45 ms.
ITU-R BT.1359-1 Perception Limits:
┌───────────────────────────────┬───────────────────────────────┐
│ Timing Relationship           │ Perceptual Threshold          │
├───────────────────────────────┼───────────────────────────────┤
│ Audio Leads Video             │ Detectable at +45 ms          │
│                               │ Unacceptable beyond +90 ms    │
├───────────────────────────────┼───────────────────────────────┤
│ Audio Lags Video              │ Detectable at -125 ms         │
│                               │ Unacceptable beyond -185 ms   │
├───────────────────────────────┼───────────────────────────────┤
│ Broadcast Reference (SMPTE)   │ Strict target: ±15 ms         │
└───────────────────────────────┴───────────────────────────────┘

How Modern Systems Fall Out of Sync

Desynchronization is rarely caused by audio or video files themselves; it is introduced by playback hardware:

  1. Display Image Processing (Video Lag): Smart TVs with “Motion Smoothing”, noise reduction, and AI upscaling buffer multiple video frames before rendering, delaying the picture by 60 to 180 ms.
  2. Bluetooth & Wireless Transmission (Audio Lag): Bluetooth codecs (SBC, AAC, LDAC) buffer packets to prevent dropouts, delaying audio output by 120 to 250 ms.
  3. HDMI eARC / Optical Soundbar Routing: Passing HDMI through a TV before sending audio to an AV receiver or soundbar frequently introduces an uncompensated 30–80 ms handshake delay.

How to Calibrate Your System

  1. Start the Test: Press Start AV Sync Test (or tap Spacebar).
  2. Lock to the Rhythm: Listen to the 4 preparatory beats (steps 1 through 4) to anticipate the upcoming tempo.
  3. Observe Step 5: Watch the central flash reticle and listen to the primary audio transient click on step 5.
  4. Adjust the Offset Slider:
    • If the click sounds before the flash, slide toward Audio Leads (-).
    • If the click sounds after the flash, slide toward Audio Lags (+).
  5. Read the Compensation Value: When the flash and click feel unified as a single sensory event, the offset readout indicates your hardware chain’s timing error.

Applying the Correction:

  • AV Receivers / Soundbars: Look for Lip Sync, Audio Delay, or AV Sync in your sound menu and enter the offset in milliseconds.
  • Smart TVs: Enable Game Mode or PC Mode to bypass video frame processing and eliminate up to 100 ms of display latency.
  • VLC / Media Players: Press J or K during video playback to adjust audio track synchronization in 50 ms increments.

Frequently asked questions

What is audio-video (AV) synchronization and lip-sync error?

Audio-video synchronization—commonly known as lip sync—is the precise temporal alignment between the visual image on screen and its corresponding acoustic soundtrack. When sound and picture are misaligned, speech appears disjointed from mouth movements, explosions hit before sound waves arrive, and immersion breaks down.

What causes audio and video to go out of sync?

Desynchronization stems from disparate processing pipelines: modern 4K/8K displays perform heavy image processing (frame interpolation, dynamic HDR tone mapping, upscaling, local dimming) that delays video by 50–150 ms. Conversely, wireless Bluetooth audio (A2DP SBC/AAC) introduces 100–220 ms of transmission buffering, delaying the sound relative to the picture.

Why does human hearing tolerate audio lagging video better than audio leading video?

In the physical world, light travels at ~300,000 km/s while sound travels at ~343 m/s (~1 ms per foot / 3 ms per meter). If a speaker or person is 15 feet away, their sound naturally reaches your ears ~15 ms after you see them speak. Because the human brain expects sound to arrive after or simultaneous with light, hearing audio BEFORE seeing an action is physically impossible in nature and feels intensely unnatural.

What are the official ITU-R BT.1359-1 broadcast sync limits?

International Telecommunication Union (ITU-R BT.1359-1) standards define: (1) Detectability threshold: Audio should lead video by no more than +45 ms, and lag by no more than -125 ms. (2) Acceptability threshold: Audio should lead by no more than +90 ms, and lag by no more than -185 ms. Broadcast masters (SMPTE ST 2097 / ATSC A/85) target a strict ±15 ms window.

How do I use the 5-step calibration pattern to set my delay?

Press 'Start AV Sync Test'. Follow the 4 prep ticks (steps 1–4) which establish a steady tempo, then observe step 5 where the visual flash and audio click occur. If the click happens before the flash, audio is leading; if it happens after, audio is lagging. Drag the offset slider until the click and flash feel perfectly simultaneous. The slider value then gives your system's exact compensation requirement.

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