How to Fix AV Sync in Live Production

Quick Answer

If the offset is fixed, you can usually correct it with a measured audio or video delay. If the offset changes over time, a static delay will not solve it. In that case, you need to fix the underlying timing, buffering, or network problem first.

What Is AV Sync Error?

Audio/video sync error — commonly called lip sync error — occurs when the audio and video components of a signal are misaligned in time. In broadcast, this manifests as dialogue that appears to come from a speaker either before or after their mouth moves. Even sub-frame offsets (as little as 15–20 ms) are perceptible to trained viewers, and anything beyond 40–50 ms is immediately uncomfortable for general audiences.

The root cause is always the same: video and audio take different amounts of time to travel through the production chain. Video often takes longer due to the computational overhead of scaling, colour conversion, compression encoding, and hardware output buffering. Audio, when processed in dedicated digital audio workstations or routing matrices, typically travels faster — arriving ahead of its corresponding video frame.

Why It Happens

AV sync problems usually come from one of two patterns: a fixed offset or a variable offset. Fixed offsets are caused by predictable latency in one path. Variable offsets come from jitter, unstable timing, dynamic buffering, or load-dependent processing.

The distinction matters. A fixed 40 ms error can be corrected. A drifting 40 ms error needs root-cause work before any compensation is trustworthy.

Step 1: Measure Your Actual Offset

Before applying any compensation, measure the actual offset in your system. Guessing and adjusting by ear is unreliable. Use one of these methods:

Method A: Camera + Clap or Flash Reference

Use a clapper, hand clap, slate, or a short flash-and-beep reference that you can send through the real chain you want to measure. The useful number is the offset you can see or hear in the actual output path, not a browser-only estimate. Record the millisecond reading you observe in the captured output.

Method B: Clapperboard Reference

Film a traditional clapperboard on the camera and feed it through your production chain. In editing software, scrub frame-by-frame to find the frame where the clapper hits, then align it with the audio transient. The frame difference × (1000 ÷ frame rate in fps) = offset in ms.

Method C: Oscilloscope / Audio Analyser

For highest accuracy, route a single reference tonal burst through video as a visual flash and through audio simultaneously. Capture both on a dual-channel oscilloscope or a high-speed DAW to measure the time difference between the two events to sub-millisecond precision.

Step 2: Identify the Source of Delay

Once you know the magnitude and direction of the offset (audio early = video delayed; audio late = audio delayed), identify which component in the chain is causing it. The most common culprits are:

• Capture card processing: Capture devices can add meaningful latency, but the range varies widely by model, buffering mode, resolution, and driver path.
• Video software processing: vMix, OBS Studio, and similar systems add latency through their render and compositing pipelines. The real value depends on load, buffering, and output path.
• NDI High Bandwidth: Usually low-latency, but still non-zero and implementation-dependent.
• NDI HX3 / HX2: Usually adds more delay than NDI High Bandwidth, with the exact amount depending on device implementation and compression strategy.
• Dante / AES67 audio network: Compensation depends on the devices involved, the path design, and where the audio is finally received or mixed.
• Audio interface buffer: Buffer size still matters, and multiple processing stages can stack into a meaningful offset.

Step 3: Apply Compensation

There are two compensation strategies: delay the faster signal to match the slower one, or reduce the latency of the slower path. In live production, reducing latency is often preferable since adding delay worsens the overall end-to-end delay for the operator. However, for broadcast output the critical requirement is sync, not minimum latency.

Fixing Sync in OBS Studio

1. Open the Audio Mixer panel.
2. Click the gear icon (⚙) next to the audio source that is early.
3. Select Advanced Audio Properties.
4. In the Sync Offset column, enter a positive value in milliseconds to delay the audio.
5. If audio is late (video is early), you cannot delay video in OBS — reduce the audio processing latency instead, or add delay to the video path via a hardware frame synchroniser.

Fixing Sync in vMix

1. Go to Settings → Audio.
2. Find the affected input and click its audio settings.
3. Use the Delay slider to add milliseconds of audio delay to align it with the video output.
4. vMix also has a global audio delay output option — useful when the entire production is delayed uniformly by video processing.

Fixing Sync in TriCaster

1. In the Audio Mixer, locate the affected input channel.
2. Access the channel's delay settings (available in the routing matrix view).
3. Set the audio delay in frames. Convert your ms offset to frames: frames = ms ÷ (1000 ÷ fps). Round to nearest whole frame.
4. TriCaster can also apply delay via its internal frame synchroniser on each input — useful for SDI sources with known per-input delays.

Fixing Sync in Dante Networks

Dante latency settings and compensation behavior depend on the devices involved and where the audio is received or mixed. When audio travels over Dante while video takes a different, longer path:

1. Verify the relevant device latency settings and the actual audio path first.
2. Decide whether the needed compensation belongs in the networked audio device, the destination device, or the downstream mixer/application.
3. Use latency settings that match the stability requirement of the network path rather than treating one published value as universally correct.

In real systems, different receivers may need different treatment. Do not assume that one Dante setting solves every destination in the chain.

Step 4: Verify in the Real Output Path

After applying compensation, verify the result with a real recording or programme output capture and check that the remaining offset is within ±5 ms for programme output, or ±15 ms at minimum to meet EBU R37. Re-check after any changes to the signal chain.

Persistent or Variable Sync Drift

If your sync error changes over time or is inconsistent, you are experiencing jitter. A fixed offset can be compensated statically; jitter cannot. Common causes:

• Network congestion causing variable NDI or Dante packet delivery timing
• Missing or unstable PTP/IEEE 1588 clock synchronisation on Dante networks
• GPU render time variation causing frame latency fluctuation in software mixers
• Temperature-driven clock drift in non-genlocked systems over multi-hour productions

For jitter, the solution is to address the root cause — stable network QoS configuration, proper clocking, consistent buffering, and timing discipline — rather than applying static compensation.

Common Mistakes

• Applying delay before measuring the real offset.
• Treating variable drift like a fixed offset problem.
• Assuming a named product always adds the same number of milliseconds.
• Fixing one destination while ignoring the rest of the production chain.

References

• OBS Knowledge Base
• vMix Help
• Audinate Learning
• Audio Latency Calculator