Latency and sync test file: how to use it

By James Jones, founder of Granary Digital · Updated 5 November 2026

Sync problems on a live show are easy to miss until a speaker's lips and voice drift apart on a big screen. This test file lets you measure the gap instead of guessing. It shows how far the picture and the sound arrive apart through the path you test.

What the test file contains

The file is 30 seconds long, in 1080p25 MP4. At the start of every second there is a white flash frame and a 40 millisecond, 1 kHz beep, so there are 30 flashes and 30 beeps. The flash and the beep come together at the start of each second. A frame counter runs on screen, so you can read the frame number from a recording without counting by hand.

Before you start

• Match the frame rate. The file is 25 frames a second. A chain running at 50 or 60 frames a second will convert it, and the conversion can add its own delay, so test at the rate you intend to run.
• Use the same route the show uses: source, mixer, encoder and screen, in the same order.
• Record the programme output, at the point where it leaves the mixer or the encoder. Testing only the source tells you little about what the audience gets.
• Have a clean monitor you trust, and keep the sound level set where the show runs it.

Step by step

1. Play the file from the source you will use on the day, full screen.
2. Record the output at the point where the programme leaves the mixer or the encoder.
3. Find one flash and its beep in the recording. Note the frame numbers from the counter.
4. Measure the gap in frames between the flash and the beep. Multiply by 40 ms to get the time.
5. Repeat on several flashes to check the gap is steady.
6. Set the audio delay, or the video delay if the mixer has one, to match.
7. Run the test again with the whole chain running, and with the speakers and screens in use, to see whether the gap changes.

Worked example

A stream goes out through an encoder. The recording shows the flash arriving three frames after the beep. At 25 frames a second, three frames is 120 ms, so the picture is 120 ms behind the sound. The sound needs a delay of about 120 ms on the programme output. After the change, the test shows the flash and the beep in the same frame, and it stays that way on the next several flashes.

Common mistakes

• Judging sync by eye on a laptop screen, which hides a gap of a few frames.
• Testing the source and never the output.
• Changing a delay once and not testing again after anything else in the chain changes.
• Using the file at the wrong frame rate, then reading the result as a fault in the chain.
• Forgetting that the audio path adds its own delay, especially on a wireless link.

For the wider picture of how a live chain fits together, read the do you need a vision mixer guide and the live streaming problems guide. The NDI, SDI and HDMI guide explains how each link adds its own delay.

Related guides

• Vision mixing basics: ATEM and vMix
• Streaming bitrate and bandwidth calculator
• Live event run sheet template, and how to fill it
• NDI, SDI or HDMI: which to use and when
• Free software for live production: OBS, Companion, CasparCG
• All guides and tools

The picture at the top of this page is an illustration, not a photograph of a client or a real job.