# What is AI lip sync?

> AI lip sync changes a speaker's mouth movement in a video to match new audio. When you need it for dubbed or avatar ads, a cost example and a quick quality check.
- **Author**: Marxx
- **Published**: 2026-11-11
- **Category**: AI
- **URL**: https://marxx.ai/library-glossary/ai-lip-sync

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AI lip sync changes the mouth of a person in a video so it matches new audio. You use it after dubbing, or to make a still face or avatar speak. Without it, a dubbed video shows lips moving to the old words.

## How it works

Most tools take a video and a new audio file. They work out the mouth shape for each sound and redraw the mouth area frame by frame. The rest of the picture stays as it was.

## When you need it

| Your ad | Lip sync needed? |
|---|---|
| Voiceover over product shots | No. No face is speaking |
| Talking-head video, dubbed into a new language | Yes. The mouth shows the old words |
| AI avatar reading a script | Usually built in |
| Founder video with a changed line | Yes, if the line is on camera |

## A worked example

Say your talking-head ad runs 30 seconds at 25 frames a second. That is 30 x 25 = **750 frames** to redraw. Say the tool charges INR 10 for each second of video.

**Cost = seconds x rate**

- One language: 30 x INR 10 = **INR 300**
- Three languages: 3 x INR 300 = **INR 900**

The numbers are made up. The point is that cost grows with length and with every language. Cut a 30-second ad to a 20-second one and you pay a third less in this example.

## Quality check

1. Watch at full size on your phone.
2. Look at fast speech and sounds like "p", "b" and "m", where the lips must close.
3. Look for blurry teeth or a mouth that flickers.
4. Make sure the face looks at the camera and no hand covers the mouth. These shots are often hard for tools.

If it still looks off, cut away to product shots during the hard lines. For more ways to avoid a fake look, see [how to avoid AI-looking ads](/guides/how-to-avoid-ai-looking-ads).

Lip sync is one step in the dubbing flow. Read [AI dubbing](/glossary/ai-dubbing) for the whole flow, and [how to dub ads into Hindi](/guides/how-to-dub-ads-into-hindi) for a worked guide.

Marxx has a production canvas for image, video, voice, dubbing, avatars and UGC that publishes campaigns, ad sets and ads to Meta. See [the production canvas](/features/production-canvas).

## FAQ

### Is lip sync the same as dubbing?

No. Dubbing replaces the audio. Lip sync changes the picture so the mouth fits that audio.

### Does lip sync work in every language?

It depends on the tool. Test a short clip in your language before you process a whole ad.

### Can I lip sync an AI avatar?

Avatar tools usually build the mouth movement in. See [AI avatar](/glossary/ai-avatar).

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