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I wish to present a small list of video clips. Using the built-in tools that use a file reference to an mp4, I find that, in preview or live, there is a significant timing mismatch between video and associated audio. In contrast, when using a URL to a third party file store, the lip synch is good.
What is required to restore lip synch. ?

Windows 11
WYSIWYG v18.4.2

How is this related to the use of the software? What has lip sync got to do with it. If they are out then it will be related to the video/audio and how it was encoded as the software has no control over this.

It is related because the built-in tools cause the loss of synch. When the same file is played externally (not thru' WYSIWYG) or via a URL, lip synch is maintained.

The video will be published as is, without any modifications. It will be played using the browser's built-in video player, over which WWB has no control. Video playback is entirely determined by the browser's functionality.

It is related because the built-in tools cause the loss of synch

I am sorry but you are wrong! the tools or the software do not modify the files in anyway they are published "AS IS". You have contradicted this yourself by saying the following when "live" they are out, so by "live" you mean published so this says it is the file at fault and not of the software. And as Pablos reply it is more related to the browser and how it rendered the file.

I find that, in preview or live there is a significant timing mismatch between video and associated audio

Just out of curiosity, how large is each file approximately?

Thanks for the replies. I've been experimenting. Chrome and Edge play the video in synch. Firefox (my default browser) on Windows/Android does not.
So, how do I cater for 'clients' with a variety of browsers ? Is there a more portable or more suitable format than mp4 ?

I would say that my files will be 800k to 1G each.

Ooops 800M to 1G

I'm not an expert at it but if anything would have to buffer that it could cause a delay. Try making a test video that is around 100 MB to see if your problems are gone then.

Most video tools have an option to save in web safe format (mp4).

One question that needs to be asked. Does your host/server allow the use of mp4 files for playback?. You have written 1g do you mean 1gb?. If so then 1 GB = 1000 MB or 1 000 000 000 bytes is quite large and your host may have a file size restriction, this would/could explain the out of sync as there is a delay. You should contact your host and ask if they have any file size restriction and if so it might be best to upload the larger files to YouTube as then you should have no issues.

that is a large file size for a video almost the size of a full movie. It needs fast and really robust server which can take the load. If you experiment it with a 6mb CUT CLIP you may be able to see some difference in performance.

Thanks all for your informative replies. In the main, I have confirmed the observation that playback is browser dependant. Despite Firefox being more sensitive than Edge, Chrome and Safari, I eventually found that there was something wrong with my mp4 ! Having fixed that, all is now well

It is related because the built-in tools cause the loss of synch

So...it was not related to the software after all!! but as mentioned browser, and a issue with the file. anyway it good to know you have fixed it...