AV Out of Sync after Muxing

[hpman247]

I am having an odd issue. When I demux a mkv, avi, or MP4 (any format) and separate the .h264 file from the either .ac3 or .aac file, and try to remux to another format, my audio and video is out of sync and the video length is always like 15% longer than it was before I demuxed it and remuxed it.

For instance, I have an MKV that is about an hour long with aac audio. I demuxed it with MKVExtract to get the indivudual files and used YAMB to remux to a .mp4 file. it doesn't work, and I'm just wondering why it is so out of sync. I noticed in VLC that the original file says 29.97..... for the frame rate and when i remux it to .mp4 it shows only 25 fps.

This happens even for instance if I use YAMB to extract the seperate files from an mp4 and even remux to an mp4. It's as if extracting the video file inherently lowers the frame rate and makes the file longer or something. It's so damn confusing and I have no idea why it works like this? Any ideas?

[Baldrick]

Have you tried any mkv to mp4 converters? Too see if you get same problem.

https://www.videohelp.com/tools/sections/all-in-one-mkv-to-mp4-avchd-ts

[hpman247]

Thanks so much for that link. I will look over all those programs. I am curious tho. I have some videos in an AVI container with AC3 audio and H264 video. Whenever I attempt to remux to an mp4 the AV is out of sync, and whenever I remux to a MKV everything is in sync but the video is really jittery.

I think I have solved the problems with the MP4s and MKVs, but the issue with the AVIs is still very much present. I have tried many things like Super, Yamb, AVIDemux, and etc with no luck on those particular AVI files.

If there is any information that you all need to help just let me know and I will try to provide it!

[jagabo]

I noticed in VLC that the original file says 29.97..... for the frame rate and when i remux it to .mp4 it shows only 25 fps.

Set the frame rate to 29.97 in Yamb: highlight the video track, press Properties.

[heretic754]

I had the same issue. I eventually discovered that the MP4s I made had a 'variable' frame rate (even though nominally it was 29.97). My sync was ruined when I demuxed then muxed back together, and would discover that the newly muxed file was 25fps. I had to re-rip my DVD's to MP4, and make sure 'constant' frame rate was selected. I was then able to demux and mux without sync issues.