Sync problems after merging mkv video with mp3 audio

[billman1234]

i hope my thread is in the right place
i have this problem:
i have some episodes of an anime in mkv format, with jaapnese audio and french subs
i also have these episodes with french audio in an avi format
the mkv files have much better quality
so i want to put the french audio on the mkv videos
so, i extracted the mkv video with mkvextract gui
and i extraxted the audio from the avis with AoA audio extractor basic in mp3 format
when i tried to merge them with mkvmerge, it worked on the first 10 episodes
but from ep. 11 and on, audio and video are not sync, and whereas in the beginnin they are sync, as the video goes the audio is getting more and more desync.
what should i do ?

[poisondeathray]

Don't extract the video from the .mkv. If you have timecodes, and don't use the time codes when muxing it will go out of sync.

Instead of extracting the video from the mkv, just extract the audio from the other video, use mkvmergegui on the mkv, checkmark the stuff you want, and add the new audio track(s), and uncheckmark the stuff you don't want

But if they are different lengths, or from different versions of the same film, you cannot do much. You would have to shrink/expand either the video or audio to match. There are some options to do this in mkvtoolnix, or you can use external programs

Also make sure the fps matches in the settings when you mux

[billman1234]

thank you ! i've tried what you have told me but they are still desync!!!
maybe they are different versions of the same film! i know that the avi files from which i took the audio have 23.978 fps but i don't know the fps of the mkv files! do you think it is important?
do you know any programs that can expand/shrink video or audio so that they match ?

[billman1234]

furthermore, the audio is in mp3 format! do you think another format (like aac) would be more appropriate ?
and i still can't understand why it worked well in the first episodes and later on it seems impossible to sync!

[poisondeathray]

use mediainfo on the mkv, and the french audio , see if the times match (duration), also check the fps of both videos

you can shrink/expand the audio with mkvtoolnix; just highlight the audio track and go to format specific options tab, enter the fraction in the stretch by box

e.g. if the video was 99/100 the length of the audio (audio is slightly longer), you would stretch the audio by 100/99, or something close to that..you'll have to do some experimenting

Or you can use an audio editor like audacity to resample and stretch/shrink the audio to match

furthermore, the audio is in mp3 format! do you think another format (like aac) would be more appropriate ?

This shouldn't matter. If you convert the audio, all you would do is lose quality (it would still be out of sync)

[Drazick]

Is there a tool which let you preview the syncing before muxing?

Thanks.