Remuxing mkv result in unsynchronized AV

[thenokiottos]

Hello guys, after 5+ hours of converting a series I found with joy that all audio/video were unsynchronized, plus with the pesky kind that grows progressively from beginning to end. I searched the forum but all the threads were either unsolved or found a solution that didn't meet my needs.

From what i found with experiments the fault is not with the converter (MeGui), because even demuxing the files in .h264 and .ac3 streams, then remuxing them with MkvToolnix results in the same problem. Instead, if i leave the video stream checked in mkvtoolnix and (for example) uncheck one audio stream, then remix, the problem doesn't happen.

A particular i noticed was that in MediaInfo there were two framerate fields, a normal one (23,976) and a "original" one (23,970). I think this could be related. Here's the report:


Could you give me a hand? Thanks in advance.


EDIT: Seems that using Handbrake and setting framerate to 23.976 - constant fixed the problem.

[Baldrick]

Yep. Try avoid variable framerate when you convert.

I think handbrake always uses variable framerate as default so you must change to a constant value unde the video tab.

[thenokiottos]

The strange thing is that even the original video had constant framerate, at least MediaInfo said so.

[hello_hello]

Which indexer did you use?

For ffmsindex, MeGUI automatically adds frame rate conversion to the script, so if the source is variable frame rate, it should be converted to constant. You need to check the script to make sure MeGUI has chosen the correct frame rate (it won't effect the audio sync if it hasn't but the picture mightn't look smooth). If you index with L-Smash, MeGUI doesn't add any frame rate conversion so you need to add it yourself for variable frame rate sources.

It's possibly not variable frame rate, but there's gaps in the audio stream, so when you extract the audio as a single file the gaps aren't accounted for and it doesn't stay in sync when remuxed. You can get around that by not extracting the audio. Instead, open the re-encoded version of the video with MKVMergeGUI, add the source MKV, deselect the original video stream and remux. The audio sync should remain unchanged.
Or... extract the audio from the source MKV with the HD Streams Extractor under MeGUI's Tools menu. It uses eac3to to extract and eac3to will attempt to fix any problems it finds, such as gaps, by filling them with silence. You can then check the log eac3to saves to determine if that was the problem.

If it's not audio gaps and the source is variable frame rate you can re-encode it with MeGUI without adding any frame rate conversion to the script (if MeGUI adds frame rate conversion to the script after indexing with ffmsindex, remove it), and extract the video timecodes from the source MKV with gMKVExtractGUI or MKVCleaver. You can then remux the encoded video using the extracted timecodes and that'll give you a remuxed variable frame rate version, the same as the source.

If none of the above solves the problem (my money's on "gaps in the audio" at the moment) maybe post the full MeGUI log file.

Handbrake does seem to handle "problem" and variable frame rate sources pretty well. You can usually achieve the same with MeGUI, you just need to work out what the problem is.

PS A method of "checking" for VFR is to remux the source MKV with MKVMergeGUI while specifying the correct constant frame rate. If you don't specify one when remuxing, MKVMergeGUI leaves the frame rate untouched. If you do, you get a constant frame rate. If specifying the correct frame rate messes with the audio sync when remuxing, the source is variable frame rate.