Need help muxing WEBM/VP9 to MKV (MKVToolNix GUI)

[bitrape]

Hello! I am currently attempting to demux VP9 video and Opus audio from a Webm container using MKVcleaver. I then remux the video and audio stream into MKV container using MKVToolNix GUI.
But when I play the MKV file, the video is lagging. I don't just mean the video/audio are out of sync, the video is literally dragging in slow motion!
I tried demuxing just the VP9 stream and putting it in MKV container by itself, but the problem is still there.
So it seems when the VP9 stream is extracted from its Webm container, it gets messed up somehow.
Any suggestions?

[Baldrick]

Can't you import the video directly in mkvtoolnixgui? Instead of demuxing.

And post more information from your source file. Use mediainfo, open video, view->text and copy everything.

[LigH.de]

AFAIR, WebM is already a special MKV variant. You you already have MKV, just with more tightly specified attributes and a different filename extension.

In general, you should always try to import still multiplexed files into a container remultiplexer; if the source format is supported, doing so ensures that the sync is preserved.

[blud7]

You need to open the file directly (import directly) into mkvtoolnix (mkvmerge) AND specify a frame rate.
Play back the original file in a media player like MPC or VideoLan etc and check the frame rate. Then specify when re-muxing.

[bitrape]

The video stream and audio stream are in separate Webm containers. This is how they were when I downloaded them off Youtube. Anyways I tried putting both Webm streams directly into MKVtoolnix and muxing them together. When I played the MKV file and the lagging problem is gone but now the video keeps distorting every few seconds!

Here's the info for the original files

If it will help I can upload the muxed MKV file that is having this new 'disorting' problem.

[LigH.de]

You downloaded from YouTube ... so it was a DASH stream. What did you use to download it? There is good software which can handle DASH streams well, e.g. the "Complete YouTube Saver" (CYS) addon for Firefox lets you select a variety of stream combinations (you also have a separate option to prefer Opus audio streams in WebM formats) and can save them multiplexed if you also install ffmpeg (its configuration contains links to Zeranoe builds).

[bitrape]

I used Flashgot 1.5.6.13 addon for Firefox.

[ricardouk]

use complete youtube saver...specify ffmpeg folder in options and then youll have the option for muxed webm + opus

here's the video:
https://db.tt/AZL0tkcE

[bitrape]

I don't have ffmpeg. I've tried to use it in the past but I could never figure it out. I guess that's a discussion for another thread. Thanks for the help.