[SOLVED] How To Synchronize Audio in a Matroska (MKV) media file container

[Fermulator]

When one views MKV files, sometimes the audio can be "out of sync" with the video. This post supplies easy to follow instructions on how to inspect the problem, and repair your media file.

The following instructions use "variables" as we learn data for future reference in the format of MY_VARIABLE_NAME.

How to synchronize audio and video in a Matroska (MKV) media file container

Phase 0: Inspection:

Tools Required: mkvinfo, mkvmerge, vlc

1) Identify the file:
- What is the file we're having problems with?

EXAMPLE: MKV_FILE_NAME="MyVideo.mkv"

2) Identify the problem:
- Is the audio delayed, or ahead, and by how much time? (ms)

We can use vlc media player to dynamically figure it out:
VLC can increase or decrease the audio delay on the fly using keyboard shortcuts "j" and "k".

• "j" -> moves audio ahead (anti-delay)
• "k" -> moves audio behind (increase delay)

Play with this until you're satisfied, and note the value.

EXAMPLE: AUDIO_SHIFT=-300ms
i.e. The audio was 300ms behind.

3) Discover the Audio and Video Tracks:
- MKV is a container, it will have one or more audio/video tracks within it. To discover them, run:

$ mkvmerge -i $MKV_FILE_NAME

$ mkvmerge -i MyVideo.mkv
File 'MyVideo.mkv': container: Matroska
Track ID 1: video (V_MPEG4/ISO/AVC)
Track ID 2: audio (A_AC3)

WARNING: Ensure that the audio/video tracks noted here matchthose from step (2) previous.

EXAMPLE: AUDIO_TRACK_ID=2

Phase 1: Adjust Audio Timecodes

Tools Required: mkvmerge

1) Perform the audio sync shift

$ mkvmerge -o ${MKV_FILE_NAME}_FIXED.mkv -y $AUDIO_TRACK_ID:$AUDIO_SHIFT $MKV_FILE_NAME

$ mkvmerge -o MyVideo_FIXED.mkv -y 2:-300 MyVideo.mkv
mkvmerge v4.5.0 ('Speed of Light') built on Feb 23 2011 00:12:51
'MyVideo.mkv': Using the Matroska demultiplexer.
'MyVideo.mkv' track 1: Using the MPEG-4 part 10 (AVC) video output module.
'MyVideo.mkv' track 2: Using the AC3 output module.
The file 'MyVideo_FIXED.mkv' has been opened for writing.
Progress: 100%
The cue entries (the index) are being written...
Muxing took 70 seconds.

Phase 2: Enjoy Synchronized Audio and Video

1) Play the MyVideo_FIXED.mkv file in your media player of choice and enjoy the synchronized audio.