How to convert .mkv file into .mp4 file losslessly?

Question

I need to convert a video file from Matroska container into mp4 container. Matroska file contains one h.264 video track and one AC3 sound track. It should be possible to do this losslessly, but how to do it with native Ubuntu tools?

Transcoding is not an option.

Answer 1

I think the easiest and most reliable tool for that is avconv from libav-tools, Ubuntu's successor of the once venerable ffmpeg. It is the backend for many of the GUI tools cited in other answers.

Changing container without re-enconding content could not be simpler:

avconv -i input.mkv -codec copy output.mp4

It auto-detects a Matroska > MPEG4 container conversion based on input/output filenames. -codec copy tells all content should be copied as-is without re-encoding.

If format auto-detection fails, you can use the -f option before either input, output, or both. From the manual:

f fmt (input/output)
           Force input or output file format. The format is normally
           autodetected for input files and guessed from file extension for
           output files, so this option is not needed in most cases.

Also, after reading the comments, it is worth noting: the above syntax will copy all the files' content to the new container, but some containers may not support some content. So check if your chosen container format, be it mkv, mp4 or even avi has support for all the content in your files (video type, audio type, subtitles, etc). For example, mp4 does not support soft subtitles (.str files)

Answer 2

Get needed software

sudo apt-get install gpac mkvtoolnix

Extract video and audio from matroska file

First you need to check what tracks the matroska file contains with

mkvmerge --identify video.mkv

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

Then extract video and audio according to their tracks with:

mkvextract tracks video.mkv 1:video.h264 2:audio.ac3

Extracting track 1 with the CodecID 'V_MPEG4/ISO/AVC' to the file 'video.h264'. Container format: AVC/h.264 elementary stream
Extracting track 2 with the CodecID 'A_AC3' to the file 'audio.ac3'. Container format: Dolby Digital (AC3)
Progress: 100%

Mux audio and video into mp4 containter

MP4Box -fps 24 -add video.h264 -add video.ac3 video.mp4

AVC-H264 import - frame size 1280 x 720 at 24.000 FPS
Import results: 31671 samples - Slices: 5672 I 125048 P 122648 B - 32288 SEI - 4936 IDR
AC3 import - sample rate 48000 - 6 channels
Saving to video.mp4: 0.500 secs Interleaving

I had to add -fps 24 option because MP4Box didn't detect video fps correctly. Depending on usage AC3 audio track should be re-encoded into something else before muxing.

Now you have remuxed streams from matroska container into mp4 container without losing any quality.

EDIT: MP4 doesn't officially support AC3 audio so the audio track should be converted into a supported format (eg. AAC, MP3) if you want the file to be playable with something else than VLC.

Answer 3

You can use Avidemux

It has good GUI interface.

When converting from mkv to mp4 it is not re-encoded.

The convertion is done within a minute (for a 60-min video).

1. sudo apt-get install avidemux

2. Open the mkv file in avidemux.

3. Select safe mode if prompted.

4. Leave the video as copy.

5. Choose File-> Properties. Check if the audio codec is aac.

   1. If audio codec is NOT aac, select aac(faac).
   2. Otherwise, leave it as copy. (Sometimes using this option the audio may be out of sync or distorted, in that case select aac(faac) instead.)

6. Select mp4 for format.

7. Select save, choose location and type a filename with .mp4 as the extension.

Note: Sometimes (~10% chance) it may fail. Then use handbrake(re-encode).

Answer 4

You can try HANDBRAKE by adding its ppa by typing this code into the termianl:

sudo add-apt-repository ppa:stebbins/handbrake-releases

Answer 5

try Arista transcoder.

This is the best one I've seen. This is available from ubuntu software center