HOWTO: losslessly convert FLV video to MP4 in Linux (Debian)

Well, let's imagine we have a Flash Video (FLV/F4V) container file (possibly "unseekable" or with other minor problems) containing MPEG-4 AVC video & AAC (or MP3) audio.

We'll convert ("remux") it to MP4 without any transcoding (re-encoding), so it'll be very fast, and using only the free tools (available in the official Debian GNU/Linux repositories).

Note: unfortunately, old FLV videos containing VP6 and Sorenson video formats can't be remuxed into MP4 format; if you don't want re-encoding, you can try to remux them into MKV (Matroska); if you just want to fix the seek problems, you can just try FLV-to-FLV remuxing.

• Method #1 (recommended): use ffmpeg (it's the main utility from the FFmpeg project).
  Install the "ffmpeg" package:
  

  # apt-get install ffmpeg

  Convert a FLV file to MP4, preserving audio & video:
  

  $ ffmpeg -i input.flv -codec copy output.mp4

  And even optimizing it for faster loading:
  

  $ ffmpeg -i input.flv -codec copy -movflags faststart output_optim.mp4

  To extract audio only:
  

  $ ffmpeg -i input.flv -codec copy -vn output_a.mp4

  To extract video only:
  

  $ ffmpeg -i input.flv -codec copy -an output_v.mp4

  Note: you can also use M4A (audio) / M4V (video) file extensions for output, if you like those Apple formats.
• Method #2 (alternative): using avconv (it's a part of Libav project, a fork of FFmpeg).
  Install the "libav-tools" package:
  

  # apt-get install libav-tools

  Convert a FLV file to MP4, preserving audio & video:
  

  $ avconv -i input.flv -codec copy output.mp4

  To extract audio only:
  

  $ avconv -i input.flv -codec copy -vn output_a.mp4

  To extract video only:
  

  $ avconv -i input.flv -codec copy -an output_v.mp4

• Method #3 (a somewhat ugly one): using MEncoder (a part of the MPlayer project).
  Install MEncoder:
  

  # apt-get install mencoder

  Convert a FLV file to MP4:
  

  $ mencoder input.flv -ovc copy -oac copy -of lavf -o output.mp4

  Note: according to the warning from MEncoder itself, output file can be corrupt:

  ** MUXER_LAVF *****************************************************************
  REMEMBER: MEncoder's libavformat muxing is presently broken and can generate
  INCORRECT files in the presence of B-frames. Moreover, due to bugs MPlayer
  will play these INCORRECT files as if nothing were wrong!
  *******************************************************************************

  So probably it suggests that you should better try the MEncoder's favorite AVI format:

  $ mencoder input.flv -ovc copy -oac copy -o output.avi

  (But that's not what we want here, right?)

Source software packages availability for Debian releases:

• ffmpeg: up to Debian 6 "Squeeze", and then again from 9 "Stretch" (also in 8 "Jessie" backports);
• libav: starting from 7 "Wheezy" (also in 6 "Squeeze" backports), up to 8 "Jessie";
• mplayer: up to 7 "Wheezy", and then from 9 "Stretch".

And yes, it should work for Ubuntu too; any current FFmpeg version should also work in other GNU/Linux or *BSD distros, MS Windows, Apple Mac OS X etc.

References:

1. ffmpeg Documentation
2. Libav documentation : avconv
3. 6.1. Selecting codecs and container formats - Chapter 6. Basic usage of MEncoder (MPlayer - The Movie Player documentation)
4. Wikipedia:
   1. Flash Video
   2. MPEG-4 Part 14
   3. M4V
   4. H.264/MPEG-4 AVC
   5. Advanced Audio Coding
   6. Audio Video Interleave

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Last updated: 2017-12-28