Remuxing old FLVs to MP4, without re-encoding!


I’d routinely download YouTube videos for playback on my FreeBSD tower back in the late 2000s. Playing in the browser required Flash, and I didn’t want to run the Linux compat libraries just to play video. I fear we’re treading down the same path with DRM now officially part of the HTML specifications, but that’s for another post.

Today I stumbled across stacks of these FLV videos in my backups, some from channels long since past. I contemplated converting them to MP4 before putting them onto Plex, and so I could get previews in file managers:

$ ffmpeg -i "video.flv" "video.mp4"

But this log output caught my eye:

Stream #0:0: Video: h264 (Main) ...    
Stream #0:1: Audio: aac (LC) ...
  httphostheader  :
  encoder         : Lavf58.29.100

Sure enough, many of these videos already had MP4/H264 video and MP4/AAC audio; they were just in an FLV container. So it was just a matter of recontainerising/remuxing them by using ffmpeg’s codec copy feature:

$ ffmpeg -i "video.flv" -c copy "video.mp4"

A loop of a hundred videos took my HP MicroServer less than 20 seconds.

Not all of these videos could be processed this way; I saw a combination of fl1 and vp6f codecs reported in the video streams. Still, it was an fun exercise in nostalgia.

Stream #0:0: Data: none
Stream #0:1: Audio: mp3 ...
Stream #0:2: Video: vp6f ...
  metadatacreator : Manitu Group FLV MetaData Injector 2

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Ruben Schade is a technical writer and infrastructure architect in Sydney, Australia who refers to himself in the third person. Hi!

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