Help me to extract a BGM out of a video.

[Partogi]

So, I have an .mkv video and I want to extract only the background music, erasing both the video and the vocal leaving only the BGM. How am I suppose to do this?

Thank you.

[smrpix]

You're NOT supposed to do it. Download the soundtrack from itunes.

If the audio is stereo and the dialogue is always centered you could invert the phase of one channel to try to negate it -- but it's rarely completely successful. And you still have all the stereo sound effects to deal with. You also may have to deal with the music being mixed around the dialogue creating inconsistent levels.

[jagabo]

Try a Karaoke filter (aka vocal remover). Most audio editor have one. That is what smrpix is describing.

[Partogi]

Thanks for the replies.

It's an anime BGM from an anime sub group. Is it really illegal?

I'll try Karaoke filter.

[Ai Haibara]

If it's from a Japanese anime series, try checking the music CDs released for that series. If it's a decently popular series, chances are they've released a BGM CD for it or more, which might have the track you're looking for. It'd be easier than trying to get a good version of the music by canceling everything else out from the audio track.

[Partogi]

Yes, it's from Japanese anime series.

They haven't or won't release this particular BGM, I think.

[Hoser Rob]

I'd try audacity. It's an audio tool but I've used it to convert the audio stream in files when none of the video tools I usually use had the right audio codec. This was a Hail Mary pass that worked.

Just try opening the video file with audacity. It should open just the audio stream, which you can then process.

I can't guarantee it'll work in this case but I'd try it.

[TreeTops]

There are other free audio extractors out there. Just google it. Audacity can't open some video files.

[jagabo]

With the ffmpeg source plugin Audacity can open most media files.

Keep in mind that vocal remover filters like the one in Audacity remove everything in the center, not just voice.

[MaverickTse]

Directly working on the audio stream to get the BGM can hardly get any satisfactory result.
Japanese Animes in recent decade usually have CD releases, and they often come with Karaoke version.
If there is none in production, try searching for second hand CD if it is an old release.

Strictly speaking, directly using anime's data and re-distribute it usually violates copyright laws or policies on derived-works(二次創作).
However, some authors allow a more lenient derived-work policy that welcome the "imitation" of the original, like re-creating a song in MIDI format(notably the Touhou Project).

So if you cannot get hold of a Karaoke version, try to create a MIDI on your own. Or if you manage to get the music score for that song, type it into MuseScore.

If you have Vocaloid3, you can also use the VocalListener plugin to help getting the scores of the music.

[netmask56]

and remember if you go the invert one channel and sum to cancel out the centre vocal to leave the background only, this will be mono.