How to strip audio streams from an MKV file?

Question

I've been trying to remove unneeded audio streams from an MKV (Matroska) file. The reason why I want to do this is to avoid having to manually select the wanted stream in Windows Media Player.

The input file gives the following ffmpeg info:

Stream #0.0: Video: mpeg4, yuv420p, 704x396 [PAR 1:1 DAR 16:9], 29.98 tbr, 1k tbn, 29.98 tbc
Stream #0.1(eng): Audio: aac, 24000 Hz, 5.1, s16
Stream #0.2(jpn): Audio: aac, 24000 Hz, 5.1, s16
Stream #0.3(eng): Subtitle: 0x0000
Stream #0.4(eng): Subtitle: 0x0000
Stream #0.5: Attachment: 0x0000
Stream #0.6: Attachment: 0x0000

Since I want streams 0, 1 and 3 (sub), my ffmpeg command looks like this:

ffmpeg -i input.mkv -map 0:0 -map 0:1 -map 0:3 -vcodec copy -acodec libmp3lame -newsubtitle test.mkv

which strangely gives the error:

At least one output file must be specified

Removing the subtitles:

ffmpeg -i input.mkv -map 0:0 -map 0:1 -vcodec copy -acodec libmp3lame test.mkv

gives me this:

Number of stream maps must match number of output streams

I seems I don't really understand how the "map" option works. Would someone help me figure it out?

Answer 1

If all you want to do is remove a stream, not re-encode, you probably want to do this with the MKVtoolnix package (see videohelp.com's page).

There are a couple of GUIs that may help you (check around on the videohelp link). I'm not sure what the exact mkvmerge commandline might be, but I think something like this would work:

# first, get audio track info so we know which one to keep
mkvmerge -i input.mkv
File 'input.mkv': container: Matroska
Track ID 1: video (V_MPEG4/ISO/AVC)
Track ID 2: audio (A_AAC)
Track ID 3: audio (A_AAC)        <----------- for example, let's keep this one
Track ID 4: audio (A_AAC)

mkvmerge -o output.mkv --atracks 3 input.mkv

That should do the trick (I don't have a file handy for testing, sorry). The --atracks option tells mkvmerge to copy only the listed audio tracks to the new file. If you wanted to keep 2 & 3 but not 4, you could use --atracks 2,3.

mkvmerge has a lot of other options for setting titles, adding a delay to sync audio, etc, so check the manpage for details.

Answer 2

Try -f to force the specified format:

ffmpeg -i input.mkv -map 0:0 -map 0:1 -map 0:3 -vcodec copy -acodec libmp3lame -newsubtitle -f mkv test.mkv

Answer 3

A little late, but to help others, or maybe help myself when I forget later, really two separate issues here.

1) The 1st command line, the order of parameters may be wrong. I think the -newaudio needs to be at the end, after the output file name.

2) The 2nd command line, "Removing the subtitles", since the input file has subtitles, you need to use the "-sn" to remove their existence. Otherwise, it seems FFmpeg wants at least one specified in the "-map" list.