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I have 3 flv files which I wish to merge. I used the answer in this post:

Merge multiple flv files?

which suggested this option:

mencoder -forceidx -ovc copy -oac pcm -o output.flv input1.flv input2.flv

It seems to have worked except that the merged file is 400meg in size whereas the original 3 files were about 33meg each.

Is this expected? Is there a way to join the 3 files without the massive increase in size?

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I can't explain the massive increase in size. Check my answer for a solution using ffmpeg. – Patrick Jan 18 '13 at 15:09
Hmmm, with mencoder, isn't that actually making an AVI file (but with a .flv suffix)? – derobert Jan 18 '13 at 15:49

Create a file mylist.txt with all the files you want to have concatenated in the following form:

    # this is a comment
    file '/path/to/file1'
    file '/path/to/file2'
    file '/path/to/file3'

Then you can encode your files with:

    ffmpeg -f concat -i mylist.txt -c copy output

Edit: You may need to compile ffmpeg from source to get a recent enough version.

Source: http://ffmpeg.org/trac/ffmpeg/wiki/How%20to%20concatenate%20(join,%20merge)%20media%20files

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thanks. Unfortunately I am getting: Unknown input format: 'concat' – Carl Jan 19 '13 at 22:18
okay, wait. I think I have an old ffmpeg version – Carl Jan 19 '13 at 22:22
confirmation that it works except you do need ffmpeg 1.1 which I had to install from source. Even the repository holding the latest latest for Debian only had 1.0.1 – Carl Jan 19 '13 at 23:22

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