[FFmpeg-user] FFmpeg concat outputs bad video file

[Paul B Mahol]

On 10/6/15, Ryan Mirman <rsgm123 at gmail.com> wrote:
> When I try to concat these 3 video files, it outputs a video with a too
> short of a duration and way too high of a bitrate.
>
> I am not sure if I am using ffmpeg the wrong way, if the audio is
> corrupted, or if it is a bug with ffmpeg?
>
>
> Here are the files,
> https://mega.nz/#F!xsoGWIBA!1EQskrUQ1_09LenwhPawgw
>
> These webm files have been recorded from webrtc through firefox on
> archlinux.
> The files used in each command are found in the linked mega folder.
>
>
> This command is enough to demonstrate the problem I am having,
>      ffmpeg -f concat -i list.txt -c copy output.webm
> this gives an output file that is too short but has a very high bitrate.
> (The log for this is concat_copy.log)
>
> VLC may not play these videos at all, but firefox does for the most part
> and I have been using GNOME mplayer.
> In this, you should see a video that says it is 1 second long but plays
> some audio after that but the video stops.
>
>
> Also note that running
>      ffmpeg -f concat -i list.txt output.webm
> outputs a file with the same duration as before, but a much lower bitrate.
> This should be similar to the last output video.
> (The log for this is concat_transcode.log)
>
>
> If you first convert the videos,
>      ffmpeg -i 0.webm 0_fixed.webm
>      ...
> (the output for these are 0_fixed.log, ...)
> and this above command with the corrected list file,
>      ffmpeg -f concat -i list.txt -c copy output.webm
> it outputs a file with the correct duration of 3 seconds and a normal
> bitrate.
> This should play like a normal video. (sorry for the crackly mic)
> (The log for this is concat_fixed_copy.log)
>
>
> It is possible that the audio was corrupted from recording, but even so
> this behavior is not expected.
> This does not happen from chrome on windows.

The video & audio you concatenate must have same properties: widht,height,
    pixel format, etc .. channels bitrate and so on.