Getting error: Non-monotonous DTS while concatenating video files in ffmpeg

ffmpeg is a versatile tool which can concat multiple videos into a single video with minimal processing load. ffmpeg takes a text file with a list of video files to be joined in the order they are to be concatenated. A sample command will look like this.

ffmpeg -f concat -safe 0 -i mylist.txt -c copy output.mp4

This command can be used universally as long as mylist.txt file contains the correct list of video files that are to be joined. The text file might look like this.

# this is a comment
file 'f1.mp4'
file 'f2.mp4'

I was joining pieces of lecture videos recently using ffmpeg and it was working perfectly till it broke. I started getting the error “Non-monotonous DTS in output stream”.

[mp4 @ 0x559118faf340] Non-monotonous DTS in output stream 0:1; previous: 10392576, current: 9748800; changing to 10392577. This may result in incorrect timestamps in the output file.

The issue seemed to be due to the video file itself. The online forums discussed something about the timestamp which didn’t make sense. I checked the encoding and bitrates and all seemed to be in order except the video and the audios steams are switched in the second video. While f1.mp4 contains the audio stream at Steam #0:0, f2.mp4 contains the audio stream at Steam #0:1.

To correct this we used the map function of ffmpeg to swap the audio and the video streams.

$ ffmpeg -i f2.mp4 -map 0:1 -map 0:0 -c copy  new.mp4

And the ffmpeg -f concat started working again!!

References:
[1] https://trac.ffmpeg.org/wiki/Map