Task: there are many files in mp4 of different sizes from different cameras. You need to get one video in mp4 with black frames, if not enough in size. The main thing is to have no distortion scaling. Total proportions should be 16: 9.
tried the following
ffmpeg -f concat -i files.txt -aspect 16: 9 -c: v mpeg4 output.mp4
Obviously did something wrong. I would be grateful for the help.