# create a list of concatencated videos
            with open(video_list, 'w') as f:
                f.write('file ' + video_black + '\n' +
                        'file ' + video_timescaled + '\n')

            # ffmpeg -f concat -i list.txt -c copy merged.mp4
            args_concat = ['ffmpeg', '-y', '-f', 'concat', '-safe', '0',
                           '-i', video_list, '-c', 'copy', '-threads', '0',
                           video_with_black]
            subprocess.run(args_concat, check=True)