cd TMP
ffmpeg  -y  -i $SRC  -vf trim=0:$BLACK_S,geq=0:128:128  -af atrim=0:1,volume=0  -video_track_timescale 600  black.mp4  # >> log.txt  2>&1

ffmpeg  -y  -i $SRC  -c copy  -video_track_timescale 600  timescaled.mp4

echo "file black.mp4"        > list.txt
echo "file timescaled.mp4"  >> list.txt

ffmpeg  -y  -f concat  -safe 0  -i list.txt  -c copy  $DST