Dear droneguy,
Tq for your reply. FYI, FC is JHEMCU F7 AIO Flight controller. I'm using latest version of betaflight and loaded with firmware 1.1.1 (legacy version)
I've finally solved this problem by setting "osd_units" to "Imperial" and then solder Camera Video in to FC board and VTX video to Video out on FC board. It worked fined already.
Tq for your concerned.