@ Kwag & andybno1
thank you friends, now is clear!
and overscan seems fantastic!
copy from Kwag's post:
"Consumer TV sets use overscan to prevent black vertical bands from appearing due to variations in the horizontal deflection circuitry of TV sets. Overscan causes a loss of an average of five percent of the image on each side of the tube. So when you set overscan blocks to 1 or 2, you're encoding less film area and making the image smaller. When you play it back on your TV, it will be expanded flush with the edges and you won't loose image on the sides
Overscan 0 doesn't do any resize. 1 = 8 lines on each side, and 2 = 16 lines on each side.
-kwag"