When I use unsharp filter, I can see blinking greep bar near 
the top of the screen on all screen's wide. Sometimes it is 
solid bar, sometimes it is stripy.

It appears only if unsharp filter is used, e.g. 
-vf unsharp=l11x11:0.6:c11x11:0.6
Size of bar seems to be independant of filter parameters, but 
duration of blinking is proportional to the matrix size and 
seems have no correlation with filter strength.