Four yuv image at one time

Hi all,
i’m new with SDL programming. I’m trying to realize a viewer for my 4 input
video grabber. I acquire the image with video4Linux API in YUV4:2:0P format. I
use the SDL_CreateYUVOverlay function 4 time to create 4 overlay surfaces in
the same window. All work fine but the four images aren’t independed. In other
word the first surface created a noised effect on the others and i see the
reflex of the first sourface on the others with a bad result for my viewer :)!
Ani?yone know anything about this problem or have already used the YUV functions
in this manner or can help me?
Thanks a lot in advantage!
saluti Carlo!