Compression on footage problem

Hi there,

I've just been doing some chromakeying on footage captured on a JVC pro camera.
I've found that there are lots of compression artifacts in the footage which make it very difficult to get a clean matte, especially on the models in the distance.
Usually the footage from this camera is fine but I've noticed that on distance shots
(not too far - just background as opposed to foreground in a room type distance) it loses sharpness.

Any advice?

Thanks
Kathy