first remove any of your backgrounds, then render it in a format that can carry an alpha like uncompressed avi or quicktime,and image sequences like targa and tiff. got to the output module then at the video output expand channels and select rgb + alpha, go to depth select million + colors or trillions, at color select premultiplied or straight, each one has it’s advantages although most people says that straight unmatted is a lot better an is handled better by nle programs puls it doesn’t produce fringing on your edges.