While I concur with Alan's suggestion to record the HDSDI out of the camera to a format that bypasses the MPEG2 4:2:0 subsampling, I would also highly recommend using just about any other keyer than the plugin built into Final Cut, which is extremely primitive.
If you want to keep it inside of Final Cut check out
Conduit from dvGarage. It does much much better keying than FCP's, and it's pretty darn inexpensive to boot.
You may find that you get perfectly acceptable keys for your application even with the 4:2:0 sampling once you're using better software.
--
Jesse Rosen
Director of Technical Development
Abel Cine Tech, Inc.www.bustedskull.com
