Good basic advice from SF Cutters' Cutter-Talk:
>> I digitized the footage as DVCPRO50 widescreen, and it looks excellent, but when I go to chroma key it, I end up with stong jaggies on all moving edges.>>
If the "strong jaggies on moving edges" are simply the same sort of every-other-line jaggies you see on interlaced footage (jaggies move smoothly with the edge and are jagged in proportion to the amount of motion), don't worry about it (as long as the target output is interlaced).
If instead these are a sort of "fixed pattern steppy edge" where the edges of the key have discrete, coarse locations, with remain stationary as the edge moves within a 2-pixel radius, then it's a chroma subsampling issue, and you should try the following things:
1) Make sure you apply the "Chroma smoothing - 4:2:2" filter as the first effect on the source clip. If you're using FCP's standard keyers this makes a huge difference.
2) If interlaced, make sure you're viewing the rendered result on an interlaced CRT, not the Mac's screen, nor a flat panel with dodgy deinterlacing.
2a) You realize, I hope, that the FCP screen only ever gives you something even remotely close to a pixel-by-pixel-accurate view when the Canvas is at 100%?
3) Play with the edge parameters in whatever keyer you're using.
I've had excellent results (faster to set up and superior quality) using DV Garage's DV Matte Pro (no involvement with DV Garage except as an early beta tester and now as a paying customer).
Adam Wilt / shoots, edits / Mountain View, CA USA