I want to use 720x480 footage in a 1920x1080 hd timeline.
My original 480 footage was shot on a Sony TRV900, 720x480, interlaced,29.97, DV/DVC-Pro, non-square pixels, etc.
The timeline I want to edit into will be 1920x10880, progresive, 23.98, Apple ProRes 422, square pixels, etc.
Now I need my sd footage to be fully windowboxed and keep the 4:3 aspect ratio. But, I need it to be bigger than 720x480. But, not all the way up to 1440x1080, which would give me pillarboxes.
So, should I use Compressor to convert all the footage to 1440x1080 and then just scale it down in FCP with the motion tab to a size I like, or should I choose the size I want (say 1280x960, which I know is some weird frame size) and convert to that in Compressor?
Compressor does a better job at scaling than FCP does natively, so I think I would agree that up-scaling in compressor, then applying your crop tools and positioning tools in the FCP timeline, would get a better-looking overall result.
But I would also look at trying a custom setup in Compressor to do the whole job in one automated step.
The easiest solution would be to go ahead and convert all of your footage to 1920x1080, 23.98 ProRes 422 etc. Just know that full hd ProRes is huge and will probably take up almost 4 times as much storage as your original footage.
In Final Cut the only thing you should have to is change the Aspect Ratio (Go to the Motion tab, scroll down and click the "Distort" tab, then change the Aspect Ratio to your desired size (the 4x3 image will be stretched when it first comes into FCP).