Your explanation leaves me a bit confused.
HDTV formats are (WxH) 1920x1080 and 1280x720
That fact that you say you can't create a 1280i project leads me to believe that what you really want is a 720 project if you want to properly scale down 1920x1080 footage.
There is no such thing as 1280i.
Brian Mulligan
Senior Editor - Autodesk Smoke
WTHR-TV Indianapolis,IN, USA
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@bkmeditor