- "Export Using Compressor" from the File menu in FCP.
- In Compressor, start a new MPEG-2 setting or modify one of the existing ones - it must be a 4:3 MPEG-2, not a 16:9 MPEG-2, so be sure to make or use a 4:3 MPEG-2 setting.
- Under the Geometry pane in the Settings Inspector, there is a setting called Padding. This is the setting you need to letterbox the HD Sequence from FCP into letterboxed-SD.
- Select the aspect ratio of your *source* (16:9) in the padding drop-down. Your MPEG-2 settings already have the aspect ratio of your output. Compressor will then select the padding values you need to create the letterbox.
- From here, just export as normal in Compressor. Your resultant MPEG-2 file will be 4:3 aspect ratio (not anamorphic PAR) with letterbox and your complete 16:9 frame from your HD sequence (scaled down, of course).
