[Gunleik Groven]"But is it possible to change. There should only be some flag somewhere that flags one SD file as anamorphic. Right?"
You should be able to change your encoded movie to the 16:9 format. The first way I know how to do this is to open your MPEG2 in QT and then do an export with a custom aspect ratio change. The length of your movie file will determine on how long it takes to make a new exported movie. Probably a while.
Export using the same codec as your movie. In the dialogue window click on options, then select the size button. There you will be able to select a preset aspect ratio or you can use the custom setting. For example, if your MPEG2 file is DV NTSC 720x480 change it to 854x480 to get your 16:9 picture.
If you want to try it another way, open your movie in QT then open the "show movie properties" dialogue in the Window menu. Hightlight the video track then click on "Visual Settings" Change the numbers in the "Scaled Size" boxes, uncheck "Preserve Aspect Ratio" then click outside the numbers boxes. Save your changes. That should do it.
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