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I got my xbox 360 a short while ago, and I planned to run it from my monitor at 1920x1080 (the max on the monitor), but when I bought my xbox, I found out I didn't have an hdmi cable to hook it up with. I ordered one online, and while I was waiting for it, I hooked it up to my parents' old 720p crt. When my hdmi cable finally arrived, I hooked my 360 up to my monitor and switched the settings from 720p to 1080p. However, it always displays in 720p instead, despite saying it's in 1080p. I moved it to my downstairs tv and it displays at 1080p, but not on my monitor. I've looked on google for ways to fix this but all I could find was for what I already did. Does anyone know how to fix this?

EDIT: I tried swotching to an hdmi-dvi cable and set the resolution to 1920x1080, but it still displays in 720p