How to set a different outut resolution (XBMC)

Settings the video output resolution from XBMC is blocked somehow. It seems to be fixed at 1080p@60Hz.
I would like to use 720p@50Hz or 1080i@50Hz. How can I get this done?
Kind regards,
Michel
I would like to use 720p@50Hz or 1080i@50Hz. How can I get this done?
Kind regards,
Michel