Inspiron 15R 5521-0521 hdmi resolution

I cant seem to get a 720p (HD ready)signal via the hdmi port. i can get much higher and lower (when plugged into the appropriate hardware). Full HD ist a problem but i have a 720p HD (1280×720 progressive @50 & 60 Hz) tv in my photo studio 

i can chose wxga 1280x768 but need the 1280×720