mini dvi to hdmi connection issues

Trying to set up my 2007 imac to my Panasonic tv. I have a mini dvi to hdmi converter and an hdmi cable connected to my tv. I only get a blank screen on the tv, but it seems that the computer is connecting. When i go into sytem preferences for display monitoring I get the second screen saying Panasonic and set up which reolution I want but theres no way to detect the display. Any help getting picture would be greatly appreciated!!