Playstation 3 does not recognise HDMI connection?

When i start up my ps3 with AV cable but wih HDMI connected it is supposed to say "HDMI found would You like to change the output" or something like that. but when i start i up nothing comes i can just play on AV and not HDMI. when i try to manually change it by going into dispaly settings it also does not work. I tried holding down the power on for 5 seconds that also does not help