Changing PS3 to 720p with no display

I usually have my PS3 connect to a full HD 1080p Sony TV.

I think it is set to this mode.

I have taken my PS3 to a friends house who only has a 720p Sony TV.

Nothing displays on the screen when I power on the PS3.

However if I put in a 720p game all is ok.

When I exit the game the TV is blank whith 'HDMI' at the top left.

I'm assuming the console is 'sending out' 1080p which the TV cannot handle.

How can i set the PS3 to display 720p without seeing the screen?

I only have a HDMI cableStart Free Trial