As far as I know it's not possible from the normal user setup.
It might be possible you can do it by entering the service menu, but from memory you would need to put the TV in hotel mode which would restrict access to a lot of other features.
I'll have a look tonight and see if there is anything else in the service menu that might help (be aware if you do make any changes in the service menu you do so totally at your own risk, change the wrong thing and you can really bugger the TV up!)