I'm running Windows 8.1 Pro and a Radeon HD 6670 with the latest ATI drivers. I've noticed lately (maybe within the past month or two) that my HDMI out will turn off after not being at the computer for some time when clearly the power settings in control panel tell my monitor to never turn off. There is nothing that I can see in my ATI control center that would have anything to do with power settings, so I presume that's not the case.
Where else can I look to make sure my screen isn't turning off when it shouldn't be?