It's simply that TV signals use different standards to computer monitors, and many computer monitors won't accept TV frame rates - there's only a fairly number of monitors that will.
HD boxes usually have HDMI outputs, and these will feed either HDMI or DVI sockets on monitors - assuming the monitor will support the lower frame rate.
I've not seen any UK set-top boxes with VGA out, it would seem pretty pointless.
The monitor should support pretty much any normal frequency. I know you can go DVI out to HDMI in, but it may not work the other way. It could also be an HDCP issue.
I don't know what the situation is in Australia, but most modern flat-screen (LCD) TVs can also function as a computer monitor. Just look for one with a VGA port to be sure it will work with your computer (assuming your computer doesn't have a real HDMI port). The only drawback to using a TV as a computer monitor is that they usually look terrible at anything but their "native resolution", which on a smaller (less than 40") will probably be 720p (1366x768).
It still seems odd to me that the monitor doesn't work. The most common TV frequencies are the same as the native frequency of your monitor (60 Hz/FPS, the ones ending with p such as 720p 1080p). Possibly, as Nigel suggests, the monitor won't go as low as 30 Hz, which would be one of the lower interlaced settings ending with i such as 480i. Maybe check if the STB has a "Progressive Scan" setting that is turned off, or if it has a setting for a higher refresh/FPS/Hz, such as 120 HZ up that is too high for the monitor.
I checked at Acer and they mention nothing about frequency/FPS/Refresh being a problem, only resolution. They state plain and simply:
Frequently Asked Questions
Q: "Input not supported" message
A: When a monitor displays "Input not supported", the display resolution is set too high. To resolve this, change the resolution to a supported resolution of the monitor.
Thanks,
The Vertical frequency is 50 Hz not 30 Hz.
I don't want to us e a TV, I want to retain the monitor as for the vast majority of the time, it will be used as a monitor. It will only be used with the STB on the odd occasion such as when we have visitors.
Nigel wrote "computer monitors generally don't go low enough for TV use, only for computer use" in a post above. So I think this is the most likely explanation, however, I'll see if I can change the resolution tomorrow.
Actually it would be 25Hz not 30Hz, assuming frames per second, and 50Hz not 60Hz assuming fields per second.
I don't want to us e a TV, I want to retain the monitor as for the vast majority of the time, it will be used as a monitor. It will only be used with the STB on the odd occasion such as when we have visitors.
You don't need both, almost all TV's have inputs for VGA (and if not HDMI) for connecting a computer.
Nigel wrote "computer monitors generally don't go low enough for TV use, only for computer use" in a post above. So I think this is the most likely explanation, however, I'll see if I can change the resolution tomorrow.
You can't alter the output from the set-top box, it's part of the boxes design - likewise it's very unlikely you can alter the monitor, as it wasn't designed to do that, and they auto-set anyway.
Choices are use a TV for both, or select a monitor which will go low enough - but it rather kills the idea of using monitors you already have
If you were an old Amiga user this exact same scenario was played out decades ago, but on CRT computer monitors.