I decided to post this here, so at least there's a guide for other people on how to fix this issue. I've looked for a solution, but never found any. Luckily I solved it by accident.
The issue (100% reproducible):
- On Windows 7 64-bit with Radeon HD 4870 and Catalyst 12.3 (and all other Catalysts I've tried)
- Connect a HDMI cable from the video card dongle to TV HDMI in (Samsung Full HD LED, but I believe the TV doesn't matter)
- Use "Extend Desktop" in Catalyst Control Center on 1920x1080 resolution
- Change source on TV to HDMI, so that you see the extended desktop on the TV (the picture comes out OK in my case)
- Wait 30 seconds
- The TV screen will automatically turn black and then say "No signal"
Also, there is no dialog about things like "Are these settings ok? Reverting back in 30 seconds if no answer" or anything like that