There are some set top boxes with HDMI output to HD TVs etc. The audio and video outputs may not work immediately despite tuning in Freeview channels via the scart connection. This has happened for the Fetch TV Smartboxes offering Freeview via a scart output as well as the HDMI output cables. There is a MENU option for PREFERENCES, ways to customise and these include scart and HDMI settings as well as TV settings for 4:3 or 16:9 or other screen formats to work OK for your TV.
So I had to check PREFERENCES then Audio and Video settings, enable HDMI video output, enable audio HDMI output, then they worked OK.
Link to User Manual:
mitlondon.com/main.php?l_code=03&itemcode=DHP8100U/DHR820xUIn some cases, a Troubleshooting guide in the User manual may point out this sequence for customisation for HD TVs. The MIT website will offer extra info as would the Helpline.
There are times I've disconnected HDMI cables to reverse them and reconnect them and the HDMI worked OK, but to do this and be successful might simply be as a result of switching off the equipment and switching back on again.
With HDMI, my HD TV has a blank screen for a few seconds before appearing OK.
By the way, some set top boxes reset themselves to default settings, so it helps to verify that other settings are working as they were manually set to.