What's 'RF loop-through' or 'RF modulation'?
If a DTT decoder has 'RF loopthrough' listed amongst its features, it means that there's a physical connection between the aerial input connector and the aerial output connector. So if you've inserted your main TV aerial lead into the set-top box through 'Aerial In', and then linked the box to your TV or VCR via 'Aerial out', it won't matter whether your box is switched on or off - your TV and VCR will still get a signal.

If the DTT decoder has 'RF modulation' (also referred to as 'RF modulated output'), that means that it can 're-broadcast' the Freeview or Top Up TV signal onto a new TV channel which you can tune your TV or VCR in to. This is particularly useful if you have an old VCR or TV which doesn't have SCART sockets.