Digital television signal is not necessarily HDTV. An HD signal is a digital signal.
It ****ed me off that satellite TV services make this claim about all digital, when it relaly means nothing. With a digital signal, it's either on or off. You see the broadcast or you don't. With analog signals, you can see a broadcast if there's a weak signal, albeit with static or 'snow'.
Now, is your USB dongle thingie capable of handling HD signals? I have no clue.