Originally Posted by mcan
I don't think it's that...
Your explanation is part of it. But the reason you only get one signal from the box is because it's digital cable. The old analog cable signal was compatible with the TV tuner, NTSC I think it is. So if you had a cable ready TV you might not even need the box, unless you had scrambled premium channels.
Digital cable sends out a decrypted signal that is not compatible with anything but [I'm guessing] channel 3/4 on the TV Tuner. The channel 4,5, 9 of the old analog days is nothing like the 4,5,9 of the digital cable box.
So you'd need the ability not only to decrypt and send two digital channels, but to assign those channels back to NTSC/analog UHF/VHF nomenclature.
If that distinction makes sense.
Count yourself lucky. My HD Dish receiver only has S-Vid/Component/DVI outputs. No Coax out at all, which means I have to split my antenna to go into the satellite for OTA-HD content and into the TV for OTA-SD content. And I can no longer send my Satellite signal back to the computer through the miles of Coax I installed in the basement rafters last year.