Just guessing here, maybe there is a setting on the unit (via software?) where you choose either component or HDMI. May not be able to do both simultaneously. If not, another reason could be the output resolution of your cable box is not accepted by the hauppage?
720p may be better than 1080i, from what I heard. It probably doesn't do 1080p. I don't think any cable boxes output 1080p. I've heard no broadcasters are doing 1080p either. Requires too much bandwidth?
Authorized Service Provider (Contractor) for MTS Allstream. Any views expressed are my own.