I'm considering getting a TiVo HD DVR. I've read some posts about using a "cable card". Granted, I really have no clear idea on what a cable card does or offers (I've heard you can't do PPV). So, I was wondering, can a TiVo HD DVR be attached to a regular digital cable box? Something like how I have my Series 2 set up now? Or does it require cable card(s) in order to work? Thanks for any help you can offer.