I keep reading about QAM cards and other stuff when browsing thru 2nd hand D* HD receivers. What is QAM, and do I need to worry about that if I'm interested in viewing OTA HDTV signals and Discovery HD and that's pretty much it? Also, in these second hand receivers I notice that there's no cards in most of them. Does that mean I have to buy that in addition to the receiver? Is such a thing purchasable and will D* activate that as a secondary receiver on my account?