I know that the network HD channels (ABC, NBC, etc) are broadcasted over the air. I also know that you can receive HD through an HD cable box.

My question is...is HD broadcasted through the regular cable lines without a cable box. I'm thinking on getting an HD3000 or an Airstar HD5000 (I'm open to suggestions on this), but I'm trying to better understand how I'll be hooking it up. If I get two HD tuner cards, do I need two cable boxes? Obviously, any of the higher premium HD channels can only be accessed via the cable box, but 95% of the things that I want to watch will be regular network broadcasts.

It looks to me like the Airstar can only function without a cable box, as it has only has a coax input. (Unless it is intended to use the coax out on the cable box)

The HD3000 has a coax, an S-VIDEO, and a composite input, but HD can be transferred via S-VIDEO or composite, so it must be the coax that is used.

Summing this up, is it possible and practical to use these tuners without a cable? (Save money on multiple HD boxes, and avoid the hassle of IR blasters)

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Currently running: R5.5, HD5000 x 2, PVR150, Athlon 64 3000+, Chaintech VNF4, 1GB RAM, 2 x 250GB in LVM, MSI NX6200TC -> AA 9A60 -> HDTV

Yes, it's called QAM. That's how a TV with an HD tuner (ATSC/QAM) can plug into the cable directly, without the box, and receive HD. Search AVSForum or find a local board in your area, to see what's available as QAM. Usually, most channels are "in the clear", but some like HBO or ESPN may be encrypted.

See:
http://www.pchdtv.com/faq.php#faq0000008

If you want to build a myth box, and connect right to the cable without going through a box, make sure it supports QAM. The HD-3000 is one such card.

Thanks, now the QAM posts make sense to me. I've been searching on that but didn't realize exactly what it was.

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Currently running: R5.5, HD5000 x 2, PVR150, Athlon 64 3000+, Chaintech VNF4, 1GB RAM, 2 x 250GB in LVM, MSI NX6200TC -> AA 9A60 -> HDTV

we also have it at this forum
http://mysettopbox.tv/phpBB2/viewtopic.php?t=8104