You're right that it isn't possible to build a MythTV HD-DVR without getting Un-encrypted HD signals via COAX, but that doesn't require a cablecard or some advanced interface to get right - just an inexpensive capture card that takes Component in, that AFAIK is not currently available. MythTV supports IR Blasters already, which is how I use my current TIVO with a cable box in the first place - were I to get an HD Cable box it could theoretically work the same if there was a readily available HD PCI or PCIe card with component in. IE the HD Signal is decrypted in the cable box -> component in on mythtv box -> tv, channels changed by mythtv box with IR Blaster.
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You're right that it isn't possible to build a MythTV HD-DVR without getting Un-encrypted HD signals via COAX, but that doesn't require a cablecard or some advanced interface to get right - just an inexpensive capture card that takes Component in, that AFAIK is not currently available. MythTV supports IR Blasters already, which is how I use my current TIVO with a cable box in the first place - were I to get an HD Cable box it could theoretically work the same if there was a readily available HD PCI or PCIe card with component in. IE the HD Signal is decrypted in the cable box -> component in on mythtv box -> tv, channels changed by mythtv box with IR Blaster.