From what I've read Comcast uses two levels of encryption, a basic one for tier one channels (extended basic in marketing terms) then a more comprehensive key encryption for their digital preferred channels.
I use a combination of two set top boxes, a PVR HD for HD channels and a PVR500 for SD channels, and two Clear Qam adapters for locals.
The Clear Qam are very good quality MPEG 2 HD or SD streams with AC3 audio, the PVR500 is reasonable, but one my boxes has interference on the svideo output.
The HD-PVR is really good, giving an H264 with AC-3 passthrough TS stream. However, every now and then the cable box glitches and the HD-PVR stops streaming. PVRInput isn't aware (yet - although Lars is looking into it) that it's not sending anything to VDR. This is a problem when recording as VDR restarts, if I'm doing concurrent recordings then I lose bits of other recording during this time. As an aside, AC-3 is passed through from the original ATSC-QAM stream, I had to patch VDR again to stop checking for ATSC on Audio Type 0x81 or VDR would ignore the audio dpid of the channel and set it to 0. This also had to be done for the VNSI plugin in XBMC-PVR.
I really want to thank Lars for teaching me about TS streams..
New question - what hardware support is there for VDR and ATSC? I'm moving to the US, so will be ditching my DVB-C and DVB-S hardware - what should I be buying to work with VDR?