Tom is using his computer capture card along with dTV to ack as a video deinterlacer + scalar. If you don't understand the significance of that, then you are not using the HTPC to it's full advantage. You may want to read the archives to get a more complete understanding of the important tradeoffs between HTPC and standalone video processors. One easy benefit of the HTPC is that you get top notch DVD playback that would require several thousand dollars worth of stand alone equipment. Of course, all of this is a moving target as the recent thread about progressive DVD playback with "very well known" video deinterlacing chip built in has proven.
Passing the signal through the receiver first allows the receiver to serve as the AV switcher which is exactly what it was designed to do.
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Reading up on dTV would be something you might want to do also. dTV is attempting to do with commercial off the shelf components, what would have cost at a minimum $600 just last year. The very best video deinterlacer and video scalars from "very well known" companies will still set you back many thousands for $$$$. Of course you get what you pay for. But since dTV is free, who's complaining???
When it comes to HDTV, this is brand new ball game. It's only a lucky happenstance that the WinTV-D uses a version of the BT878 for analog video. In fact the WinTV-D is based on a reference design from Philips, so it's not a one off design, although Hauppague did make some specialized modifications to the reference design. The WinTV-HD is based on a completely different design from Teralogic. As such, the software used to interface with the two cards is very different. dTV currently doesn't work with the WinTV-HD in any way shape or form, but as mentioned before it's just luck that it can work with the WinTV-D.
Recording HDTV is really just recording the RAW data being transmitted by the TV station. This is actually pretty easy. Recording analog TV turns out to be more difficult since the CPU of the computer must compresses each frame of video in real time. At present, MPEG-2 demands a very fast CPU for ever standard definition video capture. Compressing HD video in real time would take about a 6 Ghz CPU.
One last thing to keep in mind is that the HTPC is by far the most flexible and cost effective platform for building a top quality home theater. All of the this flexibility does come at the cost of actually needing to know some of the details of how all this technology works. If that isn't interesting to you, then HTPC may not be the way you want to go with your home theater.