For six months I've been taping movies and OTA with my Panasonic combo, and for all this time I've had an intermittent problem with digital noise. I've taped far more movies (between 150 and 200) than I've actually watched so far, but almost every tape I've watched exhibits a little bit of reproducible digital breakup (video or audio or both) every few minutes.
My recording setup was based on ideas that I gleaned from this Forum and included a Tru-Spec channel combiner (to combine OTA signals with the Dish 5000's channel 3 output and avoid the use of an A/B switch) and a Radio Shack RF amp (to boost the evidently-low signal output of the modulator).
In trying to solve this problem I've had three STBs and four VCRs serviced by Panasonic (but they all had problems anyway), I've replaced my 18-inch dish with a 32-inch DBS dish ordinarily used only in places like Alaska or Hawaii (but I should now be well-protected from rain fade), and I've ordered a Blonder-Tongue broadcast quality RF amp to replace the Radio Shack.
I write software for a living and I really do know the proper way to diagnose problems like this, but for some reason the obvious escaped me and I never thought to suspect the channel combiner. It now looks as though that is indeed the source of the problems.
Allegedly, the combiner filters out channel 3 from the OTA input, filters out everything but channel 3 from the channel 3 input, and combines them with low loss. But there's a strong local analog OTA signal on channel 4, and I suspect that adjacent channel interference has been doing me in.
Given that Channel Master's "Join-tenna" (a similar product) is known to cause interference with signals up to five channels away, it shouldn't be surprising that the cheaper Tru-Spec would have similar problems.
Though it's too early to be sure, so far it looks like using the HD modulator output directly or via a simple A/B switch eliminates the digital breakup.
So it's back to the same simple setup that I imagine is being used by most people here who have the Panasonic combo. I'll used a damned A/B switch. I'll lose the ability to program unattended recordings of Dish HD and OTA signals on the same tape, but I've only rarely used that anyway. A far more likely error is that I'll forget to flip the switch and mess up recordings that way. But an occasional missed recording is preferable to tainting every movie I do manage to record (which is what seems to have happened).
Now I begin the obsessive re-taping of all those movies that I thought I already had on tape: watching for them to show up again on the HBO or Showtime HD schedule, and programming them when they do.
Well, it's my hobby.