Object Lesson: Keep It Simple - AVS Forum
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post #1 of 4 Old 05-08-2001, 09:37 PM - Thread Starter
rrg
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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.


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post #2 of 4 Old 05-09-2001, 04:31 AM
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There is a better way. Its slightly insane.

First, due to fcc rules the output of the hdtv modulators
is only about 6db above the minimum threshold of some
STB's. By the time you run it thru combiners, most
of which are at best lousy, you end up with a signal
too close to the noise line.

Now for the solution. Radioshack sells an A/B switch
that is IR remote controlled. Its well shielded and
looses only about .1db.

All you need is something that can output the correct
ir code at the right time to switch between the
two signals. Numerous software solutions exist,
anywhere from something that runs on a pc, to something
that runs on various handheld devices.
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post #3 of 4 Old 05-09-2001, 07:51 AM - Thread Starter
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I've thought of using a remote-controlled A/B switch. In fact I'm already using one in my home theater to switch between C and Ku inputs to my Unity Motion receiver. But it has a really kludgy feel to it, and this particular Centronics switch seems to be affected by other IR remote signals, so that (for example) I'll be tweaking convergence on the display and find that the Centronics has switched inputs.

It just feels like one more thing that can go wrong, plus I'd have to provide a programmable IR source (like a Pronto remote) and put it in the right place. The attraction of the combiner was that it was a relatively seamless solution: just tune Ch. 3 like any other channel.

For more money than I'm willing to spend, the solution could be made seamless: I could get broadcast-quality single-channel filters from Microwave Filter and combine signals that way. I could point a second antenna at Philadelphia and combine the Philly digitals with the NYC digitals on the same wire. How about getting three more Dish 5000s and modulators, converting the modulator outputs to unused UHF channels, and combining all four Dish HD channels with everything else on one cable?

This would make it unnecessary to flip an A/B switch or re-point a rooftop antenna, but at a cost in money, time, and trouble that would be loony. I'm not (yet) that loony.

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post #4 of 4 Old 05-09-2001, 09:06 PM
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Quote:
Originally posted by rrg:
For more money than I'm willing to spend, the solution could be made seamless: I could get broadcast-quality single-channel filters from Microwave Filter and combine signals that way. I could point a second antenna at Philadelphia and combine the Philly digitals with the NYC digitals on the same wire. How about getting three more Dish 5000s and modulators, converting the modulator outputs to unused UHF channels, and combining all four Dish HD channels with everything else on one cable?

This would make it unnecessary to flip an A/B switch or re-point a rooftop antenna, but at a cost in money, time, and trouble that would be loony. I'm not (yet) that loony.

I may have to do some of that here in the Orlando area. It seems that the Tampa PBS station will be up sooner than two years from now (the current date for our local station), so I may have to combine the Tampa feed in. I'd consider getting a second Dish 5000 putting one on channel 3 and one on channel 4 (if I knew that it would work), and then feeding the combined signal to both of my HTPCs for recording (making it possible to record HBO and Showtime or any two shows from OTA at the same time).

/carmi

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