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Discussion Starter · #1 ·
I'm still trying get my arms around this whole DTV reception issue. Some questions that hopefully some of the experienced technical folks might be able to answer:


Why is it that I can watch an OTA show, that's getting signal strength in the 60's, for 45 minutes without a single problem and then suddenly I get those blasted green squares, tiling, or worse, complete dropout? Then the thing comes back and is stable again for a long period of time.


There's a small airport nearby. Is it possible that planes approaching or departing the airport are causing infrequent multipath distortion that's causing what I described above?


It seems to me that there must be some way to avoid these infrequent dropouts if the signal is strong enough. Is this a tuner issue or a broadcast issue? And should I expect this to get better in the future or just live with it?
 

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I'm assuming you're using the DTC-100, don't know if "signal quality" means the same thing between STBs.


If so, a signal in the 60s is somewhat marginal. You would be better if it were up in the 80s. If you are more than about 40 miles from a station, and have a marginal antenna, or are behind some hills, you are seeing variable path loss, which will result in exactly the kind of thing you are seeing. When it goes into the "greenout" mode next time, switch quickly to your signal strength meter, and see if it is dropping down into the 40s or so. You are probably going to need a little more antenna gain to get past this one. Chances are that the private airport near you isn't causing the problem. Observing a UHF NTSC station from the same transmitter site during a fade will give you a clue. With airplane multipath, it will usually be a quick fluttering picture (ghosts coming and going rapidly), to a slow flutter, then back to a rapid flutter. Standard atmospheric path loss fluctuation will cause the picture to fade gradually (over tens of seconds), followed by a gradual fade back to normal. All of these must be monitored on an NTSC signal close to the frequency of your DTV channel.


Bob Smith
 

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Discussion Starter · #3 ·
Some additional information that I should have put it:


I'm using a Dish 6000 receiver with the 8vsb OTA tuner.

Have a Channel Master 4248 antenna on a rotator mounted to the side of the house, going up about 5' or 6' above the roof line.

Using a Channel Master 7775 UHF antenna pre-amp.


I'm about 45 miles from the transmitter farm, and there is mountaineous terrain between me and the towers.


The airport near me is not a "private" airport. It's classified as an "international" airport (can't imagine why) and flies planes in and out as big as a 737. (It's the Allentown airport)


So are you saying that my problems here are simply caused by random atmospheric changes between me and the towers?


And observing an NTSC channel at the same time isn't an option. I have those channels coming in through my cable service, and god knows where their receiver is.
 

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Mountains are the big clue. I'm betting on random atmospheric changes. Since there are mountains that, I'm assuming, are taller than the transmitter antennas, you are getting reception through diffraction, an effect that causes waves to "bend" downward (not really, but good enough for this discussion). The interface between the mountain peaks and the atmosphere is very frequency, and source height dependent. Antennas at different heights or different channels on the transmitting tower will be affected very differently as they pass this interface, it will be different dependent upon the moisture content of the air, etc. Your best bet at this point would be to get as much antenna gain as possible, and probably use a good low noise preamp as well, and get it as high as is possible. Look at www.starkelectronic.com, they have a good selection, good specifications, and good prices.


You can hook up the UHF antenna you have to the input of your NTSC receiver and see what kind of reception you have. Look for good quality NTSC pictures with few or no "ghosts" about the image. Look for the "airplane flutter" effect during poor DTV reception to eliminate that as a possibility. How far is the airport from you? If more than about 10 miles or so, the planes are too high to become an efficient reflector and cause multipath.


Good Luck,


Bob Smith
 

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Discussion Starter · #5 ·
The airport is about 12 miles away. I've also found that birds like to roost on the antenna. My wife always laughs when the signal drops and I go running out the door to see if there's a bird sitting up there on the damn thing http://www.avsforum.com/ubb/biggrin.gif


Are you suggesting another pre-amp in addition to the CM7775 that I already have? The specifications of what I already have are in a previous post.


It's interesting how the mountains and atmospheric changes can really affect the signal. I found one night that I got a better signal with fewer drop outs by pointing the antenna about 20 degrees off from where the transmitter is supposed to be. Still, it's a real pain in the ass...
 

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Sorry, didn't remember you were already using a great preamp---That's not the problem. Sounds like you might have to go for more antenna gain.


Here, I have a ridge between me and Sutro Tower where I get my DTV broadcasts from.


One solution that works some of the time for me, is to have 2 antennas, one mounted lower than the other, with a separate feed and preamp installed. I have problems with channel 29 (of course, this is the CBS feed) from time to time. When this happens, I switch to the antenna that is lower on the pole (highest antenna, 30', lower one, 15'). Usually this will bring in CBS during a fade.


I am using the 4 Bay Channel Master antennas, I have 4 of them installed at different heights for experimentation.


My next move is to install a the 7' parabolic antenna from Channel master. Trying to avoid this, because it is such a big ugly thing, but think I need to to get the signal far enough out of the noise to allow the equalizer to work.


You need to have enough signal strength to be able to equalize the nulls in the signal that occur during the frequency dependent fades of diffraction reception out of the noise floor.


What I mean is that if you have 10 dB of margin, and a null occurs in the passband with 11 dB of loss, you will lose the signal. If you had 12 dB of gain, you would be OK.


The antenna gain and noise figure of the preamp determine how well your system will pull signals out of the noise floor. This means you need as much antenna gain, and as low a noise figure as possible. More gain after this point won't do anything (and probably will cause things to get worse).


I'd first try a higher gain antenna, don't remember what you have. Height also helps in most cases. The alternate antenna solution I suggested might also help.


Keep me posted,


Bob Smith
 

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Just noticed you're using a 4248 antenna, hard to top that one.


Maybe you should try one of the cheapo 4 bay CM antennas at a lower height and switch to it during problems. These antennas are really hard to beat for the price. Trouble is, you will need another preamp, you don't want to mount the preamp very far away from the antenna.


Are the mountains between you and the transmitter very big? Are you close to them, or are they distant from you?


Bob Smith

 

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Discussion Starter · #8 ·
I have a hard time calling them mountains, after having lived in Utah. They don't compare to the Rockies. They are eastern mountains, the Appalachian chain, I believe. The Poconos are to the north, the Appalchians on the other three sides. I receive my OTA mostly from Philly, which is south. The south ridge (South Mountain) is about 12 miles or so away from me, then there is "mountaineous" terrain from here all the way to Philly, gradually getting lower and lower until it approaches sea level near Philly.


I guess I should consider myself lucky because cable television got started in this area because OTA signals are so hard to get. In fact, Radio Shack is about the only place you can actually buy an OTA antenna around here. My hope is that eventually:


a) The technology gets better and can handle multipath distortions better

b) My cable company decides to start carrying the digital channels and pass the HD signals

c) I can get the locals from satellite

d) The locals install repeaters or translators so that those of us up here on the fringe of their viewing area can recieve reliable OTA signals

e) The locals increase their broadcast power. CBS is broadcasting the digital signal at around 700Kw. If they upped it to, say 1100Kw, it may help.

f) all of the above


I thought the whole point of the digital transition was to allow anyone, everywhere to receive OTA transmissions. The restrictions of receiving local signals from satellite are ridiculous, IMHO, and aimed at protecting the revenues of local cable companies, who have no problems rebroadcasting local network programming in an area that CAN receive them OTA as well. Of course, that's another story and another post. I just wish the FCC would get up off their collective asses and start working on "rules" that benefit the CONSUMER, and not business. If they did that, businesses would adapt. It's happened before, it will happen again.


In the meantime, I guess my best hope is that when the leaves fall in October, my reception will improve!
 

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Well, DTV is better when there is any signal at all. It's kind of like radial tires, they will hold very well up to a point, then watch out. DTV will hold on to a signal well below that acceptable for NTSC (about 13 dB Signal to Noise Ratio, far below that you would watch on NTSC), but then will go freeze frame or blank when it's gone.


I'm not sure if you told me what the elevation of your antenna is, that may make a big difference.


You might consider the Channel Master Parascope antenna, I think it has about the highest gain of any UHF antenna (at consumer prices), but is big and ugly.


I wish someone would make the correlation between "signal strength" numbers and Signal to Noise Ratio, I may calibrate the DTC-100 that way soon. That would give information as to how much more antenna gain is needed to overcome an outage.


Bob Smith
 

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Again I read your post after responding. You have an antenna only about 4 or 5 feet above the roofline. Is there any chance of raising it a little higher??? Radio Shack sells telescoping masts, not the highest quality, but probably good enough. Since the UHF antenna is so small, you can probably get the 40' mast and overlap the sections so it is only 30' high or so. With this mast, you can experiment with different heights, from rooftop to 40' with guys. I think having the antenna a little higher may make a big difference.


Bob Smith
 

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Discussion Starter · #11 ·
Bob


I'm kind of stuck at the moment in terms of raising my antenna. First, I should point out that the antenna is mounted to the side of my house at the highest peak of the roof. Because I had to run the cable up under the eaves and to the other side of the house where my receiver is, moving the cable will be a major pain in the butt. Plus, there isn't enough cable to raise the antenna another 5 feet. I used a radio shack heavy-duty 10' mast, and was thinking of stuffing a 5' mast under it to raise it up higher. The RS masts are narrowed at one end so they can be fitted together. The problem comes back to the antenna cable.


I would have to cut a length of cable and add it to the existing cable to get enough height. I do believe I have enough rotor cable stuffed inside the house to make it up higher. This would add an additional splice to the cable, not sure if that would make a big difference or not. Running all new cable is not my idea of fun. I don't want to mess with guying the thing (there's just not enough OTA HD to warrant that kind of work, and I really don't want to get up on my roof. It's got a VERY steep pitch), so I'm limited to about 10' over the roofline tops.


The only other option that would eliminate the need to add extra cable would be to put a higher mast on top of the rotor. That way, I would only have to cut a new longer cable to go between the antenna lead and the pre-amp. The problem with that, obviously, is that Channel Master says a maximum of 6" of mast above the rotor. I've heard other people put more mast up there, but I'm not comfortable with that. I don't want to burn out my rotor, which would make my wife real happy. NOT!


It's a two-story house, so the top of the roof is about 30' off the ground, with another 6' or 7' above the roof line. We're looking at about 35' above the ground, I'd say.
 

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I've posted this on the HDTV Programming forum but would like some feedbeck here as well after look at this string - Thanks.


Before spending hundreds of dollars trying to clean up you signal you may want to look at the effect of impulse noise on your reception. The following; http://www.pbs.org/digitaltv/dtvtech/news15.htm has a description of it. I have been trying to filter this with my Dish6000/OTA module ever since I bought it about 4 months ago with no success. I've engaged DN and its still an open call. Every time I run a load of laundry, use the microwave or turn on/off a flourecent light my tv blinks out - call it want you will. Signal strength can be 90+ and it still happens. Looks like a problems with 8VSB technology to me. I would be interested to know if anyone else if having this problem and attributing it to multipath. Look for a pattern of behavior associated with a household appliance and try to repeat the behavior. I did with a flourecent light and watched the signal go from 85 to less than 30 - and bingo - TV interuptus. I've grounded everything in sight, spent hundreds on TV antennas with no success. Is anyone else seen this on the Dish6000 or other receivers? - Thanks
 

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Randy,


I'd hate for you to climb the roof and stand a chance of breaking your neck for something that may have no effect whatsoever. I am not sure how much difference 5 or 6 feet may make, with diffraction reception, may make a huge difference, good or bad, or no difference at all.


Putting the antenna up above the rotator is probably not a good idea, though the spec for the rotator is probably for a full sized antenna, and you're only using a very small UHF antenna.


Don't know what to say at this point. You're doing everything right, and still having marginal results. A bigger antenna, like the parascope would give you more gain, and fewer "greenouts", but may not be enough.


You could try buying one of the inexpensive 4 bay antennas, putting it on a mast, and try different positions in the yard, but I'm guessing that's not a good option for you.


Placing an identical antenna about 2' below the one you have, and using an antenna coupler would give you a little over 2 dB of gain, but for the trouble, probably wouldn't have much affect.


I think short of putting a big ugly mast/antenna combination up, you've just about beat this horse to death.


Sorry I can't be much more help.


rknapp,


Impulse noise shouldn't be a big issue with 8VSB, it is a serious issue with COFDM. The coding used with 8VSB is especially robust against impulse noise. I'm suspecting the power line filtering into your STB may be suspect. Try a good quality line filter. If it is having this much effect, you need to check the quality of feedline/position of antenna/ or other things, it is NOT 8VSB causing the problem. I live in the country with electric fences and power lines that put lots of noise on my NTSC reception, and never have problems with it causing my STB to lose synch.
 
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