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Discussion Starter · #1 ·
Ok, so I'm not totally in the know about the 8vsb standard, but as I recall, one of the objections to using it was the multipathing issue. But can multipathing be corrected or at least mitigated by hardware or software?


The question is, could future firmware upgrades from Dish help my multipathing problems? Since my antenna is wall mounted on the side of my house, I'm hesitant to add more mast to raise it higher. I've only got another 4 feet to go before I'd need to guy the thing, and that's something I just don't want to do. I'm hoping for a software solution!
 

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Of course if there isn't a software fix DISH 6000 owners are still lucky - they can change out the tuner cartridge and save having to buy a whole new STB.
 

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Discussion Starter · #3 ·
Actually, I'm using:

Channel Master 4248 antenna

Channel Master 7775 UHF pre-amp

Channel Master rotor


I'm wondering if raising the antenna a few feet will help. I'm kind of stuck in terms of moving the antenna, as it's mounted on the only side of the house wall that I can get access to that won't be blocked by other objects. If I point my antenna to NYC from here, I get spikes of signals up to around 60, but no locks. It's clear there's signal there, but just too damn much multipathing to receive it clearly. What was at first a real joy and triumph is turning into a pain in the ass without a reliable signal. Would, say, another 5' of mast help with the multipathing?
 

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Randy,


The only way to know for sure is to try. I bought a bunch of Channl Master 5' masts for about $5 each, they interconnect very well. For testing only, I have added 20' of mast to my antenna and mounted it to the top of my rotator, DO NOT TRY THIS IF IT IS WINDY. If it worked out, I would have put up guy wires. In my case, I found that a total of 8-1/2' worked best, this puts my antenna about 4' above the peak of my roof. I have a custom made telescopic mast (adjusted to 3-1/2') below my rotor and a 5' mast above the rotor. I can adjust the antenna height in 6" increments. Anything above 10' and ch 60 will hardly lock. Ch 60 comes in best at 8', but then ch 65 drops out, so I had to compromise and get all channels at 8-1/2'. Now I have equal signal strength on ch 60 and 65. So give it a try, you got nothing to lose.


Glenn
 

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In the near term, most of the improvements will be realized in the demodulator (baseband) section of the tuner, not in the RF/IF section. But these parts aren't firmware upgradable (yet), so it will take new hardware.


One of the quasi-myths about 8VSB is that it can't deal with multpath, and that a multicarrier modulation such as OFDM is immune to it. This is innaccurate. OFDM can make receiver design simple for dealing with short-duration multpath by building a "guard interval" into the signal, effectively throwing away part of the bandwidth in order to use it for this guard interval time. As long as the multipath delay is less than the guard interval, the receiver can basically ignore it. Thus OFDM works well for situations where the station coverage is expected to be small (so long-delay echo is never really experienced), and you can afford to throw away some bandwidth (e.g., bit rates needed to suppot HDTV aren't a priority). In addition, if you want to work in rapidly changing (dynamic), short-duration multipath conditions, such as you get with mobile operation in the concrete canyons of Manhattan and a few other select areas in the country, you can allocation some of the OFDM subcarriers as pilot tones, thereby making signal lock easier, at the expense of even more bitrate.


On the other hand, if the multipath is a longer duration than the gaurd interval, as what you typically see in suburban areas in the U.S. (where most TV viewers are located), then OFDM-type schemes are toast.


In contrast, 8VSB theoretically can handle any amount of multipath, but the receiver has to do the work. The receiver needs to have an equalizer longer than the duration of the multipath signals it encounters, and it has to properly adapt the equalizer.


Unfortunately, probably mostly do to cost reasons, today's consumer 8VSB receivers don't seem to be designed to be up to the job. None of them are designed to work as well as the "reference" design used by ATSC in their field tests of 8VSB. There's a new chip by Broadcom that has alot of these characteristics (very wide equalizer range, NTSC rejection filters, etc.), but I don't know of any consumer designs yet using it.


Ultimately, Moore's Law will make computing power cheap enough so that the receiver chips can be made more powerful, but they don't seem to be there today.


Also, maybe in the longer term, modern antenna techniques, tightly coupled to the receiver, could be brought to bear. Imagine STBs with built-in phased-array UHF antennas that automatically "aim" themselves toward the station. Such techniques have been around in military and other communications areas for awhile, but TV antenna reception technology has been stuck in 1960's-era technology, largely due to the prevalence of Cable.


------------------

You have a right to install OTA and dish antennas on property under your control.


See http://www.fcc.gov/csb/facts/otard.html
 

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You're in the same boat I am, Randy. What is annoying PQ degradation with analog is an outright reception killer with digital. The 8VSB tuners DO apply some sophisticated filtration algorithms to deal with multipath, but it is limited in its effectiveness. The rallying cries among early adopters is "The next generation of 8VSB chips will deal with it!" And, "As the broadcasters gain more experience with this new medium, signal acquisition will improve!" A downloaded software solution? That would be nice, but only time'll tell if that will prove to be feasible. Those of us in digital hell are all still waiting. Of course there're always a select few who can stick a wire coathanger up and get nine digital channels with perfect consistency. A pox on them.


What YOU may be able to do is try a more directional antenna. Corner reflector yagi style models are usually the most directional, and the bigger, the more directional. The Channel Master 4248 is reasonably priced, well designed, and well built. Some have tried a relatively recent British design - the Blake JBX21 ( http://www.blake-aerials.co.uk/ ). This one's pricey at about $150.00 with shipping from England and all. Glenn_L tested several including the ChannelMaster and the Blake and feels the Blake is somewhat better. Antenna placement for UHF signals is critical. Sometimes moving the damned thing fore/aft or side-to-side mere inches makes all the difference. Sometimes it doesn't do squat. It's all (expensive) trial and error.
 
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