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622 OTA Overload/Multipath Problem


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31 replies to this topic

#1 OFFLINE   mraub

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Posted 18 March 2007 - 04:53 PM

One of our local broadcasters just turned on their high power DT transmitter this week. They went from about 30Kw to 500,000 Kw, with plans to go to 1,000,000 Kw soon. As soon as they went to 500,000 Kw almost everyone with a 622 within 10 miles of their tower could not get a stable lock on their signal. The station got so many complaints that they finally went back to their low power transmitter (and back to the old drawing board I presume).

Has this been a recurring problem with the 622 as more stations ramp up to their full digital power? Actually there is another local station that pegs the 622's meter, but is completely stable. The problematic station came in at 93-94, but lost lock about once a minute.

A $60 attenuator set to the station's frequency should fix the problem, but I'm hoping some sort of adjustments to the station's transmitting equipment can fix it.

Thanks for any help.

MIKE

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#2 OFFLINE   Geronimo

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Posted 18 March 2007 - 05:06 PM

One of our local broadcasters just turned on their high power DT transmitter this week. They went from about 30Kw to 500,000 Kw, with plans to go to 1,000,000 Kw soon. As soon as they went to 500,000 Kw almost everyone with a 622 within 10 miles of their tower could not get a stable lock on their signal. The station got so many complaints that they finally went back to their low power transmitter (and back to the old drawing board I presume).



Has this been a recurring problem with the 622 as more stations ramp up to their full digital power? Actually there is another local station that pegs the 622's meter, but is completely stable. The problematic station came in at 93-94, but lost lock about once a minute.

A $60 attenuator set to the station's frequency should fix the problem, but I'm hoping some sort of adjustments to the station's transmitting equipment can fix it.

Thanks for any help.

MIKE


Those transmitter power levels seema bit high to me. Am I reading them wrong? I mean a million kilowatts seems way over the top.
I never cared for all the signatures that insult posters with other points of view.

#3 OFFLINE   mraub

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Posted 18 March 2007 - 07:05 PM

You're right--it is 1,000,000 Watts ERP. Still a lot of power.

#4 OFFLINE   Mr.72

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Posted 18 March 2007 - 08:04 PM

Yes I think this is a problem with the 622's OTA tuner/receiver, that it is prone to distortion and causes problems with too much gain or too-hot signals. I have been discussing this in another thread about Austin FOX/KTBC station problems. So far all of my observations support the theory that it's the receiver being overdriven causing the problem but it could be a number of different things.

#5 OFFLINE   Barrysb

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Posted 18 March 2007 - 08:50 PM

Yes I think this is a problem with the 622's OTA tuner/receiver, that it is prone to distortion and causes problems with too much gain or too-hot signals. I have been discussing this in another thread about Austin FOX/KTBC station problems. So far all of my observations support the theory that it's the receiver being overdriven causing the problem but it could be a number of different things.


This is not the case with my 622. I have a problem station which is not my strongest signal. I've used an attenuator to get the signal strength indicator down to a range of 68-72 and the drop-outs (lost sync) disappear. It seems the receiver wants to work near its max sensitivity when it encounters a problem signal.

#6 OFFLINE   Mr.72

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Posted 18 March 2007 - 10:10 PM

I don't know. I suspect it's distortion from a stronger signal causing interference with the weaker one.

What are the channel/frequencies of the strongest signal you get, and the problem signal?

#7 OFFLINE   samchecker

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Posted 19 March 2007 - 07:51 AM

Yes I think this is a problem with the 622's OTA tuner/receiver, that it is prone to distortion and causes problems with too much gain or too-hot signals. I have been discussing this in another thread about Austin FOX/KTBC station problems. So far all of my observations support the theory that it's the receiver being overdriven causing the problem but it could be a number of different things.


Now this is an interesting possibility...I've posted here before about my OTA problems with the 622 vs putting the signal straight into my Sony DLP set. Just this weekend (at the suggestion of an engineer from one of the local stations) I switched from an amplified indoor antenna to a good old VHF/UHF rabbit ears and loop antenna, unamplified. The results I get are just as good if not better than what I got from the amplified antenna (I'm less than 10 miles from pretty much all the transmitters around here). Now, though, my biggest problem is from the only local digital that transmits in the VHF band...I'll see a signal strength of 93 and then lose the thing altogether. Hmm.

At the risk of jumping across threads, is this something the new 4.01 software is going to address?

#8 OFFLINE   Mr.72

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Posted 19 March 2007 - 08:00 AM

I can't say whether it can be solved with software. Some of the 211 users who had similar problems say the latest software update went a long way towards fixing it but so far the jury is out on the 622 since 4.01 is truly vaporware.

#9 OFFLINE   mraub

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Posted 19 March 2007 - 08:19 AM

Thanks for the replies. Though others have had this problem, it seems rather sporadic given the number of 622 users who live within 10 miles of a high power transmitter. I wonder if the problem relates to some characteristic of the transmitted signal, though I don't have enough technical knowledge of digital modulation techniques to even speculate on specifics.

Though this station is very close, a couple of others I watch are >50 miles away, which limits how much I can downgrade my antenna (an unamplified 1 bay in the attic of my two story house). It would be a nice feature to be able to adjust receiver sensitivity on a channel by channel basis, but I don't think any receiver offers this feature and I doubt it could be added to the 622 by a mere software upgrade.

MIKE

#10 OFFLINE   Mr.72

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Posted 19 March 2007 - 10:57 AM

yes if I am correct, it relates directly to the FREQUENCY of the signal.

you have two types of distortion common when you overdrive an amplifier: harmonic distortion (new frequencies appear at multiples of the original frequency, higher), and intermodulation (aka "beat frequencies", that is the difference between two frequencies represents as a new frequency).

So for example, say you have a signal at 500 MHz that's over-driving the preamplifier. It is going to likely make a lot of 3rd harmonic at 1500 MHz, plus likely that's not a balanced signal so it will show up as 750 MHz. So if you have some channel you want to pick up at 750 MHz, then when you tune to 750 MHz your receiver will see both the signal you want at 750MHz PLUS the distortion garbage from the overdriving of 500 MHz. The receiver can't tell the difference between the good (signal) and bad (distortion) so it just says "I give!" and doesn't tune the channel.

So it's very frequency-dependent.

Likewise if you have two strong frequencies close to each other like say 600 MHz and 800 MHz then you will get the difference between those as a new signal at 200 MHz.

I think many of you guys on this forum are used to thinking of a channel as something you decode or subscribe to in a data kind of way, but with OTA a channel is a frequency, which is a purely analog, RF issue.

#11 OFFLINE   Barrysb

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Posted 19 March 2007 - 12:06 PM

I don't know. I suspect it's distortion from a stronger signal causing interference with the weaker one.

What are the channel/frequencies of the strongest signal you get, and the problem signal?


Here's a list of my local local analog and digital channels, along with the digital signal strength as measured by the 622 with and without 21 dB attenuation. All are coming from about the same location about 33 miles away.

15 - analog
16 - digital 79, 100
27 - analog
29 - digital 66,93
31 - digital 72, 96 (the problem station on 622 but not a Mits LT-37131. OK with 21 dB attenuation)
33 - analog
38 - digital 78, 100
40 - digital 75, 100
41 - digital 70, 100
43 - analog
46 - digital (my 622 is not seeing this signal for some reason, will try rescanning)
49 - analog
50 - digital 62, 92 (weakest station received, will break up occasionally with 21dB attenuation)

#12 OFFLINE   Mr.72

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Posted 19 March 2007 - 12:59 PM

are these the channel numbers (like you type "16" on your remote) or are they the actual UHF frequency assignments? You can look it up on antennaweb. If these are the UHF frequency assignments then most of your problem is likely cross-band leakage since all of these channels are so close together.

See each UHF channel is alloted a frequency "space" between two different boundary frequencies. In the good old days tv broadcasters would put the video signal on one boundary and the audio on the other. It's like if you're standing arms length to people on either side of you, if everyone put the audio in the left hand and the video in the right hand, then the video from one channel was as far away from that of the other channel as you can get it, thus no interference.

With digital TV they broadcast a big fat wideband signal that stretches from end to end of their slice of the spectrum, so it easily rubs right against the adjacent channel and I presume this causes all kinds of fits for receivers.

Anyway, that 38-40-41 adjacency has to cause a lot of problems. That looks like a big fat wideband signal, essentially, from 614-635 MHz without any hole in it. This will create all kinds of intermod artifacts below it and certainly could stomp on the lower band signal at 31. the first lower IMD harmonic is very close to the top end of the ch. 31 band.

anyway, I suspect this is not the actual digital UHF channel assignment, but just your channels you actually tune to (you know, they renumber digital channels ... so it might say "31-01" but in fact it might be on channel 61 for all you know).

#13 OFFLINE   mraub

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Posted 19 March 2007 - 01:29 PM

Mr. 72,

You seem to have a good technical grasp of the problem. Is there anything a transmitting station can do to minimize distortion from appearing in receivers trying to demodulate their signal. Even if they have to use the full frequency spectrum, can they reduce the drive on their signal (I'm thinking in terms of a Vu meter in analog; there may be no digital equivilant). Can Dish do anything with a software upgrade to make the OTA receiver less prone to the effects of distortion?

The station apparently got a lot of complaints from 622 owners. They made the change just before our local college basketball team made what ended up to be a very short-lived appearance in the NCAA tournament. It's kind of a Catch-22 for the station: A good number of local viewers aren't happy with the high power signal and those further from the tower can't get a good lock because the signal is too weak.

MIKE

#14 OFFLINE   Mr.72

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Posted 19 March 2007 - 02:53 PM

yeah the only thing they can do is reduce the power, relative to the other channels.

I am purely speculating about the 622 receiver circuit. It likely employs an automatic gain control (AGC) that sets the input gain (sensitivity) for the entire band all at once, but it might actually set it when it switches channels. Either way, the problem is that once it cranks up the gain to pick up the weaker station then the distortion pops up. The way to fix this is with a piece of hardware connected to the receiver called a "preselector" aka "tuner" that isolates the band you are trying to pick up when you switch to that channel. So if you switch to channel 31, then the preselector would block the signal from other channels and only let channel 31 get in.

Like I say, this is real speculation on my part. I didn't design this 622 receiver, never opened the box up, and don't really *know* the specifics of the implementation.

if this is the problem, however, unless there is some unutilized feature in the receiver chipset that can be enabled with software, then I can't see how they'd fix it with software. I hope they do, though! Because they won't likely fix it in hardware. They're not likely to send you or me a new receiver with improved (more expensive) antenna tuner circuit just because we can't pick up a channel that is not provided by Dish.

#15 OFFLINE   Barrysb

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Posted 19 March 2007 - 03:46 PM

are these the channel numbers (like you type "16" on your remote) or are they the actual UHF frequency assignments? You can look it up on antennaweb. If these are the UHF frequency assignments then most of your problem is likely cross-band leakage since all of these channels are so close together.

anyway, I suspect this is not the actual digital UHF channel assignment, but just your channels you actually tune to (you know, they renumber digital channels ... so it might say "31-01" but in fact it might be on channel 61 for all you know).


I listed the actual channels not the virtual channels. You seem to know a bit about this but isn't this what we have the FCC for? I got to believe a group within the organization spent a lot of time to work out all of the possible interference problems. Maybe this condition will disappear when the analogs go black, However, the CE at the station is very doubtful it will. But my problem is still related to the 622 not able to handle the signal from this one transmitter, whatever is causing the problem and forcing the receiver to work at its highest sensitivity hides the condition.

#16 OFFLINE   Mr.72

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Posted 19 March 2007 - 03:57 PM

Well in audio, they used to say "you cannot drive a Mack truck through a mouse hole".

There is a rather technical reason related to gain-bandwidth product why a lower-level overall signal would result in reduced performance. Basically though I think the hot channel is hitting the rail (overdriving it ... like it's beyond the max input voltage of the receiver input), and the other channels are a moot point. The receiver can up the gain enough to pick up lower channels but it cannot pad down the hot channel enough to keep it from clipping. So when you present it with a full-amplitude signal, it clips. When you put a pad on it (attenuator), then the signal is low enough to not clip but the amplifier still has sufficient gain to pick up the lower channels without any trouble. If you allow it to overdrive then it will cause distortion that might interfere with other channels (as well as knocking out the channel you are trying to pick up).

And I should continue to point out, I could be absolutely wrong about all of this!! I could design an RF receiver, but I did not design THIS ONE.

#17 OFFLINE   Barrysb

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Posted 19 March 2007 - 09:22 PM

Well in audio, they used to say "you cannot drive a Mack truck through a mouse hole".

There is a rather technical reason related to gain-bandwidth product why a lower-level overall signal would result in reduced performance. Basically though I think the hot channel is hitting the rail (overdriving it ... like it's beyond the max input voltage of the receiver input), and the other channels are a moot point. The receiver can up the gain enough to pick up lower channels but it cannot pad down the hot channel enough to keep it from clipping. So when you present it with a full-amplitude signal, it clips. When you put a pad on it (attenuator), then the signal is low enough to not clip but the amplifier still has sufficient gain to pick up the lower channels without any trouble. If you allow it to overdrive then it will cause distortion that might interfere with other channels (as well as knocking out the channel you are trying to pick up).

And I should continue to point out, I could be absolutely wrong about all of this!! I could design an RF receiver, but I did not design THIS ONE.


But notice that my problem channel is not the weakest nor the strongest channel, and I'm able to receive all others without problems.

You may not be aware that the DT receiver has both a non-coherent and coherent AGC. The coherent function is looking at sequence information contained in the demodulated digital waveform, which is determined at the transmitter. These sequences are detected in the receiver, compared to a reference value and the differences are integrated to control the IF amp to produce the proper signal levels.

My theory is something in the signal is screwing up the coherent AGC. Inserting attenuation causes the AGC to shut down thus stabilizing the decoder.

I could be wrong as well. This digital stuff is not easy to understand.

#18 OFFLINE   redbird

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Posted 20 March 2007 - 12:03 AM

You could have bad multipath on that channel. With analog, it just produces a ghost but with digital, the receiver sees a degraded data stream it may not be able to process.

#19 OFFLINE   Barrysb

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Posted 20 March 2007 - 04:31 AM

You could have bad multipath on that channel. With analog, it just produces a ghost but with digital, the receiver sees a degraded data stream it may not be able to process.


I don't believe it is multipath. All my UHF stations transmitting near the frequency of the problem station are ghost free. Plus there are other subs in this DMA at different compass points from the transmitter have the same problem.

#20 OFFLINE   mraub

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Posted 20 March 2007 - 10:24 AM

One of the engineers at the station that caused the 622 fits posted an explanation on the AVS Forum. I'll copy it here, in case it might be of help to other 622 owners when working with a local station to resolve reception problems:


We heard from the Nexstar corporate engineer today that Thales, the manufacturer of our transmitter, has acknowledged that some of their exciters are causing problems in the Dish 622 receiver. They've apparently known about this for the past three weeks and are working with Dish to resolve it.

When we hit the air March 11, we had no way of knowing those specific receivers would be affected. I did not author the statement on our website, but the general manager and I made the decision to stay on the low power transmitter. Our city of license is Champaign; it was a hard call, but we decided it would be best to serve Champaign with the best possible signal at the expense of other outlying areas.

Thales is supposed to send us a replacement exciter ASAP to rectify this problem. At that time we will return to the high-power rig and stay there. I've had it on today so we could conduct some field tests, but it'll be turned off within the hour to go back to the low-power.

We do apologize for everyone's problems, but in this instance it was out of our control.





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