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post #4651 of 4676 Old 07-12-2016, 09:36 AM
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Well... I removed the preamp (and attenuator) and to my surprise the other channels remained stable. Fox does appear to be my weakest main station as far as signal strength, but the tuners are handling it well. The offending overload channel, real 51, seemed better at first with the strength still very high but without constant drop-outs during the evening, but the test of mid-morning reception proved that I have not found a solution. Mid-morning reception is the worst on that station for whatever reason - completely unwatchable.

I still feel that my problem is overload. The other tuners in the house seem ok so far, so I am not going to adjust the splits for now. Currently, the antenna is feeding directly into an 8-way splitter (75ohm rf terminators on 3 ports). Living room line is further split behind the entertainment center for the TV and Tivo.

Next thing I am going to try is the 20dB variable attenuator on the living room line, after the 8-way, before the TV-Tivo split.

This might be a silly question, but the attenuator has a dial with MIN (ccw) and MAX (cw). Does MIN/MAX refer to the output signal level or the level of attenuation (i.e. MAX could mean maximum attenuation resulting in the minimum output level....or.... MAX could mean maximum output level by applying the minimum attenuation)?
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post #4652 of 4676 Old 07-12-2016, 02:58 PM
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This might be a silly question, but the attenuator has a dial with MIN (ccw) and MAX (cw). Does MIN/MAX refer to the output signal level or the level of attenuation?
It's not a silly question, it's an intelligent question. It could be either one, because the markings are ambiguous. Did it come with an instruction sheet?

If you connect it before a TV that has a signal strength meter or your TiVo with its Diagnostics Screen, you should be able to tell which is which.

You have measurement tools built into your equipment, use them.
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post #4653 of 4676 Old 07-14-2016, 10:53 AM
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Originally Posted by rabbit73 View Post
You have measurement tools built into your equipment, use them.
No instructions, but using the tivo diagnostics when turned to MIN the SNR went basically to 0 on most of the channels and they would not tune. The strongest signal, the overloading station, was the only one that would tune, but even it was bad. On MAX, everything would tune but the overloading station was still showing reception problems. My conclusion is that MIN/MAX refers to signal output level (min = max attenuation).

Now, it is my understanding that the attenuator is reducing the amplitude of the signal AND the noise, so it should not directly affect the SNR (which seems to be my strongest correlation). I should see the signal strength go down and the SNR not change much (?). However, the variable I may not be considering is the receiver noise, which is not being attenuated. Speaking of, is it normal to feel a very small current (just a tingling, really) on the core of the line coming FROM the tivo? Is that indicative of receiver noise?

Can't really find a balance with the equipment I have on hand. The Tivo diagnostic screen is showing an SNR for real 51 without amplification or attenuation (and after only 1 split) of 31-33. I wish the diagnostic screen had a different type of signal parameter than %, which doesn't help much with overload.
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post #4654 of 4676 Old 07-14-2016, 12:50 PM
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Thanks for making the tests and reporting the results.
Quote:
My conclusion is that MIN/MAX refers to signal output level (min = max attenuation).
That makes sense; they marked it for the average user who would think of more or less signal. A tech would think of more or less attenuation.

I don't see why they couldn't have added Signal, MIN and MAX, or at least put it in the instructions.
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Now, it is my understanding that the attenuator is reducing the amplitude of the signal AND the noise, so it should not directly affect the SNR (which seems to be my strongest correlation).
It is true that the attenuator reduces the amplitude of the signals and the noise by the same amount, but how the SNR is affected also depends upon additional factors.

In cases of severe overload from strong signals, you can see the SNR of weak signals actually increase as you increase the attenuation. This is because when you increase the attenuation one dB, the strong signal is one dB weaker, and the weak signal is one dB weaker, but the noise floor drops 3 dB if you have IMD (intermodulation distortion) products in a preamp or tuner. Those distortion products are spurious signals created within the preamp or tuner that raise the noise floor to reduce the SNR of the weak signals. So, the net SNR gain is 2 dB for one dB of attenuation. Here is an example from my overload tests:



The FCC expects the transmitted signal to have an SNR of at least 27 dB, so when you get to that point as you increase attenuation, you will usually see a 1 to 1 correlation between attenuation and SNR. As you increase the attenuation, you will see a sudden drop in SNR (and increase in uncorrected errors) at the "Digital Cliff" just before dropout, because the digital signal requires a SNR of at least 15 dB, and a Noise Margin of at least 0 dB.

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post #4655 of 4676 Old 07-14-2016, 02:43 PM
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[QUOTE=rabbit73;45360313]Those distortion products are spurious signals created within the preamp or tuner that raise the noise floor to reduce the SNR of the weak signals.[QUOTE]

These distortion products can be a manifestation of overload? If 20dB is not sufficient to reduce the overload distortion on real 51, that would explain why the weaker signals drop completely off at max attenuation: the signal level is reduced by 20dB but the noise floor caused by distortion remains high.

If overload IS still the issue and I continue to get drop outs on the strongest signal at max attenuation, it's possible that additional attenuation will reduce overload, lower the noise floor, and bring the SNR of everything back to reasonable levels... maybe. Or maybe I need that band-stop filter.
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post #4656 of 4676 Old 07-14-2016, 03:46 PM
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If overload IS still the issue and I continue to get drop outs on the strongest signal at max attenuation, it's possible that additional attenuation will reduce overload, lower the noise floor, and bring the SNR of everything back to reasonable levels... maybe. Or maybe I need that band-stop filter.
Yes, that is the maybe. More attenuation should tame the strongest signals, but how the weakest signals will be affected is unknown. It depends upon the difference, in signal strength, between the strongest signal and the weakest desired signal, which is called the dynamic range. To that you must add the minimum required SNR which gives you the SFDR (Spurious Free Dynamic Range).



If it is not possible to find an adjustment of the attenuator that will allow you to receive the strongest and weakest desired signals without trouble, that is when you must attenuate your strongest signal with a custom single-channel bandstop filter.

You need at least two strong signals to create IMD, but you can have distortion products created by fundamental overload from just one strong signal.

I think you should add a little more attenuation as a test. Inexpensive fixed attenuators are available in values of 6, 10, and 20 dB.
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post #4657 of 4676 Old 07-14-2016, 04:02 PM
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Since CH 51 is so strong, I will now cover overload in greater detail before answering your remaining questions.

THREE TYPES OF OVERLOAD

There are three types of preamp or tuner overload, in order of increasing signal strength:

1. The strong signals almost cause enough intermodulation distortion (IMD) to interfere with the reception of weak desired signals, but the spurious signals are at or below the noise floor of the weak signals. This is the point that holl_ands uses in his preamp charts to obtain max SFDR (Spurious Free Dynamic Range). No damage will happen.

As the strongest signals continue to increase in strength, more of the weaker signals are damaged until you reach:

2. The strong signals cause overload to the preamp or tuner that makes it impossible to receive any signals. No damage will happen. The strongest signals are still there, but they can't be decoded because the IMD products have damaged them so that they contain more errors (high BER....bit error ratio/rate) than can be corrected by the FEC (forward error correction).

3. The signals are so strong that the input transistor is toast. You are not likely to encounter OTA signals that strong, unless you live next door to a high power transmitter and you have your high gain antenna aimed at the transmitter's antenna.

As a general rule, tuners can tolerate stronger signals than preamps before overload. The difference in strength is approx. equal to the preamp gain.


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post #4658 of 4676 Old 07-14-2016, 04:21 PM
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Speaking of, is it normal to feel a very small current (just a tingling, really) on the core of the line coming FROM the tivo? Is that indicative of receiver noise?
If you mean you feel a tingling on the coax shield to the TiVo, that is not normal and it is not indicative of receiver noise. You are feeling leakage current from AC operated equipment that is not grounded, but you said the coax IS grounded.

If you can not measure it, you can not improve it.
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post #4659 of 4676 Old 07-14-2016, 04:32 PM
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However, the variable I may not be considering is the receiver noise, which is not being attenuated.
That is correct; very perceptive. The tuner Noise Figure (~6 dB) is added to the Thermal Noise Floor, which is about -106 dBm. To that you must add the minimum required SNR of 15 dB.

-106 dBm + 6 dB + 15 dB = -85 dBm

That means the weakest signal you can receive is -85 dBm at the tuner input. There are two ways you can visualize that:





But, if you look at a tvfool report, you will see minimum required NM of zero is at about -91 dBm, because the report doesn't take into consideration the tuner NF. If you use a preamp, it will get you down to signals that weak because the tuner NF is buried in the amplified noise floor.

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post #4660 of 4676 Old 07-14-2016, 05:17 PM
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Can't really find a balance with the equipment I have on hand. The Tivo diagnostic screen is showing an SNR for real 51 without amplification or attenuation (and after only 1 split) of 31-33. I wish the diagnostic screen had a different type of signal parameter than %, which doesn't help much with overload.
Don't worry too much about the signal strength in percent; either you have signal lock or not with digital.

You will soon find out how weak a signal can be and still be received, so it is satisfactory as a relative indicator. You would have to spend a lot of money to buy a signal level meter that reads in dBmV as used by antenna installers, and I'm not convinced it would give you much more than what you already have. IIRC, the TiVo doesn't read much higher than about 70% no matter how strong the signal. What you need to watch are SNR and uncorrected errors. Here are two posts by a TiVo Roamio user with before and after readings:

http://forum.tvfool.com/showpost.php...8&postcount=15

http://forum.tvfool.com/showpost.php...6&postcount=21

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post #4661 of 4676 Old 07-15-2016, 04:27 AM
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I need a clarification on this, so I will quote you again:
Quote:
Originally Posted by jj94tt View Post
Speaking of, is it normal to feel a very small current (just a tingling, really) on the core of the line coming FROM the tivo?
I think you mean cord, don't you?

And if it is coming FROM the TiVo, what kind of cord is it and what does it go TO?

If you feel a tingle (small shock?), that means the coax is NOT grounded with a grounding block connected to the house electrical system ground.

And it means that it will be more difficult to tame ch 51.

And it means that you are at risk from electrical shock.
http://www.digitalhome.ca/forum/81-o...ml#post1457594

http://www.digitalhome.ca/forum/81-o...ml#post1457668

Please clarify.
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post #4662 of 4676 Old 07-15-2016, 05:28 AM
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Originally Posted by rabbit73 View Post
And if it is coming FROM the TiVo, what kind of cord is it and what does it go TO?
The core of the coax connected to the tivo. I discovered this while connecting/disconnecting the various components I am using to try and solve this issue. I disconnected the tivo on the splitter end (to add or remove the attenuator) and the dangling line brushed my arm and I felt a tiny shock, similar to a static shock. It surprised me at first, so I touched the core of the coax and sure enough, a little sustained tingle.

The grounding block is upstream of this, outside the house under the antenna, connected to the utility grounding rod. When the tivo coax is unhooked at the splitter, it is disconnected from this ground. I was surprised to feel a current coming FROM the tivo. I presume this can be attributed to leakage current.

I will check those connections outside the house tonight.
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post #4663 of 4676 Old 07-15-2016, 06:48 AM
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Ah, yes; thank you. When the TiVo coax was disconnected from the antenna system, it was no longer grounded. What you felt was the leakage current from the TiVo, because its power cord was still connected to AC.

If you mean the center conductor of the coax when you say "core," there should not be any voltage on that.

All AC operated equipment has leakage current, and this current is allowed within certain limits. It would be necessary to measure the TiVo leakage current when it is not grounded to see if it is within the limit.

http://www.simpsonelectric.com/image...9-2_manual.pdf

The average person cannot feel leakage current less than 1 mA (1/1000 of an amp). I was able to feel 200 microamperes, which is 0.2 mA. GFCI outlets trip open at 5 mA.

It is possible to make an inexpensive test circuit to measure leakage current:

http://www.repairfaq.org/sam/safety.htm#saftes

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post #4664 of 4676 Old 07-15-2016, 06:54 AM
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Originally Posted by rabbit73 View Post
If you mean the center conductor of the coax when you say "core," there should not be any voltage on that.
Yes, the center copper conductor.
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post #4665 of 4676 Old 07-15-2016, 10:46 AM
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Do you have an AC voltmeter to measure the voltage between the center conductor and ground and the shield and ground?

If you can not measure it, you can not improve it.
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post #4666 of 4676 Old 07-19-2016, 06:56 AM
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Originally Posted by rabbit73 View Post
Do you have an AC voltmeter to measure the voltage between the center conductor and ground and the shield and ground?
I will check this tonight. I also disconnected the TV from the tivo coax line, maybe I was feeling the combined leakage from both components before.
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post #4667 of 4676 Old 07-19-2016, 07:43 AM
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Originally Posted by Calaveras View Post
There's no way you can use a preamp with your channels 50 and 51. Something is going to get overloaded. I'd get a Jan Jenca channel 51 band stop filter and put it ahead of the preamp. It should eliminate the channel 51 problem and reduce channel 50 too while having minimal impact on channel 48.
I contacted Jan Jenca regarding the channel 51 band-stop filter. 2 options:
CH 51 attenuate – 17dB...............15,90€
CH 51 attenuate -32 dB...............17,90€



Again, my channel 51 has a NM of 75.5. I have to decide on which band-stop attenuate level, then should I try the band-stop ahead of the AP8275 or is that preamp still overkill for my situation regardless? Should I be thinking of the 32 dB attentuate band-stop with no preamp and a powered distribution amp?


Antenna to distribution block is probably 35-40 feet. Longest run after distribution is probably another 50 ft. The tivo is probably 30 ft after distribution. I have 5 TVs plus the tivo to feed with two additional bedroom lines that are capped at this time (8 total coax runs).


Thanks for all the help so far.
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post #4668 of 4676 Old 07-19-2016, 08:07 AM
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Quote:
Originally Posted by jj94tt View Post
I contacted Jan Jenca regarding the channel 51 band-stop filter. 2 options:
CH 51 attenuate – 17dB...............15,90€
CH 51 attenuate -32 dB...............17,90€



Again, my channel 51 has a NM of 75.5. I have to decide on which band-stop attenuate level, then should I try the band-stop ahead of the AP8275 or is that preamp still overkill for my situation regardless? Should I be thinking of the 32 dB attentuate band-stop with no preamp and a powered distribution amp?


Antenna to distribution block is probably 35-40 feet. Longest run after distribution is probably another 50 ft. The tivo is probably 30 ft after distribution. I have 5 TVs plus the tivo to feed with two additional bedroom lines that are capped at this time (8 total coax runs).


Thanks for all the help so far.

I would use the 32 dB band stop filter. That would still give you a NM of >40 dB which is huge. I would not use the AP-8275 under almost any circumstances. I had one long ago and even though it is spec'd at 29 dB gain on UHF I measured 36 dB which is far too much. I had severe overload with it. I'd consider the Clearstream Juice preamp and a CM3418 8-way distribution amp. That should maintain pretty equal Noise Margins to all your devices.
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post #4669 of 4676 Old 07-19-2016, 12:20 PM
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Originally Posted by Calaveras View Post
I would use the 32 dB band stop filter. That would still give you a NM of >40 dB which is huge. I would not use the AP-8275 under almost any circumstances. I had one long ago and even though it is spec'd at 29 dB gain on UHF I measured 36 dB which is far too much. I had severe overload with it. I'd consider the Clearstream Juice preamp and a CM3418 8-way distribution amp. That should maintain pretty equal Noise Margins to all your devices.
Of the channels that I want/need, FOX has the lowest NM of 29.4 dB. With the band-stop filter, I am thinking that I may be able to make due without a preamp, but with a distribution amp to basically restore the losses from the 8-way split. Or I could try the Juice preamp with the passive splitter.... do you think my best bet is with both the preamp AND the powered dist. amp?
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post #4670 of 4676 Old 07-19-2016, 12:41 PM
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Quote:
Originally Posted by jj94tt View Post
Of the channels that I want/need, FOX has the lowest NM of 29.4 dB. With the band-stop filter, I am thinking that I may be able to make due without a preamp, but with a distribution amp to basically restore the losses from the 8-way split. Or I could try the Juice preamp with the passive splitter.... do you think my best bet is with both the preamp AND the powered dist. amp?

You can probably do without the preamp but I'd keep the DA.
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post #4671 of 4676 Old 09-13-2016, 06:12 PM
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I was always under the impression that the number of televisions tuned to an over-the-air signal couldn't affect the strength of said signal. However, ever since the digital switchover I've noticed a strange phenomenon that I can't explain. I get fairly strong signals on all of the major networks. I can watch any of the following virtual channels and all of their sub-channels at practically any time of any day: 2, 4, 11, 13, 16, 19, 22, 31, 40, 53, 59, 66, and 69. The thing that puzzles me is this: During most OTA televised Steelers games, on whatever local channel they might air, the signal cuts out, digitizes, freezes, and does everything else one might expect due to weak reception. It also happened during the last Super Bowl broadcast and the recent Summer Olympics. It doesn't matter if the game's on 2, 4, 11, or 53. They all do it during the game but clear again up shortly afterward. A friend of mine who's somewhat into conspiracy theories says it's because Comcast pays off the station to lower the transmitter power during the games so we'll all be forced to switch to cable. Uh, no. I'm not a big football fan myself, so I could just find something else to watch if my wife wasn't so into the Steelers. I'm tired of hearing her complain about the signal breaking up when Big Ben is throwing. It did it again last night. Could it possibly be because so many other households are tuned to the same station at the same time? I typically would have argued against that possibility but I can't think of any other reason for it. Any ideas?
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post #4672 of 4676 Old 09-13-2016, 07:48 PM
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weather, airplanes, something. It's against FCC rules to arbitrarily change power. Most likely, the games happen to coincide with some other issue. Where do you live in relation to the rivers, airport and so forth?

Walking the fine line between jaw-dropping and a plain ol' yawn.
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You also haven't mentioned what kind of antenna you're using.

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Originally Posted by DrDon View Post
Most likely, the games happen to coincide with some other issue. Where do you live in relation to the rivers, airport and so forth?
I'm midway up the hill above ATI Steel's Brackenridge plant. Ground elevation is about 150 feet higher than the Allegheny River. The antenna is about 30 feet above the ground in my attic.

I've often checked for local interference by trying to view another channel during the games and there isn't any interference on any channel except whatever station is carrying the game at the time. So if it was a neighbor running a blender, for instance, it should affect most all of the channels. I can't watch TV at all when he mows his lawn with an electric mower, for instance.

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Originally Posted by Trip in VA View Post
You also haven't mentioned what kind of antenna you're using.
I'm using a 1990s Archer VU-90 hung from the ceiling in my attic. As I've stated, I get very good reception almost all of the time. It's a directional antenna, and I have it pointed to optimize channel 4-3, which is my toughest station to maintain a reliable signal.

My main TV doesn't have a signal strength meter, but a digital converter we have in the spare bedroom has a meter with red, yellow, and green indicators and a numeric value up to 100 maximum. With the exception of channel 22 (high 50s and yellow but still good enough to watch reliably) and WTAE's 4-3 (mid-70s green), all of my channels are well into the green (typically mid-80s to low-90s) and I can watch TV without glitches almost all of the time.

That's why it's so frustrating that whenever the Steelers play I get so many dropouts. Channels 2 and 11 both show 90-93 on the signal meter, but even they drop out when CBS and NBC host the Steeler games, etc. WPXI was glitching like crazy during the Rio Olympics.
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post #4675 of 4676 Old 09-14-2016, 06:38 AM
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Originally Posted by Soon2Bdark View Post
I'm midway up the hill above ATI Steel's Brackenridge plant.
Hey cool I grew up a few blocks away, up by the hospital.

Here is some discussion from a few years back I remembered:

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Originally Posted by rpesq View Post
more KDKA incompetence:
- for past week, channel 19 Nighty Sports Call, the ticker that shows the scores looks absolutely pathetic, barely readable.
- today (Sunday), watching the Steelers pregame show at 11:30am and now the CBS Noon pregame show, the screen is highly sharpened, dot matrix look that some crackpot at KDKA must REALLY love because this problem CONTINUES to arise occasionally -- same display issue that I reported for many many months here at avsforum. Extreme jagged edges, ultra sharpened, dot matrix look. Even an issue with the commercials, too, so it is inherent in your broadcast config.

Doesn't anyone at KDKA actually watch these broadcasts? Do they all have cheap TV's with poor resolution such that they cannot see how awful this looks? Seriously, if you have to report this issue this many times over MANY MANY months, you are seriously incompetent.
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Originally Posted by pittsoccer33 View Post
Glad I stumbled in here today and saw this post.


Just this morning I purchased as newer amplified antenna. On my HTPC I can choose either the OTA signal or the Comcast one. And the OTA broadcast during the Steeler game looks awful - I thought I might have a deinterlacing issue. But the cable feed is perfectly fine.


I just flipped back to my antenna feed and I was going to try getting some MPEG screen grabs, but it looks to have improved. The Steelers secondary unfortunately hasn't.
Quote:
Originally Posted by Cableguy Greg View Post
KDKA was having an issue before and up until half time of the game. When I talked to the Comcast Headend, the picture appeared fine there. Armstrong, FiOS, Consolidated and OTA all had issues. The problem appears to have been with KDKA itself. When they found out about it, it was corrected. This same issue has happened multiple times with their feed and the CW feed that they provide.

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post #4676 of 4676 Old 09-14-2016, 07:36 AM
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Originally Posted by pittsoccer33 View Post
Hey cool I grew up a few blocks away, up by the hospital.

Here is some discussion from a few years back I remembered:
pittsoccer33:

I remember that, too. Those were all video issues at the source.

Mine is a specific signal strength loss during events when lots of people are watching.
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