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Is there such a thing as having too strong a cable signal?

post #1 of 15
Thread Starter 
Thats what Comcast Cable guys is telling me after testing my lines. I was getting some tiling and picture freezing. The conclusion was that AGC was boosting an already too strong a signal which was the cause of my reception problems. I have +17db at the main TV and about the same at the drop. +23.4db at the street.

Never heard of anyone having too strong a signal. The cable guy could be 100% correct, I just never heard of this before. I was expecting to get am amp added, now they want to reduce the signal at the street.

Is this possible?
post #2 of 15
Sure it's possible. And you can get similar results from over-boosted OTA signals. -- John
post #3 of 15
Cable boxes and T.V's are designed to receive a certain signal level...I'm guessing somewhere between 0-5 dbmv. Too much signal can definately cause tilling and distortion. Most T.V's probably will not be affected by this if the cable is going directly to the T.V set, but I've seen many Sony sets that start acting up when the signal is too strong. Some just seem to be some sensitive than others.

As far as fixing this, i guess they can fix it at the street, but then that would also affect your neighbors but might not have the same problem than you.
The best fix for you is for them to 'pad' your drop. Or they can even put an attenuator, or a splitter for that matter, right behind your t.v. to lower your signal.
post #4 of 15
Thread Starter 
From what I understood, they are going to reduce the AGC that is specific to my drop, this is located at the mini-kiosk in my front yard as all cables are buried underground. They gave me some extra splitters to reduce the signal inside the house.

What is strange is the the TV has a signal strength meter that reads 64% on digital channels. If the signal is so strong, I would think the meters would read 100%.
post #5 of 15
I think the max signal on a particular drop is about +13.5 dBmV, under FCC rules.

Your "meter" is probably showing a low "quality" reading, due to overload. It's not usually just a "signal strength" indication.
post #6 of 15
Those are very strong signals. The typical level at the entry to the house is around 0dBmV. The digital cable boxes like the signal to be less than about 10dBmV and higher than -10dBmV. If you aren't adding amplification, try a fixed attenuator of different values and compare results.
post #7 of 15
Thread Starter 
Wouldn't you expect that the signal strength meters on my TV and DVR would reflect an off the chart signal as 100%, instead the TV says 64% while my DVR says 88% (measuring same digital channels on both).

TV is Pio Elite 1140 and DVR is a Sony DHG-HDD250. Are these signal strength indicators so sophisticaed that it is determining the quality rather than quantity of the signal and representing that as a percentage number?

I guess if that is true, if I add attenuators to slow down the flow, it should increase the numbers. I will try that tonight, let you know what I find.

On the other hand, it may be that these meters are so unsophisticated, that they are hardly any indication of anything meaningful.
post #8 of 15
They are usually related to how hard the error correction is working. Much easier to tie to a display rather than a signal measuring circuit.
post #9 of 15
the FCC doesn't specify a "max" dBmv level. FCC proof of performance testing is based on a 30 meter drop. a minimum of 3 dBmv at the end of the drop, and minimum of 0 dBmv at the input to a STB. the FCC specs are only in regards to analog signals. no POP specs for digital. the STB's i deal with will handle a tremendous amount of signal b4 having issues, (+ 15 dBmv). yes, high signal levels can be as bad as low. as bfoster stated what your tv is telling you isn't exactly the "truth". its a tv, not a signal level meter. high levels can greatly reduce a dig. signal SNR (or MER). which can result in tiling etc. if the problem lies in the plant then "padding" the drop won't help. the distortions caused by the high signal can't be fixed by lowering the signal in the house. the correct levels have to be obtained on the input/output of the line amps. if you are on a "high output " tap which would result in higher than normal signal levels, then padding the drop or cutting in a higher value tap would help.
post #10 of 15
Interesting thread..

I am having similar problem lately. Sometime I get freezing frame and other times I get "Please wait while we locate the channel"...

Using Motorola DCT6416III box. According to the INBAND Status, I am getting really poor signal, but it is approx 19db - 21db...

So if I understand this thread, if the signal is weak, remove splitters. If signal is too strong, add more spliter to lower the strength?
post #11 of 15
on a 256 QAM, a snr or mer < 33 db or so will cause problems. on a 64 QAM, < 28 db or so will cause problems. low/high or the correct signal level can give these readings. it all depends where the problem is. tap to house or out in the plant. the equipment in the house will tolerate a wide range of signal levels, but if you get to the extremes you will see issues.
post #12 of 15
I have been told by a few Comcast guys that a large temperature swing (esp within a day or so) can cause issues with the signal strenth. It could be impacted even if your line is underground if the source is above ground down the road.
post #13 of 15
Quote:
Originally Posted by dmichael View Post

Wouldn't you expect that the signal strength meters on my TV and DVR would reflect an off the chart signal as 100%, instead the TV says 64% while my DVR says 88% (measuring same digital channels on both).

It's odd, but too much signal strength can actually hit equipment so hard, it registers as a reduced signal.

A similar thing can happen with satellite dishes. They encounter what is known as "splatter", where the signal is so strong, it actually breaks up and becomes harder to lock in on.

Your TV may be experiencing this same effect, where the signal is so powerfull, it's breaking up and appearing to exibit poor signal strength.

Think of it like this:

Before I had a TV with multiple inputs, I used a video switch box. The problem is, every time a trailer for some suspense or horror movie would come on, the video would flip and roll. Why? Those silly white flashes they put in where too "white" for the switch box. The box didn't have enough headroom to handle that high of a video frequency, so the image went wonky.

When I had cable at my last apartment, I had the same issue with too strong a signal. I'd get knocked off the 'net all the time with my cable modem. As a result, I had to have an attenuator on the line to drop the signal down enough so the cable modem could use it.
post #14 of 15
Quote:
Originally Posted by lance_60031 View Post

I have been told by a few Comcast guys that a large temperature swing (esp within a day or so) can cause issues with the signal strenth. It could be impacted even if your line is underground if the source is above ground down the road.

Yep - cold days can really boost your signal when the wires get brittle, while a really hot day can slow things down.
post #15 of 15
Thread Starter 
Quote:
Originally Posted by alien loner View Post

on a 256 QAM, a snr or mer < 33 db or so will cause problems. on a 64 QAM, < 28 db or so will cause problems. low/high or the correct signal level can give these readings. it all depends where the problem is. tap to house or out in the plant. the equipment in the house will tolerate a wide range of signal levels, but if you get to the extremes you will see issues.

You touched on something here regarding SNR. I have a sony DVR that displays the 256 QAM SNR. Normally the SNR is 32db(in band), OOB is usually half the amount of IB. When I have issues of tiling and freezing, I immediately check the Sony and it will invariably show a SNR of 30 db or less. Many times its around 28 db. I've never seen a value higher than 32 and change.

Comcast has come out again recently because even after they claimed to have turned down the potentiometer on the house tap, I am still getting tiling. They claimed the signal to the TV was at 0 DB, which is perfect according to them and that the F-connectors were not tight. I usually use a wrench to tighten, but I don't like to make it too tight, so they just turned the wrench about a quarter turn and left. Have a nice day.

I have FIOS available in my town, and if they could just get 1 channel in HD(NESN), I would not hesitate to switch. I'll give Comcast one more shot, after that I will be forced to switch.
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