Re: [IRCA] Noisy cable TV

[Rick Kunath <k9ao@xxxxxxxxxxx>]

Here are a few thoughts...

> The idea of disconnecting at the pole to see if cable is what's causing the 
> DSL problems is a good one.  Move the probable noise source as far away as 
> possible.

I'd disconnect the CATV drop at your demarc and see if the noise changes 
first. You can do that yourself. Do an inspection of the grounding 
scheme for the three utilities, power, telco, and cable, and describe 
for us what you have.

> I walked my portable radio along under my cable drop out to the 
> distribution pole in the back yard, and the noise followed me all the 
> way.

Can you null the noise using the ferrite loopstick in the radio, and 
does that null point to an area on the pole? A bad electrical insulator 
maybe? Unattached ground on the pole, unattached ground on the CATV 
hardline? Is there a CATV ground at this particular pole?

The noise couldn't be caused by an imbalance in the telco drop causing 
it to radiate could it? If you had one side of the DSL drop opening, or 
a bad DSL filter, you might unbalance the DSL drop and cause it to fail, 
generating the noise as a byproduct. Your DSL drop is a balanced 
feedline. (That's how it rejects common-mode noise.)

If you have CATV egress you'll be able to hear video carriers leaking 
out of the cable, and a good spot to listen is in the 145.25 MHz area, 
but you can find a CATV frequency chart and look for any of the TV 
carriers with a VHF radio or scanner. If you have egress, you'll hear it 
for sure doing this.

> Not definitive since the power lines go that way too, and the phone 
> lines are buried along the same path, but still.  While there are power and 
> telephone lines on the pole, there is a junction of a major cable line from 
> the south that splits in two directions east and west for local feeds, from 
> that pole.  Walking across my back yard east under the three utility 
> services' overhead lines, the noise dropped off as I moved away from the 
> pole.  Tomorrow I may try walking along the perpendicular cable line to the 
> south as well as the westward feeder, just to see what happens to the 
> noise.  With my luck it is all fed directly into my house!

Is there a CATV node or line amp on this pole? These will have AC power 
fed forward to them from a CATV power supply located on a pole some 
distance away, maybe a long way away. If there is a CATV connector 
suckout you may have arcing going on, though if this is the case, you 
should be able to see it on your television signals. Do you? You might 
have cracked hardline, loose hardline connectors, a defective 
directional coupler at the pole, etc.

What you are describing though, sounds like it could be conducted power 
line noise. Any ideas there?

> It is interesting to note that the occasions of extreme noise began around 
> the time that very utility pole was replaced and all the services moved to 
> the new one.  Possibly that procedure damaged something and created the 
> noisy situation in so doing.  I had always been suspicious of that but had 
> not traced the noise to the extent I have the last couple days, since it 
> has gotten far worse.

Have the CATV line tech inspect the pole carefully while he is on scene. 
Wiggling the hardline and the various connections, etc. Also have him 
listen for arcing above his head in the area of the power line while he 
is up the pole.

> 
> Your comments on sunshine and day/night problems is also very 
> interesting.  During a recent period of cloudy rainy days, my DSL was 
> pretty good, though lightning in the area would disconnect it.  The last 
> few days, during which I've had no daytime service, the days have been 
> bright and sunny.  I had been hoping once the thunderstorms subsided, my 
> DSL would stop frequently going out.  Instead, as the nice weather moved 
> in, I lost DSL entirely, during the day.  Now that you mention the notion 
> of daytime heating aggravating the problem, the correlation becomes evident.

Unless you have an egress issue with the CATV plant, nothing failing 
CATV node or amp wise should be able to escape the CATV plant and affect 
you. The cable plant does self adjust itself with temperature changes, 
and a mis-behaving ASC or AGC circuit in a CATV active device can cause 
various issues, but you'd see these on the video for sure. Are you?

Suckouts due to bad connectors are worse with cool weather as the 
aluminum hardline changes dimension quite a bit with temperature 
changes. Again, is the sufficient strain relief at the new pole? Look at 
other poles for an idea about what the installation should look like.

> I'll keep these ideas in mind and hope the cable guy is willing to listen 
> to suggestions.

Let's hope you get a line tech and not an installer, who won't have the 
necessary equipment or experience to really handle the issue. If you do 
get an installer, insist that they send out a line tech to finish the 
job if necessary.

<Snip>

>> At 03:57 PM 10/2/2006, you wrote:
>> It almost sounds like the cable feed is leaky somewhere and emitting
>> extraneous noise that is most likely either their Internet feed or control
>> signals (for addressable cable boxes, pay-per-view,etc).  Both of those
>> operate on very low frequencies that often include MW and LW on many CATV
>> systems.  Or it could be a bad repeater/amplifier on the cable trunk in your
>> neighborhood.

If there is leakage, you should be able to detect it using the ideas 
described back up the page a ways. Since the VHF leakage will travel a 
lot less distance wise, you should be able to narrow down the area with 
the egress problem a lot easier using VHF. Who knows, it may be a self 
install of something by a neighbor or something on the CATV aerial 
plant. If you have MW leaking out you'll have other stuff leaking too.

>> If it's strong enough to disturb your DSL, that's pretty bad.  I would
>> recommend that the CATV Technician disconnect the cable feed at the pole and
>> see if the noise goes away.  Then check your DSL to see if it's working.  If
>> everything seems normal, have him reconnect the cable and see if the problem
>> returns.  If it does, you will have then proven that the interference is a
>> Cable TV problem.

Maybe, but it could be more complex. DSL does use RF frequencies, and 
CATV return path leakage just might be able to affect this, again, look 
for telltale egress signs. If you can find the leakage evidence, you can 
find the problem, and the cable tech will have to fix it right then and 
there if the leakage is bad enough, that or shut off the entire area.

>> My guess is a noisy amp in the neighborhood whose signal level is so strong
>> that it blasts through the cable.  

Probably not the cause, even with a misbehaving amp, there still should 
be  no egress, and no effect to anything outside the cable plant. 
Besides, if this was the case, you'd have seen them there fixing the amp 
already as there would be video (likely) and cable modem complaints from 
users. But who knows...

>>Why it would only happen during the day
>> is anybody's guess.  Given that you're in Florida, it could be that
>> something in the CATV system nearby is heating up during the day and causes
>> the noise.  

CATV plant losses decrease with lower temperatures. As temps increase 
active devices that measure the ambient air temperature increase the 
amplifier gain and the slope (higher frequency pre-emphasis) to 
compensate. Hardline also changes dimension with heat and cold. This can 
get interesting when compensation circuits get out of whack, but I 
guarantee you'll have visual evidence of this on your television or 
evidence of dropouts big time on the reverse path if this is happening.

You have got a lot of good ideas from folks and hopefully some of these 
will lead you to the problem.

Rick Kunath

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