Re: [IRCA] Noisy cable TV

[Rick Kunath <k9ao@xxxxxxxxxxx>]

Comments in-line...

W. Curt Deegan Wrote:

<Snipped here and there>

> There is no ground on the new utility pole that I noticed.  I'll look again 
> tomorrow.  The noise can be nulled and points to the pole -- and to the 
> cable coax from the pole -- but it's not possible to be more specific.  The 
> noise does not appear on power lines or phone lines in the house, just the 
> cable coax.

That's good info. There usually isn't a ground on every pole.

> It's tough to compare the pole installation to others because this pole has 
> a junction of a major -- 2+ inch thick -- TV cable, splitting and going off 
> in two different directions.  There is a large black block hanging on the 
> cable with two less thick cables and smaller leads coming out, I assume 
> this is a junction for the local drops and the split.

There are two distinctly different line levels associated with a CATV 
system. One is the longer distance trunk level, and this (after coming 
off of Fiber to RF) is not used to feed drops to users, but rather to 
feed trunk amps, which are daisy-chained together for a reasonable 
cascaded number. At any trunk amp signals can be split off and sent out 
at the higher level used for CATV distribution, and then on through 
directional couplers (taps and splitting devices) at poles, through line 
extenders, etc. fanning out and off to users.

> The only pole 
> mounted box is telephone.  There is no power transformer, the local drops 
> tap directly into the three power wires on the pole.  All of the power line 
> fixtures are new along with the pole.  The phone drop was replaced and 
> buried several months ago.

Also good info.

> Now that you mention it, there are occasions especially during the day, 
> when digital TV signals lock-up or pixelate for several seconds, I've not 
> thought to relate that bad behavior to the other problems.  I assumed this 
> to be satellite signal reception problems at the cable company.  

It might be. Artifacts (that's what the CATV tech will call the blocky 
pixelation) is usually caused by an insufficient amount of data to 
reproduce the on screen picture. This can be caused by noisy data on the 
cable, interference on the satellite, or by low line levels feeding the 
digital CATV converter box in your home. Have your cable tech tell you 
the levels at several places in the television spectrum at your home. If 
the levels decrease to a point unable to be demodulated with enough data 
to display a full picture, you'll get artifacting. And, the levels might 
be changing, causing this. All levels on a cable system change a small 
amount with changing outdoor temperature, but not by great amounts. 
However, if your levels are just at the lower edge of usability, normal 
level changes during the day could cause intermittent and unreliable 
operation. A digital CATV signal pretty much either works or it doesn't. 
At the hairy edge, you'll see the artifacting before it locks up 
completely and stops decoding.

> No so long ago, tuning to unused channel 117 brought up a camera focused on 
> an oscilloscope face.  No doubt related to system performance but I have no 
> idea what the trace represented.  Unfortunately, when I check now my TV 
> screen goes black, indicating something is there not just noise, but not 
> the nifty scope display.

That's fairly common. They put up a display that can be looked at using 
a signal level meter with a little LCD TV screen on it, or a small TV 
receiver. That way you can see what you are doing on the reverse path 
from the field.

> Grounding by the various services at the back of my house is best described 
> as inspired.  The house is nearly fifty years old and some of this stuff 
> has been there since day one, some of it is just a few months old, and the 
> rest lies somewhere in between.  There are the three utilities as well as 
> junction boxes for the pool pump fuses and the timer.  The cable TV 
> grounding block is actually connected to a conduit descending down from the 
> pool pump fuse box and seems to contain a ground wire, but is not itself 
> part of the ground.  This immediately caught my eye, but will need closer 
> examination to decide just what is grounded to what.

The key to proper grounding and noise reduction is making sure that you 
have a single-point ground. This is almost universally at the power 
utility ground point. You'll have a heavy ground connected to your power 
panel and that should be the ground for the telco and the CATV also. The 
actual demarcs for the telco and the CATV should be near enough to the 
power ground to bond to that as their ground. What you don't want is a 
separate ground rod for any of the other utilities like the telco or the 
cable, or a connection to plumbing pipes. This is sure to cause ground 
loops and is really shoddy installation. Telcos are usually pretty good 
at doing grounding right and marking their tap to the power ground 
conductor with a nice warning tag to call them if it is loose or 
damaged, but some CATV installers are not so conscientious. I suspect 
that your local electrical code also requires this single-point ground, 
so you likely have plenty of leverage to make the installer fix it if it 
does not comply.

Noise is still likely another issue, but the grounding can make a 
difference in radio noise. And, poor grounds can generate noise due to 
poor connections, etc. Disconnecting your cable drop at your home's 
ground block can tell you whether this is the case in short order.

Generally you'll have what is called a ground block at the CATV demarc 
point. This is an F-81 double male connector with a ground screw and a 
mounting bracket. This will be tied into the power ground, then off from 
there to other splitters as appropriate for your devices. Sometimes 
they'll use the first splitter as the ground block. This is acceptable, 
but I prefer a separate ground block with a short coax jumper coiled 
several times in about a 3-inch diameter coil of 2 or 3 turns, and then 
on to the next splitter (nylon cable-tie it to hold the shape.) This 
acts as an RF choke to lightning surges and is a bit of added 
protection. A better and safer way to do the ground block is to use one 
of the Regal gas-discharge ground blocks. These look like a 2-way 
splitter with just the 2 output ports and no input. The case does the 
same job as a traditional ground block, but the device also has a gas 
discharge tube to shunt any surges on the center conductor of the 
coaxial cable to ground too. These are cheap, and really offer some 
added protection to your connected appliances. They work similar to a 
PolyPhaser on a radio antenna feed line. I've seen one of these Regal 
devices commit suicide and save the home electronics with no damage 
after the aerial CATV drop was vaporized due to a strike to the power 
pole at the rear of a home.

Here's what they look like:

http://tinyurl.com/mbdmy

They also come in models with a built-in 2-way splitter, but as I 
mentioned, I prefer the coax coil as additional isolation on the shield, 
so don't recommend these.

Here's the parent page of the above:

http://tinyurl.com/zzb5r

They're cheap protection compared to the price of a new television set 
these days.

> The most telling clue I think, is the strong noise that can be heard 
> placing an AM radio (I don't have a VHF radio to try, plus I have digital 
> service which I assume would act differently than analog) next to the Cable 
> TV coax at the front of the house, completely away from all the other 
> services at the back.  The noise is raucous there and can be tracked back 
> through the house right under where the cable coax crosses from front to 
> back along the roof.

Again, disconnecting it at the house and taking a listen would be 
interesting. If the noise is still there, it isn't coming from the house.

Good luck on getting this fixed.

Rick Kunath
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