On Fri, 14 Feb 2014 11:16:43 -0800, (David
Platt) wrote:

In article ,
Jeff Liebermann wrote:

It's also a problem in the other direction, which is called ingress.
Nearby transmitters, such as mine, can easily get into the cable
system, and interfere with reception.

Heh. I remember a discussion hereabouts, some years ago... it might
even have been you posting, Jeff?... in which the poster reported
having detected cable-leakage problems in his town, reported them to
the cable company, and was being ignored. He found that even the
cables system's central facility (where the downlinks and primary
head-end amplifiers were) was leaking like a sieve.

Not me. I've reported specific leaks, with photos of my test
equipment and attached reports to Comcast. However, that's when I had
a friend employed by Comcast to expedite the process. Even so, it
would usually take a month for any kind of response. I no longer have
an inside track into Comcast, so I don't know what will happen if I
tried that today.

He visited the office, asked to speak to the lead engineer, and
explained what he'd found. The engineer poo-pooh'ed and said "No, our
system is tight, we're very careful about that." The OP took out his
2-meter radio, keyed down... and the signal monitors in the next room
went crazy, as the 2-meter signal leaked into the system and saturated
the first-stage amplifiers. Picture quality went to hell all over
town and the phones started ringing...

That definitely wasn't me. I would never do that. The most sensitive
point of the cable system is at the head end. That's where all the
gain is buried, where all the numerous RF sources come together, and
where there's a high probability of nearby RF sources creating
problems. I also doubt the story. I'm quite familiar with the
Comcast head-end in Scotts Valley. Nobody gets anywhere near the
equipment unless they have a very good reason for being there. A
leakage complaint doesn't qualify. Also, there's no "lead engineer".
All the Comcast engineers are either in Denver, or outside
contractors.

If you sit in the parking lot at the Scotts Valley office, you'll hear
some RFI coming from the building on VHF and UHF. I've walked around
2 sides of the building with a portable spectrum analyzer and didn't
see anything really disgusting. However, I will admit that everything
above 700 MHz was problematic due to the proximity of a nearby cell
site. That was also many years ago, so things may have changed (or
deteriorated).

So far, what I've seen of the local Comcast plant, it's quite well
done. There's some junk cable Comcast inherited years ago north of
Boulder Creek that should have been ripped out and replaced, but
otherwise, it's quite well done and shielded. Unfortunately, Comcast
is using more and more outside contractors, with minimal oversight,
resulting in some rather lousy installations.
http://802.11junk.com/jeffl/pics/Comcast/slides/Comcast-cabling.html
I have more photos that are worse, but I think you can see the
problem.

Also, there are problems with the roadside amplifier and pull boxes.
These are targets for vandals, copper thieves, and sloppy installers.
I've found them broken open and reported them to Comcast. I usually
get an instant reaction, not because of the potential loss of customer
service, but because there's high voltage on the big cable (to run the
amplifiers) which could easily kill some idiot trying to steal the
copper.

Incidentally, if you run into the test frequency that Comcast is using
for leakage testing, I'm interested. It changed a few months ago when
they went all digital and I haven't been able to pry it out of anyone.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558