[Bob Simon]

I got a new plasma TV and just upgraded cable service to digital with
HDTV. After hooking everything up, I hear a low-level hum from the
stereo. I presume this is a ground loop. Here are what I believe to
be the relevant connections:

Set Top Box
audio to receiver (2 channel only) via RCA cables
video to TV via component video cables
DVD Player
audio to receiver via RCA cables
video to TV via HDMI cable

I can eliminate the hum by doing any one of the following actions:
unplug the audio cables from the DVD player
unplug the three component video cables from the DVD player
unplug the audio cables from the STB
disconnect the coax cable feed from the STB.

When I unplug the HDMI cable from the STB, the hum gets much louder.
Why?

Based on my experiments, I believe I can break the ground loop by
replacing my DVD player and receiver with models that can be connected
by fiber. (I want a Dolby decoder anyway.)
WIll this work?

I also scheduled a service call with Cox and requested a ground
isolator. I'll see what actually happens on Sat.
Bob

[Richard Crowley]

"Bob Simon" wrote ...
I got a new plasma TV and just upgraded cable service to digital with
HDTV. After hooking everything up, I hear a low-level hum from the
stereo. I presume this is a ground loop. Here are what I believe to
be the relevant connections:

Missing: cable system to Set Top Box

Set Top Box
audio to receiver (2 channel only) via RCA cables
video to TV via component video cables
DVD Player
audio to receiver via RCA cables
video to TV via HDMI cable

I can eliminate the hum by doing any one of the following actions:
unplug the audio cables from the DVD player
unplug the three component video cables from the DVD player
unplug the audio cables from the STB
disconnect the coax cable feed from the STB.

BINGO! Get a ground isolation transformer for your
cable feed and get on with your life.

When I unplug the HDMI cable from the STB, the hum gets much louder.
Why?

Eliminate the problem at the source (the cable system)
and don't worry about it.

Based on my experiments, I believe I can break the ground loop by
replacing my DVD player and receiver with models that can be connected
by fiber. (I want a Dolby decoder anyway.)
WIll this work?

Eliminate the problem at the source (the cable system)
and don't worry about it.

I also scheduled a service call with Cox and requested a ground
isolator. I'll see what actually happens on Sat.

This is far and away the primary cause of the kinds of
symptoms you are describing. And you debugging steps
(congratulations :-) confirm it.

[Scott Dorsey]

Bob Simon wrote:
I got a new plasma TV and just upgraded cable service to digital with
HDTV. After hooking everything up, I hear a low-level hum from the
stereo. I presume this is a ground loop. Here are what I believe to
be the relevant connections:

This is specifically mentioned in the FAQ. Make an isolation device with
two back-to-back 300-75 ohm baluns that don't connect the ground of the
75 ohm side to the 300 ohm side, and put it in the line.

I also scheduled a service call with Cox and requested a ground
isolator. I'll see what actually happens on Sat.

I seriously doubt anyone at Cox has a clue, but you can try them. Welcome
to the wonderful world of cheesy consumer electronics with unbalanced
unisolated connections everywhere.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Richard Crowley]

"Scott Dorsey" wrote ...
I seriously doubt anyone at Cox has a clue, but you can try them. Welcome
to the wonderful world of cheesy consumer electronics with unbalanced
unisolated connections everywhere.

They just might have gone through this enough times (thousands?)
with home theater customers so that they now have a clue about
isolating their screwy grounded system from the rest of the world.
Or not. I'm not betting anything on them, but he might get lucky.

[Chris Hornbeck]

On Thu, 28 Dec 2006 14:03:24 -0800, "Richard Crowley"
wrote:

BINGO! Get a ground isolation transformer for your
cable feed and get on with your life.

This works, but sometimes there's an issue with
bandwidth limitations for the cheapest isolators
causing problems in "polling" the set-top boxes,
to update services available and whatnot. A good
number to look for is "5MHz-xxxMHz".

Lately I've been recommending folks just get a
cheap power strip "lightning protector" with
F-connectors on it. Also works. Plug everything
into the power strip; everything! no other
copper connections to the outside world.

Much thanks, as always,

Chris Hornbeck
"This life has been a test. If this had been an actual life,
you would have received instructions on where to go and what to do."
- Angela Chase (Claire Danes)

[Bob Simon]

On Fri, 29 Dec 2006 03:52:12 GMT, Chris Hornbeck
wrote:

On Thu, 28 Dec 2006 14:03:24 -0800, "Richard Crowley"
wrote:

BINGO! Get a ground isolation transformer for your
cable feed and get on with your life.

This works, but sometimes there's an issue with
bandwidth limitations for the cheapest isolators
causing problems in "polling" the set-top boxes,
to update services available and whatnot. A good
number to look for is "5MHz-xxxMHz".

If the isolation transformer does not have adequate bandwidth, will I
be able to immediately identify this by impaired video on one or more
cable channels? If so, which channels will be most succeptible? Will
it be most obvious on the HDTV channels?

[Bob Simon]

On Thu, 28 Dec 2006 15:37:59 -0800, "Richard Crowley"
wrote:

"Scott Dorsey" wrote ...
I seriously doubt anyone at Cox has a clue, but you can try them. Welcome
to the wonderful world of cheesy consumer electronics with unbalanced
unisolated connections everywhere.

They just might have gone through this enough times (thousands?)
with home theater customers so that they now have a clue about
isolating their screwy grounded system from the rest of the world.
Or not. I'm not betting anything on them, but he might get lucky.

I asked the L1 guy who took my call to have the tech bring an
isolation transformer. I found a recommendation on another thread
here for the Jenzen VRD-1FF so I asked the L1 guy to include that in
the notes. So they already have enough information.

The tech should be here by 3:00 Saturday. I'll let you know what
happens.

[Bob Simon]

On Thu, 28 Dec 2006 14:03:24 -0800, "Richard Crowley"
wrote:

"Bob Simon" wrote ...
Based on my experiments, I believe I can break the ground loop by
replacing my DVD player and receiver with models that can be connected
by fiber. (I want a Dolby decoder anyway.)
WIll this work?

Eliminate the problem at the source (the cable system)
and don't worry about it.

I also scheduled a service call with Cox and requested a ground
isolator. I'll see what actually happens on Sat.

This is far and away the primary cause of the kinds of
symptoms you are describing. And you debugging steps
(congratulations :-) confirm it.

Thanks. Too bad I can't use this as justification to my wife that we
need to upgrade our receiver to one that has a 6.1 decoder and toslink
inputs.

[Scott Dorsey]

Bob Simon wrote:
If the isolation transformer does not have adequate bandwidth, will I
be able to immediately identify this by impaired video on one or more
cable channels? If so, which channels will be most succeptible? Will
it be most obvious on the HDTV channels?

You will notice snow on higher numbered channels. HDTV will probably be
less affected than standard NTSC, until the noise floor is so bad that
it degrades very severely and pretty abruptly.

Note that if you have a cable ground problem (and most folks with cable do),
you may see hum bars, which will go away when the isolation transformer is
in place.

I have very little faith in the people that run cable systems.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Scott Dorsey]

Bob Simon wrote:
I asked the L1 guy who took my call to have the tech bring an
isolation transformer. I found a recommendation on another thread
here for the Jenzen VRD-1FF so I asked the L1 guy to include that in
the notes. So they already have enough information.

The Jensen isolation transformer, or the Allen Avionics model that is
similar, is probably about two orders of magnitude higher cost than the
ones the cable company have to drop in your line.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[w_tom]

Bob Simon wrote:
If the isolation transformer does not have adequate bandwidth, will I
be able to immediately identify this by impaired video on one or more
cable channels? If so, which channels will be most succeptible? Will
it be most obvious on the HDTV channels?

That isolation is already supposed to be part of the TV. Fixing it
may simply be masking another problem.

Also verify the cable is earthed to same electrode as AC electric
where utility wires enter the building. This is required by code and
could also contribute to hum. Again, the hum would be a symptom of
some other failure.

[Bob Simon]

On 29 Dec 2006 11:18:53 -0500, (Scott Dorsey) wrote:

Bob Simon wrote:
I asked the L1 guy who took my call to have the tech bring an
isolation transformer. I found a recommendation on another thread
here for the Jenzen VRD-1FF so I asked the L1 guy to include that in
the notes. So they already have enough information.

The Jensen isolation transformer, or the Allen Avionics model that is
similar, is probably about two orders of magnitude higher cost than the
ones the cable company have to drop in your line.
--scott

Scott,
Allen Avionics has two models: the DTI-1S is an inductive device and
their CATV 1300 is capacitive. Do you recommend one over the other?

Also, what bandwidth is required for this application? The Jensen
VRD-1FF bandwidth is 2 MHz to 1300 MHz while the MCM 33-8700 is 5 MHz
to 2.4 GHz.

Thanks.
Bob

[Scott Dorsey]

Bob Simon wrote:
On 29 Dec 2006 11:18:53 -0500, (Scott Dorsey) wrote:
Bob Simon wrote:
I asked the L1 guy who took my call to have the tech bring an
isolation transformer. I found a recommendation on another thread
here for the Jenzen VRD-1FF so I asked the L1 guy to include that in
the notes. So they already have enough information.

The Jensen isolation transformer, or the Allen Avionics model that is
similar, is probably about two orders of magnitude higher cost than the
ones the cable company have to drop in your line.

Scott,
Allen Avionics has two models: the DTI-1S is an inductive device and
their CATV 1300 is capacitive. Do you recommend one over the other?

I have only used the DTI-1S, but I would assume the CATV 1300 is cheaper
if it's capacitive, and it might be just as effective.

Also, what bandwidth is required for this application? The Jensen
VRD-1FF bandwidth is 2 MHz to 1300 MHz while the MCM 33-8700 is 5 MHz
to 2.4 GHz.

What's the highest channel on your cable system? Find out what the
frequency of that is, and you'll be in the ballpark.

Also, of course, these aren't useful frequency responses without tolerances.
If it's 3 dB down at 1300 MHz, I would have no qualms about using it on a
1300 MHz signal. If it's 30 dB down at 1300 MHz, I would have serious ones.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Chris Hornbeck]

On Fri, 29 Dec 2006 08:51:15 -0600, Bob Simon
wrote:

sometimes there's an issue with
bandwidth limitations for the cheapest isolators
causing problems in "polling" the set-top boxes,
to update services available and whatnot. A good
number to look for is "5MHz-xxxMHz".

If the isolation transformer does not have adequate bandwidth, will I
be able to immediately identify this by impaired video on one or more
cable channels? If so, which channels will be most succeptible? Will
it be most obvious on the HDTV channels?

Polling the boxes is a low frequency thing (as best as I can
tell) but everything you do effects high frequencies. Cable
systems run on the raggety edge of acceptable-to-anybody-
who-doesn't-yell-loud-enough-to-matter, and for good economic
reasons probably. There's *nothing* to spare. 20dB headroom?
Earth calling...

Ground loops are difficult to explain but easy to solve
once the concept is mastered. I've only raised this as a
possible issue because I've lately been converted to the
short-loop-but-still-a-ground-loop method. Seems to work just
fine at my day job, where I deal with this crap daily.

And sometimes I'll see the loss through a coupla F-connectors
and half an inch of wire (inside a "lightning protection"
strip) cause the HD channels to be intermittent. Cable
systems can be very robust, but only with planning, and
that seldom happens. Signal levels really, really need to
be within a very narrow dynamic range, a range so small we
audio folks can hardly believe it. Less than 20dB, on all
channels, between noise and overload for absolute levels
in a well balanced system. Yikes!, but do-able.

Sorry to get long-winded, but you seemed interested, and
I just happen to have my piano with me...

All good fortune,

Chris Hornbeck
"This life has been a test. If this had been an actual life,
you would have received instructions on where to go and what to do."
- Angela Chase (Claire Danes)