Electrical Engineering - Video interference "60 cycle noise" or "hum bars" retail TV display

Author

Video interference "60 cycle noise" or "hum bars" retail TV display

Jim

2007-10-21, 1:25 pm

I've have a retail customer with a large TV display. The content they
display is generated from a multimedia server with both an NTSC and ATSC
outputs. The ATSC signal then feeds a high definition receiver which
feeds a daisy chain of HD distribution amps. The NTSC signal feeds
other TV's which are just used for general information.

The TV's are displaying a horizontal bar that moves from bottom to top
very slowly. On the HD sets the bar is colored and on the analog sets
it's just a dark bar.

Every componant has been changed out (server, video output card, amps,
modulator, combiner, HD receiver, distribution amps and cables), and the
bars are still there.

Even went as far as having a new server brought in and set it up right
on the sales floor. The bars are still there!!!!! When I was feeding
this from the back room the bars appeared in about 25% of the TV's, when
I moved the server out on the floor, they appear in all of the TV's.

When I disconnect the HD receiver and connect a DVD player to the same
HD distribution amps, the bars are gone!!!

At first I was trying to case this down as a ground loop problem, now
I'm not sure. I'm thinking more like RF or EMI.

Store does have a ton electronics and some RF equipment (hand held
scanners, two-way radios, etc). No transformers close by, electric room
is 100' away.

Anyone have any ideas????

Jim

charles

2007-10-21, 1:25 pm

In article <471b66ac$0$25670$4c368faf@roadrunner.com>,
Jim <jimsnot@home.com> wrote:

> I've have a retail customer with a large TV display. The content they
> display is generated from a multimedia server with both an NTSC and ATSC
> outputs. The ATSC signal then feeds a high definition receiver which
> feeds a daisy chain of HD distribution amps. The NTSC signal feeds
> other TV's which are just used for general information.

> The TV's are displaying a horizontal bar that moves from bottom to top
> very slowly. On the HD sets the bar is colored and on the analog sets
> it's just a dark bar.

Slow moving bars will just be 50Hz mains.

> Every componant has been changed out (server, video output card, amps,
> modulator, combiner, HD receiver, distribution amps and cables), and the
> bars are still there.

> Even went as far as having a new server brought in and set it up right
> on the sales floor. The bars are still there!!!!! When I was feeding
> this from the back room the bars appeared in about 25% of the TV's, when
> I moved the server out on the floor, they appear in all of the TV's.

> When I disconnect the HD receiver and connect a DVD player to the same
> HD distribution amps, the bars are gone!!!

> At first I was trying to case this down as a ground loop problem, now
> I'm not sure. I'm thinking more like RF or EMI.

> Store does have a ton electronics and some RF equipment (hand held
> scanners, two-way radios, etc). No transformers close by, electric room
> is 100' away.

> Anyone have any ideas????

Yes. try earthing the tv set - at the aerial socket.

Many years ago I have a similar problem - but only when it rained. I
eventually discovered that the aerial socket had a standing voltage of 120v.

During wet conditions this voltage flowed up the aerial lead into my aerial
combiner, up to the lower frequency aerial and onto the metalwork of the
support bracket and down the wall to ground. In the combiner, there was a
6v pd between the input and output on the 'earthy' side. This 6v became
superimposed on the rf signal leading to the effect you describe.

the current involved was only about 100uA, so there shouldn't be any danger
to life.

--
From KT24 - in "Leafy Surrey"

Using a RISC OS computer running v5.11

Jim

2007-10-21, 1:25 pm

charles wrote:

> In article <471b66ac$0$25670$4c368faf@roadrunner.com>,
> Jim <jimsnot@home.com> wrote:
>
>
>
>
>
>
> Slow moving bars will just be 50Hz mains.

In this case it's 60Hz, Guess I should have mentioned that I was in the US.
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
> Yes. try earthing the tv set - at the aerial socket.
>
> Many years ago I have a similar problem - but only when it rained. I
> eventually discovered that the aerial socket had a standing voltage of 120v.
>
> During wet conditions this voltage flowed up the aerial lead into my aerial
> combiner, up to the lower frequency aerial and onto the metalwork of the
> support bracket and down the wall to ground. In the combiner, there was a
> 6v pd between the input and output on the 'earthy' side. This 6v became
> superimposed on the rf signal leading to the effect you describe.
>
> the current involved was only about 100uA, so there shouldn't be any danger
> to life.
>
The TV's are double insulated, no ground conductor in the power cord.
The HD distribution amps and the power are on same source and bonded
together.

This one really has me stumped.

Thanks

Jim

Stuart

2007-10-21, 1:25 pm

In article <471b66ac$0$25670$4c368faf@roadrunner.com>,
Jim <jimsnot@home.com> wrote:

> The TV's are displaying a horizontal bar that moves from bottom to top
> very slowly. On the HD sets the bar is colored and on the analog sets
> it's just a dark bar.

Classic description of hum bars which can be a bugger to eliminate.

Hope you're using good quality double-screened co-ax designed for video -
if not get some.

Sometimes the only way is a "hum-bucking" coil at the end where it is
being introduced. This consists of many turns of video co-ax wrapped
around an iron coil. If you were in the UK I'd suggest looking at Canford
Audio - they are on line. They may supply the US anyway.

--
Stuart Winsor

From is valid but subject to change without notice if it gets spammed.

For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk

charles

2007-10-21, 1:25 pm

In article <471b74ce$0$32531$4c368faf@roadrunner.com>,
Jim <jimsnot@home.com> wrote:
>
> The TV's are double insulated, no ground conductor in the power cord.

Yes, mine had no earth either. The "earth" of the aerial (antenna)
connector will be at half mains potential - in your case 55v.

Try connecting an a multimeter set to ac amps reading and measure current
to a real "earth". If there is any - then my solution is likely to work

> The HD distribution amps and the power are on same source and bonded
> together.

irrelevant.

> This one really has me stumped.

Try my suggestion.

--
From KT24 - in "Leafy Surrey"

Using a RISC OS computer running v5.11

Stuart

2007-10-21, 5:25 pm

In article <4f35550260SW_NOSPAM@dsl.pipex.com>,
Stuart <SW_NOSPAM@dsl.pipex.com> wrote:
> This consists of many turns of video co-ax wrapped
> around an iron coil
^^^^

Core, sorry

--
Stuart Winsor

From is valid but subject to change without notice if it gets spammed.

For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk

Tomi Holger Engdahl

2007-10-22, 9:25 am

Jim <jimsnot@home.com> writes:

> I've have a retail customer with a large TV display. The content they
> display is generated from a multimedia server with both an NTSC and
> ATSC outputs. The ATSC signal then feeds a high definition receiver
> which feeds a daisy chain of HD distribution amps. The NTSC signal
> feeds other TV's which are just used for general information.
>
> The TV's are displaying a horizontal bar that moves from bottom to top
> very slowly. On the HD sets the bar is colored and on the analog sets
> it's just a dark bar.

Sounds like a "ground loop" problem situation.
The dark bar on analog sets are typical noise
I have seen. I don't remeber seeiing any colourful bars on
HD system but I would thing that kind of interference pattern
is possible with some HD interfaces in use.

What signal interface yype you use for HD signal ?
(analog component video, analog RGB, VGA, DVI, HDMI)

For more information on ground loops read my document here:

Ground loop problems and how to get rid of them
http://www.epanorama.net/documents/...loop/index.html

> Every componant has been changed out (server, video output card, amps,
> modulator, combiner, HD receiver, distribution amps and cables), and
> the bars are still there.

It is a typival for a ground loop problems that chaning
different components on the system does not solve it.
Ground loop problem is a system level problem.
The question lies how different components are interconnected
(what interfaces used, what kind of wiring, cabling routes) and
how the power distribution to equipment is done (power grounding
most important issue).

As the source and destination of a video signal can be at differing ac
or dc earth potentials, earth loop currents flow and cause
longitudinal hum to be introduced into the video signal. Video hum is
low frequency (50 or 60 Hz mains frequency or it's harmonics) noise
from the ground lines which has influenced the video signal, causing
degradation of the displayed signal. Video hum is usually observed as
bars rolling vertically through the video image, video hum may also
cause video distortion or even tearing of the picture in severe
cases. Video hum may be a problem in any system where video sources
and display devices are connected to different A/C power sources with
varying grounding potentials.

> Even went as far as having a new server brought in and set it up right
> on the sales floor. The bars are still there!!!!! When I was feeding
> this from the back room the bars appeared in about 25% of the TV's,
> when I moved the server out on the floor, they appear in all of the
> TV's.
>
> When I disconnect the HD receiver and connect a DVD player to the same
> HD distribution amps, the bars are gone!!!

Definately your servers are grounded.

It seems that your HD receiver gets the other ground connection
to your system, the one at different potential. That ground connection
on HD receiver can come through mains power (if grounded power plug)
and/or the TV antenna cable (through CATV or antenna network connection).

> At first I was trying to case this down as a ground loop problem, now
> I'm not sure. I'm thinking more like RF or EMI.

Sounds to me as ground loop.

But other interfence kis possible but less likely.

> Store does have a ton electronics and some RF equipment (hand held
> scanners, two-way radios, etc). No transformers close by, electric
> room is 100' away.

Hand held scanners and two-way radios do not typically cause
this kind of problems you described.

> Anyone have any ideas????

Analyze carefully the possibility of ground loop.

--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at
http://www.epanorama.net/

2007-10-22, 1:25 pm

Jim wrote in message <471b66ac$0$25670$4c368faf@roadrunner.com>...
>
>I've have a retail customer with a large TV display. The content they
>display is generated from a multimedia server with both an NTSC and ATSC
>outputs. The ATSC signal then feeds a high definition receiver which
>feeds a daisy chain of HD distribution amps. The NTSC signal feeds
>other TV's which are just used for general information.
>
>The TV's are displaying a horizontal bar that moves from bottom to top
>very slowly. On the HD sets the bar is colored and on the analog sets
>it's just a dark bar.
>
>Every componant has been changed out (server, video output card, amps,
>modulator, combiner, HD receiver, distribution amps and cables), and the
>bars are still there.
>
>Even went as far as having a new server brought in and set it up right
>on the sales floor. The bars are still there!!!!! When I was feeding
>this from the back room the bars appeared in about 25% of the TV's, when
>I moved the server out on the floor, they appear in all of the TV's.
>
>When I disconnect the HD receiver and connect a DVD player to the same
>HD distribution amps, the bars are gone!!!
>
>At first I was trying to case this down as a ground loop problem, now
>I'm not sure. I'm thinking more like RF or EMI.
>
>Store does have a ton electronics and some RF equipment (hand held
>scanners, two-way radios, etc). No transformers close by, electric room
>is 100' away.
>
>Anyone have any ideas????
>
>Jim

A slow moving horizontal bar is a definite ground loop. I don't know what
your expertise is with ground loops, but it usually involves more than using
a ground lifter at the power plug. When equipment is connected together
using single ended coax, the shield of the coax carries ground current from
one piece of equipment to the other if the chassis are not at the same exact
potential. That ground current gets added to the video.
There are balanced line drivers and receivers for video which means that you
don't need to connect a shield at both ends. If there is one piece of
equipment that seems to cause the hum bars, you might try balancing only
that connection to keep the costs down. I think there might also be
isolation transformers for video as well.
Google has a few hits for balanced video, here's one
http://www.knollsystems.com/prod-baluns.html

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