| Understanding
and eliminating ground loop problems.
What does it look like?
From light to dark horizontal lines or bars, slowly traveling through the picture,
from the bottom to the top of your TV screen or monitor. This is sometimes called
video hum. Severe cases can cause tearing of your picture.
Before
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After
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| This
shows a moderate
ground loop problem on an
oscilloscope. Video & 60
Hz. | | This
shows a restored video
signal after the Humbucker is
installed. |
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Ground
loop Interference looks like a wavy line superimposed
on the normal video signal.
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| This image illustrates a moderate video ground loop problem. Note the
three gray horizontal bands. The bands move up through the picture. |
This
shows a restored video
signal after the Humbucker is
installed.
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What does it sound like?
Audio hum, 60-cycle noise, 60 Hz hum, similar
to the lowest note on a bass guitar or bass drum.
What causes the
problem?
Relatively long audio or video transmission cables (usually from
room to room or longer), with 120 VAC powered equipment at both ends of the cable
that is connected to different circuit breakers. This is the typical scenario
but any length cable hooked to any equipment can have this problem. The name given
to this problem is a ground loop.
What is a ground loop?
A difference in the ground voltage between two electronic devices powered by 120
VAC and connected together with your audio or video transmission cable. This difference
(common mode) voltage will drive a current through your cable and superimpose
itself onto your audio or video signal showing up as hum or lines. This is an
addition of voltages. A similar effect can occur when an AC magnetic field comes
close to your cable. The AC field can induce a current directly into your cable
and superimpose itself onto your audio or video signal showing up as hum or lines.
This is related to the workings of a transformer.
How are ground loops
corrected?
Simply disconnect one side of the coax cable and insert a Hum
Bucker between the coax cable and the equipment. The Hum Bucker
will block the current flow and restore the audio and/or video
to normal.
The
fix for standard equipment is a video Isolation transformer.
The standard Hum Bucker blocks the current's path with a Video
Isolation transformer.
A standard quality Hum Bucker is used for removing hum from most
sources of video.
It passes about 20 Hz to 4 MHz, and its hum isolation is greater than a choke.
The transformer is primarily used in CCTV: security, avionics, display, etc.
Both transformer and choke can pass camera control signals.
The
fix for premium equipment is a video isolation choke.
The Premium Hum Bucker blocks the current's path with a video
isolation choke.
A broadcast-quality Premium Hum Bucker is used for removing hum
from the finest sources of video, it passes DC to hundreds of
MHz. The choke provides protection against moderate levels of
60 Hz hum, and protection against very high levels of RF noise.
The choke is primarily used in high quality applications like
Broadcast TV & ENG (electronic news gathering) etc. because
it passes the DC component of the signal.
Do not remove the middle prong of your 3-prong electrical plugs!
This is very hazardous and a violation of UL code.
This increases the shock
hazard and will not fix the problem. It is difficult to predict where a ground
loop will occur.
Ground loops are usually an "after the fact" problem where the
end user blames the installer, the installer blames the manufacturer,
and the manufacture blames the electricians. Often nobody knows
how to fix it. A ground loop problem may occur at multiple points
in your system. Each problem must be corrected individually. This
type of Interference can affect cameras, video recorders, monitors;
video effects generators, switchers, computers and much more.
Details about the conditions causing ground loops.
The 120 VAC power
wiring in all buildings has losses in the copper wiring. You can see this in the
fast speed of an electrical saw at the outlet and a slower speed at the end of
a long extension cord. The losses show up as voltage differences. You can actually
take a voltmeter with 30-foot leads and measure a voltage between the ground on
the 120 VAC outlet in one room and the ground on the 120 VAC outlet in another
room. The ground (of course) is the screw that you can see and touch, that holds
the plastic plate onto the electrical conduit box. The voltage gets higher with
multiple power panels and high amperage loads. The equipment that is plugged into
these outlets carries the same ground voltage on their own chassis. When a video
transmission cable is connected between the two chassis it now becomes a current
carrying conductor. The voltage difference causes current to flow through the
coax inducing a (common mode) voltage into the video. This is seen and heard as
Hum and usually displays itself in the form of horizontal lines or a disturbance
floating through the picture.
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Note you can get hurt or killed in rare cases of extreme ground loop problems.
Proceed with caution. Holding one hand on the equipment while using the other hand to disconnect a cable puts you (and your heart & lungs) in the circuit. With enough voltage/current you can stop breathing or worse.
Read about it at http://en.wikipedia.org/wiki/Electrocution and Google the word "Electrocution".
You may have a defective component (TV VCR audio system etc.) causing this problem. Testing each component separately (disconnected from the other parts of your system) is the best way to tell.Do I have a ground loop problem?
You may have a ground loop problem even with all good components, all correct wiring and a building that is up to code in every way.
If you do have a ground loop problem the only cure is to break the loop.
What is a ground loop?
A ground loop is usually found between equipment using standard 120 VAC power that is connected to more equipment using standard 120 VAC power.
This connection is usually audio cables and/or video cables and/or CATV cables.
Your hum problem may be originating amongst your Camera, Receiver, TV, DVR, VCR, CATV, Audio System etc.
Ground loops are often found on longer runs of cable or between equipment using separated 120 VAC outlets.
Please note a hum may originate up stream and will be propagated down stream to the last component.
Your typical stream might be a camera connected to a wireless device connected to a video amplifier connected to a switcher connected to a recorder connected to a monitor or TV.
A filter will not remove the hum once it is in the system. The filter will only work at the source of the problem.
If the ground loop problem starts between the camera and the wireless device then no filter connected to the TV will help.
In this example disconnecting the cable from the recorder to the TV will make the hum disappear but it is not the source of the problem. In this example you can test the connection between the camera and the wireless device by using an alternate source of video or moving the camera and the wireless device to a new location or connecting the power to other locations etc.
One simple test is;
Step one, disconnect an audio, video or CATV cable and see the problem clear up.
Step two, momentarily touch the outer body of one connector to the outer body of the other connector.
Do not let the center conductors touch each other or the outer body of either connector.
If the problem returns it is a good indication that this is at least one point in the loop.
Installing a filter at this point may help solve your hum problem.
Can you do some tests to your system?
The approach is to monitor the hum as you make changes to your system.
You should do the simple test above on all the wires necessary.
You will try to disconnect wires one at a time (and in most cases reconnect them) until you find a change in the hum level.
Ground loop problems are one of the toughest problems to correct.
Good luck and work safely.
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Another
way to look at Ground Loop problems.
How to prevent audio hum and video bars.
What is a Ground Loop?
When you
hear hum in audio or see bars in video, it's often caused by a
ground loop effect between two or more pieces of gear, plugged
into two or more 120 VAC outlets.
One half of a ground loop is formed by connecting two electronic
components together with a cable, such as, a DVD player is connected
to a TV, with audio and video cables.
The second half of this ground loop is formed by the 120 VAC plugs
of the same electronic equipment and the unseen ground wiring
in the house, such as, the TV's power cord is plugged into one
outlet and the DVD player is plugged into a second outlet and
the wires in the wall connect the two together.
This loop is from a DVD player, through audio and video cables,
to the TV, through the TV's power cord to the house wiring, through
the house wiring to the outlet that the DVD is plugged into, through
the DVD players power cord and back to the DVD player where we
started.
How to Fix Ground Loops
For complex systems you may need to repeat these steps starting
with a different piece of equipment in various combinations to
locate the problem:
1. Strip the system down to one central piece, such as a mixer,
by disconnecting all interconnects and AC cords except for the
mixer.
2. Add one piece of equipment at a time; hook up AC and interconnections
(making sure all grounds are connected and in good condition)
then listen for hum or look for noise.
3. You can turn on and off the power each time you switch equipment
to avoid pops and shorted outputs.
4. Proceed until you find the offending piece(s) causing the problem.
5. Plug the The Ground Loop Isolation Transformer in all lines
between the offending equipment and the rest of the system.
6. Another common path for ground loops is through a chassis into
the rack and then into another chassis. Test this by removing
the chassis from the rack.
7. What about cutting or lifting the AC ground?
Caution: Removing or disabling the AC ground can cause electrocution
and:
· Can cause distortion due to floating signal references
· Can cause noise to be dumped into another piece of equipment.
8. What about cutting or lifting the signal ground?
Cutting the shield at one end of the interconnecting cable:
· Can distort the signal return.
· Can alter the cable's frequency response.
· Can defeat the shielding effect.
Ground Loop Isolation Transformers are the safer and better solution,
they break the ground loop and keep all AC power grounds intact.
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More filter information.
Hum Eliminators can do more than just prevent AC hum
from being picked up by your sound systems. Special units can
be used as one of the most cost-effective ways to convert unbalanced
signals to true balanced signals.
Ground Loops Hum Eliminators prevent ground loop antennae
from forming and picking up AC hum. AC hum & noise can be
caused by a loop antenna effect across signal lines between two
or more pieces of gear, or by long unbalanced cables picking up
noise. Loop antenna(e) are basically a type of radio antenna,
which tend to pick up the 60 Hz (and harmonics) AC signal being
broadcast by a building's electrical current. These loop antennae
are closed circuits usually along the ground wires and hence are
commonly called ground loops.
Balanced and Unbalanced Unbalanced signals are more susceptible
to picking up electrical noise & RF Interference than balanced
signals are. The longer the unbalanced cable, the greater the
chance of a problem. Hum Eliminators are one of the most cost-effective
ways to convert between unbalanced signals and true balanced signals.
MIXERS and RECORDING
Many mixers have unbalanced or "impedance balanced" monitor outputs.
Use Hum Eliminators to get true balanced outputs for noise-free
performance. When using more than one mixer (or submixing), there's
a good chance of getting ground loops (and buzz). Use Hum Eliminators
to prevent these ground loops. Eliminating hum is one of the biggest
challenges facing studios. Many studios will run all of their
line level gear through Hum Eliminators to help ensure noise-free
recording.
KEYBOARDS, SAMPLERS and SYNTHS
Most keyboards, samplers, and synthesizers have unbalanced -10
dBV outputs. Use a Hum Eliminator to balance the signals if used
with other -10 dBV equipment. To use a -10 dBV unbalanced device
with a +4 dBu balanced device, use a Line Level Shifter.
LIVE SOUND and DJ MIXERS
A common problem with live performance is long unbalanced cables
picking up hum along the way. Balanced signals are more immune
to picking up noise. Use the Hum Eliminator to balance unbalanced
signals. Mobile DJs or bands never know when setting up their
system if they're going to get a hum or not. The electrical wiring
changes from one building to another. What works fine at one gig
might not work at the next. Its worth the low cost to keep a Hum
Eliminator available as nobody can afford to lose a gig because
of hum problems. Some DJs run everything in their rig through
Hum Eliminators to play it safe. The main outs on most DJ mixers
are -10 dBV unbalanced RCA. When using with a +4 dBu power amp,
over 2/3 thirds of the amp's volume can be lost. Use a Line Level
Shifter to fix it.
POWER AMPLIFIERS
Running multiple amplifiers can create ground loops between the
amplifiers, which will pick up the dreaded hum and buzz. Run each
amplifier's input line through a Hum Eliminator to prevent this
problem.
GUITARS
Many guitar players use rack and pedal effects. They can get buzz
from ground loops that these devices create. Use a Hum Eliminator
to stop that buzz. When using two or more guitar amplifiers, ground
loops can form between the amplifiers. Because of the tremendous
amounts of gain in guitar amps, the buzz can get so bad that the
amplifiers will actually squeal (especially with tube amplifiers).
Simply run the feed to each amp through one channel of a Hum Eliminator.
COMPUTER SOUND CARDS
Most sound cards have unbalanced inputs and outputs. Balance these
with the Hum Eliminator. Even balanced sound cards have tremendous
susceptibility to ground loops because of the computer's power
supply. If you hear a buzz, try a Hum Eliminator.
BROADCASTING VIDEO
Press Boxes have many feeds all going to different gear, which
can create lots of ground loops and hum. Use a Hum Eliminator
on these feeds to eliminate the problem. Using a Hum Eliminator
on the audio feed of a video camera can help prevent video hum
(rolling bars).
Note:
Never use Hum Eliminators between amplifiers and speakers, equipment
may be damaged. Only use the Hum Eliminator on line level signals.
Another common path for ground loops is through a chassis into
the rack and then into another chassis. Test this by removing
the chassis from the rack. The Hum Eliminator will help but you
should also try isolating the chassis from the rack with electrical
tape and insulating the rack screws with nylon washers.
Most ground loop problems can be solved using the Hum Eliminator.
Patience is a necessity when attempting to solve ground loops!
Many popular mixers with TRS balanced line inputs do not have
common mode rejection. They drop the inverted signal, creating
unbalanced inputs! To get the benefits of balanced inputs, run
your balanced line into the Hum Eliminator and run a mono 1/4"
line from the Hum Eliminator outputs to the mixer's TRS inputs.
This way you are running a balanced line all the way up to the
Hum Eliminator and you will not lose common mode rejection.
Note
Some audio power amplifiers have a small amount of hum inherent
to the internal electronics. You can sometimes test for this by
turning off the receiver and leaving on the power amplifier (be
sure not to do damage to your system).
A question to ask yourself, have there been any changes to your
video system or your power system just before your picture or
sound got worse?
A ground loop filter will not work if you have any other wires
that start from the same source and connect to the same monitor,
this would 'short out' the filter. Such as audio cables, computer
cables, steel 2 x 4s, power extension cables etc.
A good test is to disconnect all cables from the TV except the
power cord and the cable from your ground loop corrector. The
picture may change confirming your system may have a second ground
loop, or a broken filter.
Sometimes a Cable box to your plasma TV looks like a ground loop
problem but it is not.
The problem can be the refresh rate of the plasma TV that is set
wrong.
How to test your CATV system to
determine if a CATV filter will help you.
Some times a bad CATV ground loop problem will insert hum into
your system even when your TV is turned off, and you are only
listening to a CD etc.
A good test is to disconnect your CATV cable from your TV system.
If this stops your hum it is a good bet the CATV filter will work
for you.
If the hum persists then you could disconnect more cables one
at a time, starting with the longest cable runs. The cables that
are most likely to cause your problem are relatively long audio
or video transmission cables (usually from room to room), with
120 VAC powered equipment at both ends of the cable.
If your problem clears up after unplug the powered equipment at
the far end of the cable then the HUMplug-e may be a good choice.
If you find a problem with one or more cables you will find many
choices of filters to use on this page.
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