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Diagnosing Screen Interference

When mysterious image jitter or color distortion appear on CRTs, the problem frequently falls into the hands of IT and system support technicians to solve. Monitors are swapped out, maybe even returned to the manufacturer, but the problem is not the monitor or even the video card. The repair technicians next try surge protectors and UPS units, but it's still no good. Next, the electrical contractor checks the circuits and finds nothing wrong. The local power company is called in and does a power quality analysis. Everything is within specification and they conclude the problem is not the power.

Somewhere along this process, someone voices the theory that the problem is not inside the monitor, but outside. You can’t see it, but the problem may be magnetic fields, either AC Fields from a power source or DC Fields from a magnetic source. It can be as simple as having speakers too close to the monitor, or having an electrical distribution panel on the other side of the wall.

These circumstances, though relatively rare, are rapidly becoming more frequent, especially during building remodels, upgrades or expansion. If you haven’t encountered it before, the process of discovery can take weeks – we have seen instances where it took years. And it can be very expensive, to say nothing about the disruption it can cause.

The Physics of Monitor Interference

Most computer monitors are based on Cathode Ray Tube (CRT) technology, which is to say that an image is created on the face of a monitor by a stream of electrons emitted from an electron gun located at the back end of a glass vacuum tube. The location, direction and focus of this beam is controlled by internal magnetic fields which are generated by a set of horizontal and vertical deflection coils.

The rate at which the beam is directed back and forth is its horizontal rate and the up and down control fields operate at the monitor’s vertical rate. All of this assumes that the environment in which the computer operates is free from any environmental, or ambient background fields which would interfere with this process.

Back when computer monitors had their horizontal and, most importantly, vertical scan rates were derived directly from and were equal to the power line frequency (60 Hz), any background field caused by power systems in the area would be equal to the vertical scan rate. Although this interaction between fields would distort the image slightly, the distortion would be constant and relatively small, for all but the most energetic background fields.

As monitors became more sophisticated display devices, they were increasingly color, larger in size and higher resolution devices. Each of these desirable characteristics increases the sensitivity of a monitor to an external background field. Both color and higher resolution requires a more precise alignment and focus of the beam, and manufacturers began to increase the vertical scan rate to allow for these higher resolutions.

Larger monitors are more sensitive since the distance the electron beam must travel is longer and more susceptible to being disturbed. Energy efficient monitors are configured to operate at lower anode voltages which causes the electron beam to travel more slowly, thus making it more sensitive to the external fields. Ironically, "low-radiation" monitors reduce the CRT's internal control field strength and make them less able to resist an external magnetic force.

In other words, all of the trends in CRT monitor technology tend to increase the frequency and severity of this problem. Note that this particular aspect of the large area called electromagnetic interference (EMI) is limited to CRTs. Since an LCD display has no control fields, it is not subject to the problem.

Building Sources of Magnetic Fields

Electric and Magnetic Fields (popularly referred to as EMF) are a natural consequence of the use and distribution of electricity. At ELF (Extremely Low Frequency, commercial power) frequencies, the electric and magnetic fields operate independently of each other and it is possible to shield or eliminate one but not materially affect the other. (This is not the case at Radio Frequency (RF) and higher levels where a fixed relationship exists between the electric and magnetic fields. To affect one is to affect the other.)

Electric fields are caused by voltage and are measured in volts/meter. Magnetic fields are caused by current and are measured in milliGauss (mG). At ELF frequencies, the electric field is relatively easy to shield – most common building materials will substantially reduce the strength of an electric field. However, exactly the opposite is true of an ELF magnetic field. At these frequencies, magnetic fields will pass undiminished through virtually all common building materials, materials even as dense as lead have essentially no effect on them.

With this in mind, magnetic fields will normally be found in areas adjacent to high current-carrying conductors. Examples are electrical closets and switchgear, building feeders, conduit and bus bars, transformer vaults and even the power distribution and transmission lines outside the building. Other common sources are fluorescent lights and adjacent monitors. Two monitors side-by-side can create distortion on one or both of the monitors, as they each have an internal magnetic field that can interfere with the control fields in the adjacent monitor.

Sometimes the source may not be obvious. It is quite possible that, if the building has wiring errors in it, even relatively low current distribution circuits will cause substantial fields to exist over large areas of a building. Wiring errors in a building, or even in wired partitions, can create "net-current" conditions, in which all of the current from a circuit is not returning on the same path.

Since the strength of a magnetic field is directly proportional to the amount of current flowing in the circuit, fluctuations in the use of power throughout the year or even over the course of a day can cause a sporadic interference problem. For this reason, it is common to get complaints from a computer user who only experiences jitter in the summer when the air conditioning is being used.

AC or DC?

Screen interference caused by magnetic fields generally falls into two categories. AC (alternating current) fields, typically cause the image to "jitter", while DC (direct current) fields usually cause a steady distortion or loss of color integrity. Sometimes however, you may get other signs of instability or a combination of symptoms.

DC fields are typically generated by magnets, including subway or train rails, elevators, building steel, batteries and speakers. The earth produces its own magnetic field, but most monitors already compensate for this field.

Common sources of DC fields are powerful magnets associated with medical diagnostic and research instruments, like MRI and NMR (nuclear magnetic resonance) devices. Less obvious sources are fields that are the result of building structural and reinforcement steel that has become magnetized. This can occur as a result of the existence of a prior tenant with an MRI or, one theory has it that it is possible for building steel to become magnetized as part of the welding process during construction.

CRT’s are much more sensitive to AC fields than to DC fields. Many CRT's will exhibit signs of interference in AC fields of 10 milliGauss (mG) and most will be unstable in fields of 30 mG. By comparison, most monitors will begin to exhibit interference in DC field strengths of 1,500 mG (the earth has a DC field of approximately 500 mG).

Some computer systems/monitors are more sensitive than others. For example, Macintosh and Sun Workstations tend to be much more sensitive, and interference will often be seen at thresholds as low as 3 to 5 mG. Since the normal ambient field in a building can vary from between 1 and 4 mG, the chances for interference with these devices is considerable.

Factors that affect the appearance and sensitivity of electromagnetic interference (EMI) include the type of system and monitor being used, the monitor's refresh rate, the strength, direction and source of the external fields and the user's personal sensitivity. (Extensive research has shown that there are substantial differences between people in their ability to perceive interference on a monitor.)

What can be done?

Unfortunately, there are few easy and inexpensive solutions to these problems and the building owner or systems administrator may have to choose between solving the problem and reducing the symptoms.

If an EMI problem is suspected, it can be useful to measure the fields. Frequently the local utility company will do readings either free of charge or for a small fee for their commercial customers and they will have competent people to advise their customers. Independent, professional surveys, usually with a mitigation plan attached are advisable if a sizeable problem is discovered.

You may prefer to purchase a meter to do your own readings. Meters to measure AC fields are relatively inexpensive (about $200) and can be useful in identifying hidden sources. Gauss meters for DC fields are more expensive, running into the several hundred dollar range. If you do your own survey, make sure that nearby CRTs are shut off, so that the field they generate is not measured as well as the background fields.

If the source of the interference is an AC magnetic field, there are several mitigation options available, depending on the magnitude of the problem. Generally there are five solutions, in order of increasing costs: (1) modify the refresh frequency of the monitor, (2) increase the monitor-source distance, (3) shield the monitor, (4) buy LCD displays and (5) shield the source.

On some systems, it is possible to reset the refresh rate of the monitor to line frequency (60 Hz) without serious side effects. More often, though, this will result in a sacrifice of resolution, and in almost all cases, the jitter will have been traded for flicker (the refresh rate has been increased by the monitor manufacturer to avoid flicker – the eye can detect a refresh of the image at 60 Hz). Further, if the background magnetic fields are strong enough, even setting the refresh rate to 60 Hz will not remove all of the jitter.

Another theory suggested by a leading monitor manufacturer, is to set the refresh rate to 120 Hz. Doing this would rid the monitor of both the jitter and the flicker. Unfortunately, few applications at present are compatible with the 120 Hz settings.

The second solution is to increase the distance between the monitor and the source. Often a relatively small change in distance or orientation may be enough to eliminate the interference. This solution is effective in cases where the monitor is near a transformer or switch panel, but is ineffective if the source is transmission and distribution lines running outside the building.

If the source is power lines, or if it is not feasible to move the monitor and/or workstation, the third solution to consider is shielding the monitor or the affected device. External shields made of permeable materials will attract the magnetic fields and give them an alternate path around the monitor. A well-designed shield will fit snugly around the monitor and should allow for adequate ventilation to prevent overheating.

External monitor shields are commercially available and come in a range of options, from adjustable to custom-made, mu-metal boxes. The material used to make the monitor shields is expensive, especially if you require mu-metal for exceptionally high fields.

In some circumstances it may be acceptable to purchase LCD monitors. Cost, quality and system compatibility are issues which must be resolved prior to purchase, but withstanding that, an LCD will not be affected by an external field.

In cases where the source of the problem is a DC field, you can usually degauss the monitor by pressing the degauss button or restarting the monitor, depending on the model. It may be tempting to simply degauss the monitor and consider the problem fixed, but if you leave the monitor exposed to the DC field source, over time, it will again become distorted, so you will want to shield or move the monitor in addition to the degaussing.

Finally, if the magnetic fields are affecting a fairly wide area other mitigation options are available, depending on the source of the fields. If the source is a large transformer, for example, the building owners or managers may want to consider shielding the transformer vault. If the fields are due to wiring errors, it is often very effective to have a specialist locate the errors and work with an electrician to correct them.

Study Case 1

Problem: Elevated fields over a wide area

Cause: Wiring errors in the building

Solution: Isolate and correct wiring errors

A software development company purchased an old building and had it converted to office space. As part of the renovation, they upgraded the electric service. As soon as they moved in, they discovered that many of their monitors were experiencing "jitter" and instability. After a survey, it was discovered that there were elevated fields spread out over a large area which is a key characteristic of net-current problems. Wiring errors were causing an imbalance on the electrical distribution system.

After isolating and correcting the wiring problem that caused the net-current condition, most of the interference disappeared, except in the offices adjacent to the transformer vault which was later shielded. Employees were also counseled to relocate certain pieces of equipment, including stereos, further from their sensitive workstations.

About the author

Michael L. Hiles is the president of Field Management Services, a Los Angeles based company that manufactures the JitterBox and provides mitigation and survey services. He can be reached at FMS@FMS-Corp.com or at (323) 937-1562.