by Jon W. Munderloh
Images on monitors are jittering... excessive hum in the audio system... strange problems with network data systems... perhaps caused by magnetic fields?
When reports are received of wavy or jittery images on monitors, or audible "hum" in an audio system, maintenance engineers often scramble for an explanation and a solution. Grounding systems are meticulously checked and inspected, yet the monitor distortion remains or excessive audio hum persists. Monitors are swapped, isolation amplifiers substituted, surge protectors installed, all to no avail. Everything is within specification. The mysterious interference problem persists. Eventually, everyone involved concludes that the source of interference isn’t the affected equipment, the system it’s connected to or the power source. What’s left? What could it possibly be? It just seems to be in the air. Perhaps the interference problem stems from low frequency magnetic fields emanating from some near-by power source.
Broadcast engineers and technicians are generally knowledgeable about equipment interference problems often associated with electronic grounding issues or external environmental sources (RF, microwave, radar systems, etc). However, few engineers and technicians are aware of interference that may be caused from elevated low frequency magnetic field conditions.
AC and DC Magnetic Fields
Equipment interference in a technical facility may be caused by elevated levels of either alternating current (AC) magnetic fields or by direct current (DC) magnetic fields in a building’s interior. A wide variety of equipment interference problems can occur when levels of either type of magnetic field are excessive.
Magnetic fields occur throughout nature and are among the basic forces of nature. Any object with an electric charge on it has a potential or "voltage" at its surface and can create an electric field. A change in voltage over distance is known as the electric field. When electrical charges move together (defined as "current"), additional forces are generated. These additional forces are represented by magnetic fields; hence all electric currents create magnetic fields. Depending on the source of the field, a magnetic field’s strength and direction can be static (direct current, DC) or alternating (alternating current, AC). AC magnetic fields are a natural consequence of distribution and use of electricity. At 60 Hz or extremely low frequency (ELF), the electric and magnetic fields generated by an AC circuit operate independently. In other words, it is possible to reduce or eliminate one without materially affecting the other. However, at radio frequency (RF) and higher, a fixed relationship exists between the electric field and the magnetic field. A reduction in the magnetic field also results in a reduction in the electric field.
AC electric fields are generated by voltage and are measured in volts/meter while magnetic fields are caused by current flow and are measured in milliGauss (mG). At ELF or power frequency, electric fields emanating from an AC circuit are quite easy to shield, as nearly all standard building materials will substantially reduce the electric field strength. Conversely, ELF magnetic fields are extremely difficult to reduce. At 60 Hz power frequency, AC magnetic fields pass relatively undiminished through nearly all common building materials, including lead.
Elevated ELF magnetic field conditions are normally present in areas adjacent to high current-carrying conductors. Typical high current sources in commercial buildings include electrical equipment rooms and closets, utility substation and transformer vaults, distribution bus-ducts, wire ducts and feed conducts. Electrical transmission and distribution lines passing near the exterior of a building may also create elevated magnetic field conditions in a building’s interior. In some instances, the source of elevated ELF magnetic fields in a building may not be readily apparent. Wiring errors in a building’s AC power system, even in low current distribution circuits, can cause substantial ELF magnetic field conditions to exist in large areas of a building. Such wiring errors including improper ground-to-neutral connections and crossed neutral conductors, can create "net-current" conditions wherein all of the current in a circuit is not returning via the same path. Since the strength of an ELF magnetic field is directly proportional to the amount of current flowing in the source circuit, fluctuations in the use of power during the day or seasonally, can cause changes in elevated magnetic field conditions. Because of this temporal variation, it is not uncommon for an equipment magnetic field interference problem to appear as intermittent.
Static or DC magnetic fields commonly occur in nature. The earth has a natural static magnetic field which, depending on location, will range from 400 to 500 mG. DC magnetic fields may also be generated by DC current flow in subway or train rails, elevators, and battery based power systems. Powerful magnets associated with MRI and NMR medical magnetic imaging systems typically produce elevated levels of DC magnetic fields. As a consequence, structural and reinforcing steel members in commercial building structures can become magnetized as a result of containing such equipment over a length of time. Structural steel can be magnetized by placing it in a strong external dc magnetic field that will essentially capture and align the magnetic domains in the material. Steel can also become magnetized by putting strong DC currents through the material such as grounding welding equipment to structural steel during construction.
Common Magnetic Field Interference Problems
Interference with computer or Cathode Ray Tube (CRT) video display monitors is the dominant problem associated with elevated magnetic field environments. Screen interference caused by magnetic fields is of two categories: AC magnetic fields can cause the image to "jitter" on a display, while DC magnetic field monitor interference problems are manifested as stationary image tilt or color purity problems (changes or blotches of color in various areas of the screen). Thresholds for computer monitor interference will vary by different magnetic field intensities, depending upon the type, size, make and model of the monitor. In general, CRT’s are much more sensitive to AC magnetic fields than to DC magnetic fields. Many CRT’s will exhibit signs of image jitter interference when placed in external AC magnetic field conditions of 10 mG and most will be unstable in fields of 30 mG. Some high-end large screen graphics monitors tend to be much more sensitive and interference will often be noticed at thresholds as low as 3 to 5 mG. As ambient AC magnetic field conditions in most commercial buildings range from 1 to 4 mG, the chances for interference with such monitors is high.
Typical DC magnetic field interference on CRT’s can be observed in magnetic fields as low as 1,000 mG, (500-600 mG above background Earth DC field) with increasing interference as DC magnetic field levels increase. Although relatively infrequent, residual, elevated DC fields in buildings can be in the range of 2 to 5,000 mG.
A wide variety of audio equipment may experience interference problems when located in an elevated AC magnetic field environment. Most notably, sensitive preamplifier sections of professional and broadcast audio mixing consoles may experience increases of audible 60 Hz "hum" or increased levels of signal-to-noise when located in areas with elevated AC magnetic field conditions. Such hum and increased signal-to-noise conditions are created by the induction of an interference voltage at 60 Hz in sensitive components of analog audio amplifiers. Similarly, sensitive or poorly shielded microphones and musical instrument audio pick-up transducers can experience undesirable levels of 60 Hz hum when used in environments with elevated levels of AC magnetic fields. Professional musicians have long been aware of this phenomena in performance venues and have learned to shift or orient amplifiers and sensitive musical instruments to locate areas with lower AC magnetic field levels or "null" points.
Interference problems may be present in audio/video/data cabling when placed in close adjacency to conduits, bus-ducts or other electrical distribution equipment containing high AC current conductors. ELF magnetic fields naturally emitted from such conduits or bus-ducts may be sufficient in magnitude to induce troublesome levels of interference AC voltage on adjacent signal cabling. The potential for such interference is markedly greater when signal cable runs closely parallel AC power conduits for extended distances. Although not well documented or understood, there have been numerous anecdotal reports of a wide variety of possible interference manifestations in CPU and digital equipment when placed in elevated AC magnetic field environments. Such interference problems have been known to affect the operation of high-speed CPU and certain computer disk drives, loss of data resulting in increased error rates and slower transmission speed of LAN digital signal networks. Most equipment manufactures including companies making CRT monitors, unfortunately do not publish AC or DC sensitivity levels for equipment.
Lastly, elevated levels of magnetic fields present in archive areas may affect the long-term storage of magnetic media including magnetic tape, floppy-discs, etc. Typical specifications for magnetic storage media including floppy-disks and hard-disk drives, range from about 6,000 to 10,000 mG for magnetic field levels in the frequency range of 0 Hz (dc) to 700 kHz (which includes the power-frequency of 60 Hz). Below 6,000 mG data corruption on storage media is typically not observed as reported by hard-disk drive manufacturers.
Dealing with Magnetic Field Interference Problems
Resolving equipment interference problems due to the presence of an external magnetic field source is often difficult and expensive. In some instances a choice may have to be made between eliminating the source of a magnetic field or minimizing the interference effects of the source magnetic field. The first step in identifying and resolving a suspected magnetic field interference problem, is to locate possible magnetic field sources and to measure magnetic field intensities in the area of concern. Magnetic fields are measured by a gauss meter which range in cost from several hundred to several thousand dollars. Different instruments are required to measure AC and DC magnetic field values and directions. Prior to purchasing such measurement equipment, it may be preferable to contact the local utility company and request magnetic field measurement assistance. Many utilities have an "EMF specialist" who will provide at no-cost or for a minimal fee, initial magnetic field measurement services. If a sizable problem is identified, such as an audio/video equipment room located above or immediately adjacent to a major electrical service room, it may be prudent to engage the services of a professional engineering firm. A specialized and experienced engineering firm can conduct a detailed assessment of the problem and evaluate possible solutions. If the source of interference is an AC magnetic field, three general strategies may be considered to mitigate the problem: (1) increase distance, (2) decrease the magnetic field strength, or (3) shielding.
Magnetic fields decrease in strength at increased distances from the source. It may be possible therefore, to simply move or relocate affected equipment away from a magnetic field source until interference problems are minimized or eliminated. This solution may be effective, for example, in instances where a monitor is near a transformer or electrical panel, but this effort may prove ineffective if the source is a transmission line passing outside the building. In certain instances, it may be possible to decrease the magnetic field strength from a source by implementing electrical modifications to increase natural cancellation of opposing conductors. In the instance of magnetic fields caused by net-current electrical circuit conditions, dramatic reductions in magnetic field levels typically result as a consequence of correcting wiring errors that create net current conditions. As a third possible solution, consideration may be given to shielding the affected equipment, shielding the source of magnetic fields or shielding the area in which the affected equipment is located. In the case of monitors, special external shields made of permeable materials that attract magnetic fields and provide an alternate path around the monitor are available from a number of manufacturers. Shielding large areas such as an electrical room or an entire space containing sensitive equipment is generally difficult to implement and should be designed and installed by an experienced and qualified magnetic field shielding engineering company.
Monitor interference from external AC magnetic field sources may be minimized by two additional possibilities. In some computer systems, it is possible to set the vertical refresh rate of the monitor to 60 Hz power frequency without serious compromises to the image quality. However, resolution of the monitor may be reduced and in almost all cases, cure of the jitter problem will be at the expense of increased "flicker" from area lighting. Further, if the external AC magnetic fields are strong, setting the refresh rate to 60 Hz will not remove all jitter interference. As a second possibility, it may be acceptable to replace CRT monitors with LCD monitors. LCD technology monitors are generally not affected by an external magnetic field. However, quality and system compatibility issues should be considered prior to purchase of a replacement LCD monitor.
In instances where monitor interference is from a DC magnetic field, it may be possible to degauss the affected monitor to temporarily restore color purity or install a shield around the monitor. If the source of DC magnetic fields is from a structural steel building member that has become magnetized, it may be necessary to consider degaussing the magnetized steel to permanently to eliminate the interference DC magnetic field. This degaussing option removes residual magnetism from steel objects.
In new facility construction projects, consideration should be given to careful design of electrical facilities such that high-current carrying equipment is not located adjacent to areas which may contain sensitive equipment. Documenting AC magnetic field conditions at a proposed project site prior to design and construction may insure that passing transmission lines or near by utility electrical facilities won’t present a problem. It may also be a good idea to measure DC magnetic field levels near all structural steel members during building construction. If excessively high values of DC magnetic field levels are present due to magnetized steel members, it is much more feasible and cost effective to remove the magnetism by degaussing while such steel members are exposed and accessible.
Selasa, 23 Desember 2008
Broadcast article
Thanks to DD in the comments for the heads up on Emily Bell's fantastically wrongheaded article in Broadcast (go read it first, but brace yourself). If it wasn't spreading such harmful misinformation, it'd almost be funny. I’ve sent an email complaining about it - yes, I have become that man, the one who writes to newspapers to complain about articles, the line has been crossed. Anyway, I've reproduced the email below just in case anyone else shares the same incorrect view of things:
---
Dear Ms Bell
Your article in Broadcast on the 7th November, "Striking writers are wrong to think they should be paid more", is perpetuating several common misconceptions. Even the title itself displays a spectacular misunderstanding of how things work.
"Try telling that to the Writers' Guild of America, which went on strike for the first time in two decades, as it insists that screenwriters should be paid more money to cover reformatting rights across digital platforms other than broadcast TV. What a truly bizarre prospect this is - and perhaps only on the west coast of America where the relativity of wealth is so insanely out of kilter could it garner any kind of purchase as a basis for a strike."
First of all, it is not MORE money. Writers earn royalties - or, as they call them in the States, residuals - on a piece of work they authored. If a network makes money from the resulting TV show, then the writer is entitled to a share of that money. It's not extra. It's not a bonus, it's not a perk.
Say Doris Writer creates a TV show. The network puts that show on air, with adverts (or on a pay per view channel), thereby earning money. Doris gets a share of that money, as is right and proper, yes? Yes. All fair enough so far. The network then puts the show out on DVD, and Doris gets a share of the DVD earnings - much like an author gets a percentage of every book sold. Again, as is right and proper, yes? Yes. Fair enough again. Now stay with me, Emily, cause here's where it gets really complicated. If the network then puts that show on the internet, with adverts, it is earning money from that internet showing - the same way it earns money from a TV airing, or from a DVD box set. Doris then, surely, deserves to get a percentage of that too, because the network is earning money from an airing of her show. How, in the name of David Hasselhoff, is that NOT fair to you?
Many writers get most of their earnings from repeat showings - but what if that TV show only ever repeats on the net, or as an iTunes download? Under your rules, Doris wouldn't be entitled to any residuals at all. Still fair?
The writers aren't asking for "more money to cover reformatting rights". They want their hard earned residuals, in whatever medium that may be. It's not such a "bizarre prospect", and it's not only "on the west coast of America". Five minutes of research would have revealed the WGGB/PACT TV agreement, which entitles us UK writers to 5.6% minimum royalties on the "multi-media" exploitation of a standard TV episode we wrote. 5.6%. DVD, downloads, on-demand, yada yada yada. Know how much US writers get on DVDs? 0.3%. They were asking for 0.6%. They even abandoned this shockingly greedy request, in the hope of getting *something* for internet distribution. The studios refused to give them anything. Yeah, the WRITERS are the greedy ones...
"The basis of the claim for greater pay is surely a fairly obvious fallacy - that television is going to offset its declining audiences and therefore production budgets with internet exposure and that scripts acquire an additional value when extended across all platforms for which writers should be paid."
Again, it is not "additional" value. If the network makes money from a repeat - whether that repeat is on TV or the internet - then the writer is entitled to their percentage. It’s a royalty. Like it or not, people are watching less on their actual TV boxes, and either waiting for the DVD or watching online. If TV moves online altogether - sure, maybe it won't move in completely, but it's already leaving some clothes and a toothbrush behind, you know, just in case - then the US writers are going to need a solid residuals deal in place, or they'll suddenly find themselves with drastically reduced earnings.
"It is a formula which many of us might wish we had adopted - if I'm paid for a piece in print then how about a bit more for it popping up on an interweb site?"
Hey, my line of work isn't the same thing at all, but why don't I get exactly the same deal?? Doesn't work that way. You get paid for an article. They print it. That's the end of the story. It appears for one day, and never comes back. They can't "repeat" the newspaper, several weeks later, and get people to buy the same old articles all over again, earning themselves more money. They couldn't sell advertising in that repeat paper. So it's not the same thing. The website is an extension of that same paper. But if they were to collect all your articles and make a TV show about them that went on to sell millions of copies on DVD or online - you'd be entitled to royalties from that. And if they tried to do it without giving you any money, you'd be pretty angry.
"For the first time in five or six years the pendulum is even swinging back against the idea that the advertising-funded model of web content will work. At the nerdiest edge of the internet there is now an argument that advertisers will no longer have to stick their brands to content as there are so many better ways to reach the consumer. "
And here's where you lose me completely. You're saying online delivery of ad-funded content isn't going to work, is too expensive, and will eventually be abandoned by the advertisers altogether? So why are writers greedy for wanting their fair share of residuals for money that *is* earned on those online shows? If it's all going to fall apart anyway, what the hell, give them 20%, won't make a difference, eh?
It is extremely irresponsible to publish such a poorly researched, ill-informed article. In an ideal world, there'd be a retraction, and an apology to the striking writers who are trying to safeguard their livelihoods. Then again, in an ideal world, articles in a publication featuring “television and radio industry news” would demonstrate some basic understanding of, say, the television industry.
This isn't about the millionaires, this is about the people who haven't sold anything for a few years, the ones who live off residuals. Some of them have families. All of them would like to continue to be able to eat and pay their rent. If you genuinely think that's wrong, then that's the "truly bizarre prospect".
Yours sincerely,
James Moran
(Non-greedy writer)
---
Dear Ms Bell
Your article in Broadcast on the 7th November, "Striking writers are wrong to think they should be paid more", is perpetuating several common misconceptions. Even the title itself displays a spectacular misunderstanding of how things work.
"Try telling that to the Writers' Guild of America, which went on strike for the first time in two decades, as it insists that screenwriters should be paid more money to cover reformatting rights across digital platforms other than broadcast TV. What a truly bizarre prospect this is - and perhaps only on the west coast of America where the relativity of wealth is so insanely out of kilter could it garner any kind of purchase as a basis for a strike."
First of all, it is not MORE money. Writers earn royalties - or, as they call them in the States, residuals - on a piece of work they authored. If a network makes money from the resulting TV show, then the writer is entitled to a share of that money. It's not extra. It's not a bonus, it's not a perk.
Say Doris Writer creates a TV show. The network puts that show on air, with adverts (or on a pay per view channel), thereby earning money. Doris gets a share of that money, as is right and proper, yes? Yes. All fair enough so far. The network then puts the show out on DVD, and Doris gets a share of the DVD earnings - much like an author gets a percentage of every book sold. Again, as is right and proper, yes? Yes. Fair enough again. Now stay with me, Emily, cause here's where it gets really complicated. If the network then puts that show on the internet, with adverts, it is earning money from that internet showing - the same way it earns money from a TV airing, or from a DVD box set. Doris then, surely, deserves to get a percentage of that too, because the network is earning money from an airing of her show. How, in the name of David Hasselhoff, is that NOT fair to you?
Many writers get most of their earnings from repeat showings - but what if that TV show only ever repeats on the net, or as an iTunes download? Under your rules, Doris wouldn't be entitled to any residuals at all. Still fair?
The writers aren't asking for "more money to cover reformatting rights". They want their hard earned residuals, in whatever medium that may be. It's not such a "bizarre prospect", and it's not only "on the west coast of America". Five minutes of research would have revealed the WGGB/PACT TV agreement, which entitles us UK writers to 5.6% minimum royalties on the "multi-media" exploitation of a standard TV episode we wrote. 5.6%. DVD, downloads, on-demand, yada yada yada. Know how much US writers get on DVDs? 0.3%. They were asking for 0.6%. They even abandoned this shockingly greedy request, in the hope of getting *something* for internet distribution. The studios refused to give them anything. Yeah, the WRITERS are the greedy ones...
"The basis of the claim for greater pay is surely a fairly obvious fallacy - that television is going to offset its declining audiences and therefore production budgets with internet exposure and that scripts acquire an additional value when extended across all platforms for which writers should be paid."
Again, it is not "additional" value. If the network makes money from a repeat - whether that repeat is on TV or the internet - then the writer is entitled to their percentage. It’s a royalty. Like it or not, people are watching less on their actual TV boxes, and either waiting for the DVD or watching online. If TV moves online altogether - sure, maybe it won't move in completely, but it's already leaving some clothes and a toothbrush behind, you know, just in case - then the US writers are going to need a solid residuals deal in place, or they'll suddenly find themselves with drastically reduced earnings.
"It is a formula which many of us might wish we had adopted - if I'm paid for a piece in print then how about a bit more for it popping up on an interweb site?"
Hey, my line of work isn't the same thing at all, but why don't I get exactly the same deal?? Doesn't work that way. You get paid for an article. They print it. That's the end of the story. It appears for one day, and never comes back. They can't "repeat" the newspaper, several weeks later, and get people to buy the same old articles all over again, earning themselves more money. They couldn't sell advertising in that repeat paper. So it's not the same thing. The website is an extension of that same paper. But if they were to collect all your articles and make a TV show about them that went on to sell millions of copies on DVD or online - you'd be entitled to royalties from that. And if they tried to do it without giving you any money, you'd be pretty angry.
"For the first time in five or six years the pendulum is even swinging back against the idea that the advertising-funded model of web content will work. At the nerdiest edge of the internet there is now an argument that advertisers will no longer have to stick their brands to content as there are so many better ways to reach the consumer. "
And here's where you lose me completely. You're saying online delivery of ad-funded content isn't going to work, is too expensive, and will eventually be abandoned by the advertisers altogether? So why are writers greedy for wanting their fair share of residuals for money that *is* earned on those online shows? If it's all going to fall apart anyway, what the hell, give them 20%, won't make a difference, eh?
It is extremely irresponsible to publish such a poorly researched, ill-informed article. In an ideal world, there'd be a retraction, and an apology to the striking writers who are trying to safeguard their livelihoods. Then again, in an ideal world, articles in a publication featuring “television and radio industry news” would demonstrate some basic understanding of, say, the television industry.
This isn't about the millionaires, this is about the people who haven't sold anything for a few years, the ones who live off residuals. Some of them have families. All of them would like to continue to be able to eat and pay their rent. If you genuinely think that's wrong, then that's the "truly bizarre prospect".
Yours sincerely,
James Moran
(Non-greedy writer)
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