By Richard Cadena for Elation Professional
Electromagnetic interference (EMI) is caused by the flow of current through conductors and electronic components, which, in turn, causes a magnetic field to radiate from the source, thereby inducing noise in nearby electronic devices such as audio systems. EMI can also be caused by an electromagnetic field coupling with other internal electronic components and producing noise on the electrical supply, which finds its way into other electronic devices connected to the same supply. EMI can degrade the performance of electronic devices such as audio, video, and lighting systems, or it can cause them to fail altogether. Large LED video displays can exhibit a lot of EMI, resulting in audible noise in audio systems, visible noise in video systems, and/or degraded performance of lighting systems. The solution is to buy and use LED video displays that are designed and built to limit the amount of electromagnetic emissions that escape from the panels and that filter noise from the electrical supply. These video displays are typically tested for electromagnetic compatibility by third-party testing laboratories and the manufacturers can provide documentation of the test results.
Causes of EMI
When current passes through a conductor or through components like power supplies, transistors, and LEDs, it generates a magnetic field that radiates from the source out into the atmosphere. The distance that it radiates depends on the amount of current flowing through the conductor or component, the voltage of the circuit, the frequency of the current, and the construction of the equipment, including, for example, whether or not the conductors are shielded, whether or not the enclosure is metal or plastic, how the circuit is grounded, and other considerations.
LED displays have a very large number of individual emitters, all of which are conducting a small amount of current, resulting in hundreds of amps (at very low voltage) flowing through a typical display module. For example, an LED display module with 176 pixels by 176 pixels, each of which draws 20 milliamps at full brightness, results in a current in the hundreds of amps. Most LED video walls comprise dozens of such modules, so a large video display has the potential to emit very high amounts of electromagnetic radiation.
Pulsing a circuit on and off exacerbates the issue with EMI because every leading edge of a current pulse emits a burst of electromagnetic radiation. And despite the appearance of dimming, LEDs are not actually dimmed, but they are pulsed on and off at a higher rate of speed than the human eye can detect. By varying the width of the pulse, an LED can be made to appear to be brighter or dimmer because of the persistence of human vision; the narrower the pulse, the dimmer it appears, and the wider the pulse, the brighter it appears.
The human eye can detect pulses of light up to about 30 to 50 cycles per second or hertz (Hz), but most video displays modulate the width of the pulses at frequencies from about 400 Hz to 1000 Hz. At very low intensities, the pulses are very narrow with durations in the range of tens of nanoseconds (10-9 seconds), which results in EMI with frequencies in the gigahertz range. The pulses are also in the form of a square wave, or something very close to it because the LEDs are either on or they are off . Square waves also contain frequencies called “harmonics” that are much higher than the rate of the pulses. Harmonic frequencies are whole number multiples of the original frequency, although a square wave has only odd-numbered harmonics (3rd, 5th, 7th, etc.). A 1 gigahertz pulse, for example, has harmonics with frequencies of 3 GHz, 5GHz, 7 GHz, etc. Video displays can generate harmonics with frequencies as high as 30 or 40 times the original frequency.
Results of Excessive EMI
All of this can result in LED video displays that produce very strong electromagnetic emissions. Under certain conditions, the electromagnetic emissions generated by an LED video display can radiate far enough to couple with other nearby electronic devices through induction and cause electromagnetic interference (EMI). In other instances, the electromagnetic field can couple with internal wiring and/or components, propagate through the power distribution system, and interfere with other devices that are connected to the same power source, which is another form of EMI. All it takes for this to produce audible noise in an audio system is for the audio signal cable or circuit conductors to be influenced by the magnetic field or to be connected to circuit conductors that are influenced by the magnetic field.
All electronic devices, including LED displays, produce EMI to some degree. If electronic equipment is designed and built without careful attention to limiting EMI, it can affect the performance of other electrical equipment connected to the same power source or in close proximity to it. That can, in turn, cause degradation of the performance of the equipment or cause failure altogether. In the case of audio equipment, excessive EMI can produce audible noise at specific frequencies, which is referred to as narrowband EMI, or it can produce interference across a range of frequencies, which is referred to as broadband EMI. Audible narrowband noise sounds like a tone or a combination of tones, while broadband noise sounds like white noise or pink noise.
Solutions for the Management of EMI
In a production environment, whether it’s a live event or a pre-recorded event, managing EMI is often mission critical since these events typically involve multiple crafts such as audio, video, and lighting. Some electronic equipment is carefully designed and manufactured to contain or control electromagnetic radiation while other equipment is not.
Electromagnetic emissions can be somewhat challenging to understand for the average user of LED video walls, but its behavior is well known and understood. Manufacturers have techniques to control EMI if a product is designed and built with that in mind. Such design techniques using proper grounding and shielding, decoupling and filtering, better quality components that are designed to limit EMI, proper layout of the circuit boards, and more. It costs more to produce products that limit EMI, but there is also a cost associated with trying to use equipment that interferes with the operation of other equipment. If compatibility is a requirement in a show or a production, then it’s important to use gear that is properly designed and built for that purpose.
Electromagnetic Compliance Regulations
Because electronic devices designed and built without regard to EMI can interfere with the operation of other nearby equipment or equipment connected to the same power source, there are regulations in different parts of the world that limit EMI to acceptable levels. In the European Union (EU), video displays must comply with EMC directives or standards that spell out the maximum allowable electromagnetic emissions from the product as well as its immunity to EMI. When a video wall is erected for a show in the EU, they are commonly field tested by EMC compliance officers using handheld instruments to determine whether or not the equipment under test complies with the EMC emissions standards. If the equipment fails to meet compliance, they will not allow it to be used.
In the United States, the Federal Communications Commission (FCC) regulates electromagnetic emissions pertaining to video displays. The FCC does not typically show up at an event to test for compliance in the field unless there is a reported violation or a report of interference, but if your equipment disrupts cellular phone service, Wi-Fi, air traffic control, television or radio reception, they have been know to dispatch a mobile team to track down the problem. Large video displays that are not EMC-compliant might not be prevented from entering and being used in the United States, but when they are used, they often interfere with audio and/or video equipment. And if non-compliant video modules are shipped to Europe, they will not pass the scrutiny of EMC compliance officers, rendering them unusable.
Clearly, LED video displays comply with EMC regulations are ideal for live event production. But how can you tell if it is compliant? If an LED video display is EMC-compliant then there will be documentation indicating so and it’s likely to bear the CE mark as well.
Understanding the Test Report
If a video display meets compliance testing, then there will be technical documentation that indicates that it is compliant and how well it performed in tests. There will also be a CE Declaration of Conformity with a reference to the EMC Directives, product identification to insure the documentation applies to the product in question, the date of the directives referenced in the documentation, the date of the testing, and the signature of the person who conducted the testing.
The testing includes measurements of both EMC emissions and EMC immunity. EMC compatibility testing includes measuring radiated emissions, conducted emissions, harmonic current emissions, and more. Radiated and conducted emissions are the prime causes of interference with audio and video equipment, while the other testing has more to do with power distribution and other factors. EMC immunity testing, for example, is important to understand how well the unit under test performs in an environment high in EMI, but it has nothing to do with the effects on the other nearby equipment or equipment connected to the same power source.
Compliance testing is carefully carried out under very controlled conditions that are spelled out in the EMC directives. Radiated emissions are measured using a spectrum analyzer with a broadband antenna that can accurately measure very small emissions in the range of microvolts (10-6 volts) per meter. The video display panel is connected to power through line impedance stabilizer, and at a measurement distance of 3 meters, the EUT is rotated on a turntable. Measurements are taken with the antenna in the horizontal position and another is taken in the vertical position and the spectrum analyzer graphs the EMI from 30MHz to 1GHz.
Conducted emissions are measured using a spectrum analyzer connected to the same power source as the video display panel, both of which are powered through a line impedance stabilizer. The spectrum analyzer graphs the EMI from 150KHz to 30MHz.
It’s important that the tests are conducted with the equipment under test being operated exactly as it would be used in a real show. For example, if the testing is conducted with the LED video display at a reduced refresh rate, the results can be misleading. The operation of the EUT should be spelled out in the documentation, including the brightness, refresh rate, etc.
The EMC directives classify equipment according to its intended use. LED video displays are considered class A devices since they are digital devices that are marketed for use in a commercial, industrial or business environment, and are not marketed for use by the general public or for use in the home.
The test results are displayed in the form of a graph showing the maximum allowable level of EMI in decibels of microvolts (dBμV) and the measured EMI versus frequency. As long as graph of the measured EMI falls below the maximum allowable levels, which will be superimposed against the measure EMI, then the device is compliant.
Recommendations for Best Results
If the success of your event is important, then here are some recommendations that will help you avoid problems due to EMI.
1. Look for LED video displays that meet EMC regulations. If a product meets the criteria there should be valid documentation backing up the claim.
2. Examine the EMC Test reports and look for clues as to its validity. Look at the test conditions and the results, and make sure there are no unusual circumstances.
3. Buy from a trusted manufacturer or distributor. In today’s business environment, it’s tempting to cut costs by buying direct from unknown distributors who send out unsolicited mass emails. Buying from the wrong supplier could cost you more money in the long run due to down time, technical issues, or time lost to troubleshooting.
4. Ask the manufacturer how they are able to design and build products that meet EMC regulations. They should be able to elaborate on the topic and provide you with enough information to make you feel comfortable about it.
Footnotes:
[1] LED video display modules are seldom operated at full intensity so the actual current being drawn can be considerably less.
2 The ideal waveform is a square wave, but due to the physical limitations of a circuit, it takes time for the voltage or current to rise and fall. Transistors control the voltage and current, and the maximum rate is limited by the slew rate of the transistor. Slew rates are measured in volts per microsecond (V/μsec).
© 2018 Elation Lighting Inc