News

12 May 2012

Video Matters. RF interference from LED screens is a BIG problem!

by Richard Cadena

“EMC is being enforced in Europe and it’s likely to become a bigger issue in other parts of the world, including North America. Don’t be caught with your shields down.”

If you could see electromagnetic radiation, you would be surprised at just how much of it is around you. Everywhere you look, you would see pulsing waves of energy. Fortunately, our eyes can’t see those frequencies, but they’re there . . .

Advertisement

Magnetism and electricity are inextricably linked; when electricity flows, it radiates a magnetic field. Electromagnetic radiation (EMR) is produced by electrical and electronic gadgets like radio transmitters, microwave ovens,  electric motors, and video displays.

If the radiation is strong enough and the frequency is between 3kHz and 300GHz – which is the range of radio frequencies – then it can interfere with radio broadcasts and communications. If you want to see a vivid example, check out http://plasa.me/emr. In this short video you’ll see a person keying a handheld radio next to a dimmer rack, and you’ll hear the breakers on the dimmers tripping because of the electromagnetic interference (EMI) from the radio.

BRING IN ‘DA NOISE

Pete Daniels, owner of Pete’s Big TVs in Nashville, Tennessee (www.petesbigtvs.com), only recently learned of the issue after working on a show for which he supplied an LED video display.

After the show, he was chatting with the line producer when she received a frantic call over the radio from the audio crew, who was having a problem.

“Can I help?” Peter asked.

“There’s nothing you can do,” she replied.

“The sound guys are dying, trying to find open frequencies in the spectrum to tune their wireless microphones.”

So Daniels went to see what was going on, and what he saw appalled him. Looking at their spectrum analyser, he could see what he called ‘trash’ everywhere. But it was the source of the radio frequency interference (RFI) that surprised him. It was coming from his LED videowall.

“They were getting hits on this frequency and that frequency. The

LED screen was spitting out all kinds of frequencies that everybody was trying to use.”

It was not an isolated incident.

Scott Ingham, owner of Ingham Designs (www.inghamdesigns.com), has experienced similar problems. Ingham’s company, which has offices in Austin, Texas and in Zhuhai, China, designs electronics for the entertainment industry, including low-EMI buffer boxes for video displays. He was called to rescue a large video installation at a power station cooling tower in Drogenbos, Brussels. The RFI generated by over 1000 LED tubes was jamming radio communications in the city.

“It took a while for the EMI police to find the source of the interference because the tubes weren’t always on,” Ingham said.

“But once they found it they shut it down. It turns out the manufacturer was pretty good at making antennas too.”

THE SPREAD SOLUTION

Ingham ended up solving the problem by building a new data distribution box to distribute the video content data from the controller to the displays.

“I used every trick in the book to reduce and spread out the noise,” he said. He spread the noise by modulating the data with a range of frequencies instead of concentrating all of the energy in a single frequency or a few frequencies.

“It was a bit of a band aid because I was told to solve the problem without touching the tubes, but that’s where the noise was coming from.”

The noise in video displays, he says, comes mainly from the transfer of digital data to the display and within the display. Video data is usually synchronous by the time it gets to the printed circuit board within the display, meaning that it is timed by means of a clock signal which is on a separate data line than the video data.

Both the data and the clock signals are square waves with very fast rise and fall times, which cause harmonics to be generated. Harmonics are whole number multiples of the frequency in question, so even if the data rate is not in the radio frequency range, the harmonics could be.

Also, Ingham says, the driver chips in LED displays typically have two sets of signals. These signals are pulse width modulated (PWM), meaning they are square waves with a fixed frequency but a variable duty cycle or a variable amount of ‘on’ time versus ‘off’ time within each single cycle.

“One PWM driver or buck regulator is used to set the peak current of the LED, and that can run in the 500kHz to 1MHz range,”adds Ingham. “The other PWM signal can be used to dim the LEDs and is usually 300Hz to 400Hz.”

Including harmonics, both of those signals can contribute to RFI emissions from the video display.

“The length of the PC board traces and the wires connecting them can start to become very effective antennas above 100MHz,” Ingham continued. “A quarter wave antenna for a 300MHz waveform only needs to be about nine inches long.”

It might seem as if the power supply – where most of the electrical energy is concentrated in any system, including video display systems – would generate the most powerful EMI. But Ingham believes it’s fairly easy to build a Faraday cage or a shield around a power supply to prevent the emissions from escaping.

“Power supplies can radiate,” he comments, “but it’s been my experience that most of the noise with video walls and tubes is from the transfer of digital data to and from the LED driver chips.”

THE RF POLICE

Why does all of this matter to the live event production industry? Or does it matter? Daniels says it absolutely does matter. In May, 2011 he was visiting a video display manufacturer in China when he happened upon some Germans at the same factory.

While he was inspecting one batch of displays, the Germans were looking at another. Daniels was perfectly happy with the batch he was looking at but the other guys were in the process of telling the Chinese they were rejecting theirs. Puzzled, Daniels asked them why they rejected the video screens.

“They looked fine to me, but the Germans said they didn’t meet the RF spec,” Daniels said. “And then they showed me their spectrum analyser.”

Daniels recalls how the Germans said that government representatives had been showing up at concerts with spectrum analysers, and if they didn’t meet IEC standards, they would shut them down. “They turn everything off and take a background reading, and then they turn things on one by- one. If the video screen emits too much RFI they say, ‘That’s a problem – you can’t use it.’”

A light bulb went off over Daniel’s head: “Maybe a year fromnow,” he said, “I’ll be getting a phone call from (Bruce Springsteen’s video director) George Travis saying, ‘I’m here in Düsseldorf and I have some Germans with a spectrum analyser saying I can’t use the video screen. What am I going to tell Bruce?’”

CUSTOMISING FOR COMPLIANCE

Daniels knew he had to get this problem under control, so immediately started searching for a solution. This involved finding a video display manufacturer that was small enough to cater for custom products but big enough to design and build products properly. The company found, InfoLED, already manufactured a 9mm pixel pitch display, but Daniels wanted a 7mm. The two parties decided to modify the existing product and change the module size from 600mm x 600mm to 500mm x 500mm but still use 64 pixels x 64 pixels, resulting in a pixel pitch of 7.5mm.

“I told them I needed them to make this product meet the specs for FCC, ETL, UL, TÜV compliance for real,” Daniels emphasises.

“Everyone in China claims they meet them, but they don’t.”

At the time of writing, Daniels had the finished product and was demo’ing 16 panels, with four in his warehouse in Delaware and four in Germany undergoing TÜV testing.

“I’m going to come away with a piece of paper written in German,” he said, “so when the authorities show up and say the screen is a problem, George Travis will hand them the documents saying it’s okay. The same goes for the American testing lab.”

After the first run of 16 prototypes, Daniels approved them and ran a second batch of 250 production panels. “Two-hundred and fifty were sold to a Broadway show that closed in eight days,” he said. “So we got them back, and we have been gigging them since September. We had them on the Soul Train Awards and said to the sound guys, ‘when you get around to doing spectrum allocation, we’d love to have you check the screen.’

A day later, the guy walked into the room with an analyser and there was nothing there. We just did the Latin Grammy’s in Miami – this is where we had the problem – and it ran flawlessly.”

According to Daniels, the new video panels have power supplies with toroidal transformers and lowloss capacitors. They also have very quiet driver chips running at 400Hz. “It cost twice or three times the price,” Daniels said, “but in a matter of years, everybody’s going to have to do this.”

SEALS OF QUALITY

Ironically, Daniels said that before he did the Soul Train Awards show, he would have said his gear met the standard for EMC. Now the industry is taking notice and an organisation in Germany is focused on EMC compliance for videowalls.

HQ-Union (www.hq-union.de) is a trade organisation made up of manufacturers, landlords, users and designers of the LED video industry. According to their articles of association, their primary purpose is to support compliance with European statutory norms including CE and EMC through the assignment of ‘quality seals’ indicating compliance.

An article titled Elektrosmog induced by LED-Videowalls? By Helmut Gundermann says that “More and more LED videowalls don’t comply with the allowed limits (of EMI) by CE, FCC or CCC.”

He argues that the cause of the increasing non-compliance has to do with “the liberalisation of the goods traffic inside the European Union” when the CE mark was implemented.

The CE mark is designed to show compliance with European standards but there is no thirdparty verification; it is strictly self-enforced. Further, he argues, the problem of EMC compliance is a growing one.

Early LED displays were “primitive” but the electromagnetic fields they generated were “only marginal,” the article says. Over time, resolutions and refresh rates were increased without regard to the increased radiation. Further, by ignoring shielding requirements, manufacturers can gain “an immense price advantage at the expense of meeting compliance with regulations regarding safety, fire prevention, and valid VDE rules.”

These products are marketed touting features like ‘refresh rate,’ ‘lower weight, and ‘low price’, making it hard for companies who obey the rules and produce only tested, safe and reliable products.

Gundermann offers these suggestions for making sure you are using compliant gear:

• Check in advance the competence of the provider and ask how CE is secured.

• Before you order, first have a look at a sample. Do not be scared to spend some money on an external specialist to check the sample – it’s better to be safe than sorry.

• In some countries you have a money-back guarantee if the product is not compliant, even longer than the term of the warranty because the transfer of legal goods has not taken place.

Gundermann closes the article with an invitation to become a member of the HQ-Union. Membership is open to anyone over the age of 18 who accepts and promotes the purpose of the association. Although membership does not mean that the products you produce are EMC compliant, the association does offer its members support in measurement and shielding of equipment, and organises seminars and training events, among other benefits.

EMC is being enforced in Europe and it’s likely to become a bigger issue in other parts of the world, including North America. Don’t be caught with your shields down.

technical@lsionline.co.uk

• This article first appeared in Light and Sound International.

Subscribe

Published monthly since 1991, our famous AV industry magazine is free for download or pay for print. Subscribers also receive CX News, our free weekly email with the latest industry news and jobs.