Sennheiser

Frequency coordination experts run a narrowband/broadband co-existence test

May 22, 2023 - Wireless Multi-Channel Audio technology is a highly interesting concept for large-scale RF wireless set-ups as found in big music events, theatre productions, and broadcast studios – but can the technology stand up to a practical test by two seasoned frequency managers? How does it 'get along' with narrowband systems? And how does it handle direct interference? Marco Völzke, freelance frequency coordinator, and Jonas Naesby, technical application engineer with Sennheiser and owner of a frequency coordination company, met with the Sennheiser WMAS inventors to put a prototype through its paces. "The overall idea was to stress-test the system to see how it behaves under pressure, and to see what would happen if you basically did everything wrong," shares Naesby.

The test set-up

A WMAS system was set up in an 8-MHz TV channel and had an analogue IEM system operating at high transmission power on the frequencies in the lower adjacent channel, and an eight-channel digital EW-DX wireless microphone system in the upper adjacent channel. Normally, the spectrum between IEMs and mics is not used but kept as a guard band. The team deliberately placed WMAS in this 'no-go' channel to see if it would still operate without any audio interruptions.

Info Box: The radio spectrum and the role of the frequency coordinator

The radio spectrum used for wireless microphones and wireless in-ear monitoring is generally a shared resource, with spectrum access regulated by national frequency authorities. Different radio services and applications, such as broadcasting, radio astronomy, and military share the same spectrum as wireless mics and monitors, therefore, the TV-UHF spectrum available to professional wireless equipment varies considerably from one location to the other, depending on the amount of these services and where they operate.

If wireless production equipment is to be used at an event or venue, its transmission frequencies are coordinated to ensure interference-free operation, which is in the mutual interest of all wireless users on site. The organizer of an event or a venue owner calls on an expert, the so-called frequency coordinator or frequency manager, who is the central person in all frequency matters of an event or a venue.

Frequency managers have a communicating, supervising, and managing role: They receive frequency requests from all users, plan and calculate the frequencies to be used, assign the frequency resources to the users, monitor interference levels, manage any cases of interference, and troubleshoot and supervise all frequency-related matters before and during an event. In this respect, frequency coordination for an open-air event is more difficult than that of a theatre or a broadcast studio, as a building will provide some protection from outside RF sources. In both scenarios, the frequency coordinator has to deal with interference products generated by other equipment that is important for production, for example, video signal converters or video/light signal distribution (splitters and combiners).

To stress the system even further, Marco and Jonas started adding in-band interferers into the TV channel used by WMAS, simulating real-life "accidents", such as an ENG crew arriving at a show with non-coordinated equipment, or a user mistakenly switching on a narrowband mic on an old, pre-set frequency.

The interview

Marco and Jonas, could you start by describing today's frequency situation in general? What are the trends you observe in your profession as frequency coordinators?

Jonas: Productions are not slowing down but growing, whether that's conferences or music festivals. More and more acts bring more and more wireless. At the same time, we have a decline in available frequency resources. That's why we see a growing demand for frequency coordination – it's simply impossible to run an event without strict control of who uses what when.

Marco: I totally agree. The expectations of organizers and audience have increased massively, they want big entertainment, something they have never seen before, something unforgettable. I'm not only talking about big concerts – today, even a normal congress starts with a huge opening ceremony. A lot is done to impress people, to show excellence. If you take a look at a typical medical congress, for example, you will find a big show, and it's totally normal that this congress has its own TV channel, which is broadcasting for the entire event.

Then we have many other devices that occupy our spectrum. Some wanted – like controls for moving lights or special effects – and then unwanted interference of say, a video switcher or the like, as all these devices are packed closer and closer together. You now have 360° LED walls with just a tiny opening for a person to get in and enjoy an immersive AV experience. All these elements taken together cause interference problems that can no longer be controlled. A few LED walls may not be problematic but if you arrive at 200, the interference will add up. When event organizers come up with an idea that has never been implemented before – that's when issues are likely. Creativity knows no limits, and each time I solve a challenge the next one is just around the corner.

Jonas: Completely agree, and while we see a new generation of LED screens which are way less problematic, there is a whole new series of products being pushed into production that are now radiating RF and causing interference, be it lighting, video, control, whatever.

Marco: Yes, the physics of interference don't end with special effects. At one show, we realized that interference problems were coming from our RF IDs. In some countries they are allowed to transmit at more than 30 watts! Also, the need for wireless at events is increasing due to the on-site and the hybrid part, the streaming that happens simultaneously. It's not the world before Covid or during Covid, you now have to satisfy both worlds.

On to the test: You tried out the WMAS prototype and checked how it would work alongside narrowband systems. What are your findings?

Marco: I was blown away by the performance. I've never heard such audio quality coming from a wireless in-ear monitoring system, with precise positioning and almost palpable stereo depth. The RF side is my profession, but I was first blown away by this natural sound; it's like I'm sitting at my desk with headphones on but here I can walk around. And this again gives people with a creative mind a new level of freedom. I'm really excited to see what the creative industry will do with these systems – maybe immersive audio projects. I think this is the first system that you can try out for this and impress artists and listeners. This might be a new type of event that has not been possible before.

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