28.08.18

Wireless mics: RF spectrum casts shadow

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Wireless microphones offer flexibility to the AV world, but the cloud over available RF spectrum is casting a shadow far and wide. Paul Milligan reports.

There can be few things in the AV world as useful as a wireless microphone.  During a live performance or important presentation the last thing you want to worry about is tripping over a long wire.  Being wireless solves so many mobility issues in that regard, and gives the user (and installer) a huge degree of flexibility. But as we have seen with other technologies that have moved from wired to wireless, it’s possible you’ll encounter some teething problems along the way.  When you add the ongoing confusion regarding the RF spectrum and wireless mics, where the only certainty is uncertainty, you may ask yourself why bother?

When you talk about any wireless technology inevitably you have to talk about interference, and with microphones it can be a critical issue, as Daniel Ku, senior applications manager for Mipro succinctly puts it “interference in the RF spectrum is an inevitable phenomenon.”  But as the technology has evolved does is mean it’s becoming less of an issue? Sadly, the opposite is true.  “RF interference is still the number one issue, and the problem is getting bigger,” says Tobias von Allwörden, senior product manager, professional audio for Sennheiser.  “The available spectrum is decreasing, leaving you with less room to manoeuvre.”

This sentiment is backed by another leading name in wireless mics, manufacturer Shure; “The threat of interference is arguably higher today and in the years ahead than ever before,” says Stuart Stephens, senior specialist, product manager, pro audio and retail. Where are the contributing factors for this? “We are seeing a continuous loss of UHF spectrum to the mobile sector.  Since 2012 our industry has lost nearly 50% of usable UHF spectrum and this leaves us with a more hostile RF environment than previously.”

There are other factors that can cause interference to watch out for says von Allwörden; “Sources can be LED walls which are quite omnipresent in today’s entertainment industry, and other lighting equipment and controls. They all emit spurious radiation in the UHF spectrum.”   Luckily, a few manufacturers of videowalls have become aware of this and are trying to remedy it.  Interference is becoming increasingly tricky to pinpoint says Ian Bridgewater, director TOA Corporation.  “It is quite difficult for some users to recognise other entities that would interfere with it, things like radio transmission towers, or MoD (ministry of Defence) sites that have ground radar.” 

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There have been many articles written in the proAV press about the RF spectrum over the past five years, so without going over old ground where are we at right now? “UHF spectrum continues to shrink at alarming levels.  The UK is due to clear the so-called 700MHz (703-790MHz) band by 2020 leaving users with nearly 50% less usable spectrum than we had in 2012,” says Stephens.

Uncertainly seems to be the one constant when it comes to the RF spectrum and its future. “It sometimes feels like looking into a big crystal ball.  And as there is no worldwide harmonisation, the usable spectrum also differs country to country,” says von Allwörden.

 “The future of UHF continues to be uncertain as the relentless push from mobile services providers seems to have no end,” says Stephens.  “It’s clear that mobile technology is part of our day-to-day lives and that sufficient bandwidth must be available to cater for demand.  The irony of course is that the very same spectrum that is being made available to deliver content is the same spectrum that is being used to make content in the first place and we are at serious risk of diminishing the production quality that our market is recognised for.”

The future isn’t looking rosy agrees Alex Lepges, marketing director, EMEA for Audio Technica, but he isn’t completely without hope either.  “With the growing demand for wireless content delivery (streaming video etc) it is certain we will see more spectrum disappearing for the usage of wireless microphones in the decades to come.  On the other hand other spectrum currently unused by wireless microphones might become available and there are several research studies ongoing to determine which frequency ranges could be allocated to PMSE (program making and special events).” 

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We spoke to an array of microphone manufacturers and it seems they are all keen to work together to continue the fight for more spectrum allocation.  “At the end of the day it is a lobby decision and we have to make certain the decision maker understands why we need spectrum for broadcast, major sports events, theatre and musical production, live music events and other applications,” adds Lepges.

Another issue with wireless mics is latency, how much can you limit the delay, is this still a significant issue or has progress been made in this regard? “Latency becomes an issue as soon as you want to monitor the picked up audio signal in real time,” says Lepges.  “Especially when you use in-ear solutions (earphones) a total latency of maximum 10ms seems to be the limit while some more critical users define 5ms as the total maximum. With typical latencies for digital wireless mics around 3ms to start the signal chain you can see that there is not much room left for further latency involved audio processing as the mixing console and a potential digital return channel.”  When it comes to latency says Stephens, it’s important not to get too wrapped up in a single component of the system and miss the bigger picture, “it’s the latency of the entire audio chain that is critical.  Once latency exceeds 5ms, it can be a problem when using wireless in-ear monitors, as the monitor signal arrives out of phase with the vibrations they’re hearing through bone conduction and can cause comb-filtering.”

Working in a venue with poor RF provision can be undeniably tricky, but are things improving in the ways we can correct and/or control audio dropouts? Taking time to correctly setup is the key to it all was the unanimous answer,  “Minimising dropout is really down to good installation and using the correct equipment, cable type and aerials,” says Bridgewater.  “There are techniques that we use to do forward-error-correction that will help to push out the ‘edge’ a bit to signal levels but the bigger impact will always come from the operator of the wireless system,” says Lepges.  “Placement of antennas, short antenna cable runs, good components and a smart RF gain planning next to frequency coordination is still the most important ingredient for a successful show.” 

To this end, Shure has a Timeline function as part of its Wireless Workbench software which allows an RF coordinator to perform a walk test around the venue while it records live audio and RF data, whereas Sennheiser’s Digital 600 Series has in-built error correction able to repair ‘a few ms’ of lost signal without a noticeable audio dropout. 

How do you make sure you choose the right wireless mic for your needs? First of all you need to make the decision on whether you need a lavalier, headworn or handheld mic to start with, and then the choice of wireless platform depends on how many simultaneous wireless mics you plan to use.  There are other, more human factors to consider too, “For stage purposes, you need to be conscious of the fact that any transmitter is going to be designed into a costume or wig. It should be small in size, comfortable for the performer and not get too hot when it’s been on for a few hours.  In a studio environment, there is likely to be close to 100 channels operating in the same vicinity, so a system that is spectrally efficient will help the RF coordinator’s job a lot easier,” advises Stephens.

If mics are moving towards a wireless mode of working, why is it that wireless monitoring systems are still predominantly analogue? The answer is down to latency again.  “With digital wireless mics offering 2-3ms latency and consoles offering 1-2ms latency, adding another 1-1.5ms latency from a digital in-ear system then you are approaching the 4-5ms point where a performer will start experiencing comb filtering,” says Stephens.  Money is also a factor says Bridgewater, “in-ear monitoring systems are available as both analogue and digital, but it’s a much smaller market and, as a result, there just hasn’t been the same amount of investment in the development of these types of products.”

Finally how effective is setting up a wireless mic on a boom pole in achieving rounded audio coverage? “This is a very effective way of miking for broadcast, eliminating the need to mike up several speakers.  To create your wireless boom pole, you will need a highly directional shotgun microphone, make it wireless with a plug-in transmitter at the end of the pole, and then point the mic at the speakers or actors to ensure optimum audio.  Using a shotgun is essential, an omni-directional capsule would just capture about everything and make it difficult to distinguish between dialogue and background noise,” advises von Allwörden.

The move from wired to wireless will happen in the microphone world, we know it will because it has happened to every other kind of electronic technology.  Wireless is the default because it is handy, versatile and flexible.  What is holding up the move to wireless is the fight for RF spectrum, which sadly, seems to have no end in sight, and is something we were saying at this same point five years too.