A few thoughts on the state of the UHF spectrum

By Kent Margraves

From a manufacturer and free lance point of view, Kent gives his thoughts on the current scene of the UHF spectrum in the US.

As a technical solutions provider & educator at Sennheiser I often view the industry from the manufacturing angle.  But the first decade + of my career was as a professional audio mixer and as an audio director, and I often remind myself to never forget that vantage point of our industry.   And I still free lance frequently as a front-of-house mixer, which hopefully helps avoid becoming blinded by a manufacturer-only perspective when teaching or providing technical solutions for clients from various sides of our business – systems designers, contractors, end users, etc.  Anyway, here are some loose thoughts on the current scene of the radio spectrum in the U.S. as it relates to wireless microphones and related gear.   This is not meant as a polished statement release from a manufacturer, but simply some thinking out loud as a fellow SACer living in the midst of this stuff.

From an engineering perspective, the UHF spectrum range (470 – 698 MHz) makes great sense for wireless mic operation.    It offers the best balance of factors such as good wave propagation, manageable antenna sizes, reasonable power supply demands and more.    It’s also a sizeable chunk of spectrum , even after  a portion of was auctioned off several years ago (it’s still over 200MHz wide).   But these positive factors attract manufacturers of other wireless technologies too.  Broadcasters, communications companies, and other wireless players also often covet this piece of the spectrum for their purposes.

Quality RF microphone manufacturers have been building and operating multichannel systems in the “white spaces” of the UHF spectrum successfully and reliably for a long time.   We know how.  This is not voodoo or black magic – it is well known science.

A lot has happened recently.    The U.S. TV broadcast community moved to digital transmission and consolidated below 698 MHz, freeing up the 700 MHz range (698-806 MHz).  Most of that spectrum has since been auctioned for billions of dollars for a variety of new uses, and portions are also assigned for use by emergency services in various markets.    So the portion of the UHF spectrum we can legally use for wireless mics shrunk by about one third.   Will that happen again?  Possibly.   But if it does, it would be an extremely slow process and might take upwards of a decade from now.    And if so, we’d just adapt.  Again.

Also, and separately, the FCC ruled that TV Band Devices (TVBDs) will be allowed onto the U.S. market.  These are a variety of future wireless devices, both fixed and portable, that will also operate in the white space channels.  That means we must share the white spaces (the unoccupied TV channels between active TV channels in each market area) and that’s clearly a concerning idea for the proaudio world.   For several years prior to that ruling there was a lot of fear in the audio community that RF mics would be rendered useless or unreliable with the introduction of TVBDs.    Its true that most wireless mics have operated quite successfully without need for a license, and without causing any harm whatsoever to other users.  After much lobbying, educating, and spending by key players in the industry, the FCC eventually recognized that wireless mics are widespread and are a truly vital tool in our society and that provisions should be made to allow them a reasonable chance of continued, reliable operation.

So, TVBDs must past strict performance testing and approval by the FCC.   One requirement is that these devices must access an online geo-location database (yet to be opened) that maintains a list of licensed broadcasters and licensed wireless microphones, and must avoid operating on those occupied channels at a given location.  That’s a great thing for us, at least for the wireless microphones that are licensed.     Another optional method of interference avoidance by TVBDs is spectrum sensing.  In this, TVBDs must be able to look at their environment continually and avoid operating on channels already used by broadcasters and wireless audio devices.    There are also strict rules on the operating power of TVBDs and the physical “exclusion zone” details.

Also, two vacant UHF TV channels (that’s 12 MHz) will be reserved for wireless microphones in all areas of the U.S.    It’s the first unused TV channels above and below ch.37 (608-614MHz – reserved for radio astronomy).

Wireless mic users are going to be able to reserve additional channels for their use as well.  They must apply to the FCC and demonstrate a need as defined by the FCC.  Specifically, they must first plan to place at least 6 frequencies in each TV channel that are unavailable to TVBDs.

We haven’t seen any TVBDs hit the market for sale yet, but they will sooner or later.    Several high powered companies are designing products and they’ll eventually meet the technical guidelines and deliver products.    The bottom line here is that the FCC has made reasonable efforts to protect wireless professional audio from TVBDs and to allow them to continue reliable operation in the white spaces.

Wireless mics (including inear monitor systems, IFBs, and wireless intercom) are not on death row.   But if things get tougher in terms of available spectrum, we’ll notice an increased demand for excellent technical support, spectrum planning and frequency coordination education.    The gap between high quality and mediocre wireless products may be noticeably widened.    And we, as one RF manufacturer, are constantly improving our designs to make performance even more reliable.    By using advanced filtering, high quality components, and highly linear RF circuitry with ever growing dynamic ranges we gradually raise the bar for multichannel stability – all while staying within the performance limitations  imposed by the FCC.    Frequency agility is key.  We now offer a system with a switching bandwidth of 184MHz!

One thing is certain: we (Sennheiser) are not interested in trading any amount of sonic quality for the sake of RF performance.  Sound is the whole point of these products!   We now see RF microphone systems with extremely low noise floors and audio dynamic ranges that exceed that of the Compact Disc standard.   In fact, one system offers a signal-to-noise ratio of 118dB(A).

We manufacturers generally encourage users to take advantage of a variety of resources when planning wireless audio systems.  This can include 1)several on-line free databases where one can view the state of the radio spectrum at their exact facility location, which leads to selecting the most appropriate frequency range for the intended application, 2) manufacturer support including design assistance, 3) pre-coordinated channels stored on-board products (we’ve done all the math to avoid intermodulation interference between frequencies), 4) custom designs for unusual or highly complex applications.  These are things our team does every single business day by phone, email/remote, and in person.  What we don’t like to see (and it sadly happens too often) are cases where multi-channel systems are purchased and then turned on without planning.   That can work, but it is far better to plan prior to purchasing!    How many times have we taken technical support calls to optimize a system knowing that if it had been purchased in a different frequency it would have worked with the pre-coordinated channels?

We (Sennheiser) have now started offering several advanced RF services that include customized consulting and design.  Several of us are highly trained and certified for this purpose.   And nothing satisfies us more than to walk a customer through the planning, purchasing, system optimization and finally, realization of outstanding performance!   This is in addition to excellent technical support that we’ve been providing for years to all our customers.  We also have a travelling RF Sound Academy where we spend a day transferring RF theory and relate real world applications “tips & tricks” to attendees.  Our friendly competitors also offer outstanding education opportunities.

We continue to do what we do quite well – offer multichannel FM systems in the UHF spectrum with extremely high performance standards.   We’re here to stay and so are professional wireless microphones, in-ear monitor systems, and related gear.   As the spectrum changes, we’ll adapt and roll with each challenge.   We always have.