Top RF consultant James Stoffo discusses the picture for wireless system use...
January 07, 2014, by James Stoffo
By now, we should all be well aware of the fact that our production audio “RFscape” is changing dramatically due to the introduction of digital consumer devices within the wireless microphone spectrum. The next two years will separate the women from the girls and the men from the boys when it comes to wireless audio operation in the United States.
It’s becoming more and more difficult to operate multiple wireless systems, including microphones, in-ear monitors, IFB, and intercoms, on events and installations. We have overcome the energizing of digital television (DTV), the “700 MHz” auction, and are preparing for the mass introduction of white space devices in the U.S. and around the globe. And the FCC is already speaking of further auctions within what is left of the production audio wireless spectrum.
Though these additional auctions are scheduled for next year, it may be years before they actually take effect. I heard the first mention of digital television in 1991 but it was not fully adopted until 2009. This means that we likely have time to prepare and properly plan. As long as we know what to plan for, I believe that we’ll get through the next wave of electromagnetic assault on our wireless systems relatively unscathed.
Making It Work
Let’s start with what we do know as fact. We’ve been forced to operate all of our wireless audio in about half of the spectrum that we’ve had since 1962. Next year, with the introduction of white space devices en mass, we’ll need to work with less than 10 percent of the UHF spectrum that we’ve always had. The Fixed and Portable digital consumer devices that we’ll be sharing radio spectrum with must follow very strict guidelines.
The two types of white space devices have two distinct sets of rules, so it’s a good idea to understand the differences between them. Fixed devices may operate anywhere in both the VHF and UHF radio spectra; however, they cannot operate adjacent to any local TV station. This means that you may operate wireless microphone and intercom systems adjacent to TV stations when planning for Fixed White Space Devices.
Portable White Space Devices may only operate above 512 MHz (or TV channel 21) and do not have to obey the adjacent TV channel rule. These will be the “smartphones” in your audience that may really create havoc within wireless systems. The FCC has left the pro audio community with a total of 12 MHz of radio spectrum to operate within, including mics, IEMs, IFBs and intercoms.
The only way to make that work will be to properly band plan. It’s always a good idea to separate base transmitters, such as IEM and intercom transmitters, as far away as possible in terms of frequency from base station receivers, such as wireless mics and intercom receivers. This keeps the overall RF noise floor low and guarantees the best range and audio quality from wireless audio systems.
In addition, find out which specific two 6 MHz slices of spectrum are reserved for wireless systems in the city in which you’re operating. These two TV channels will change from city to city and the information is available to anyone in advance. They will always be the two closest open TV channels above and below TV channel 37. TV channel 37 goes from 608-614 MHz, so this rule will change after the next auction—but will certainly be in effect for the next few years.
Knowing The Rules
As wireless operators, we’re at a point in time where the only way to achieve success will be to continually know what the rules are. And yes, they are changing.
For example, the next auction will place everything above 600 MHz on the chopping block. This means that we’ll lose another 100 MHz of UHF spectrum. At that point, wireless will literally need to be “reinvented” from a manufacturing standpoint to survive.
Wireless system makers are well aware of this fact. Between the introduction of digital wireless mics from AKG, Shure, Sennheiser, and Line 6, and the return to the VHF band for wireless intercoms from Radio Active Designs (a company I’m involved with), we’re seeing a rebirth of technologies in the audio community. This new approach represents a necessary evolution in our industry and has been brought on by the catalyst of FCC spectral changes and digital consumer technology.
As the majority of consumer digital technology will be in the UHF band, it’s a good idea to seek safe harbor in the VHF band wherever possible. This may include placing IFB systems in the 76-88 MHz band and intercom systems in the 174-216 MHz band. Between these two, you can buy yourself an additional 100 frequencies and get some “wiggle room” up in the UHF band for your wireless mic systems.
I’ve worked with the new digital wireless microphone systems from all of the professional manufacturers and can make a usable observation. A digital wireless system will always be more tolerant to interference than an analog wireless system. All analog wireless systems in the U.S. use Frequency Modulation (FM) technology.
Though FM sounds great and has no latency, it is not very forgiving when it comes to interference rejection. In fact, the slightest interference may cause an FM system to become unusable. The digital wireless systems on the market today are extremely tolerant to interference and can operate with a very high general noise floor inside of their radio band.
Since the portable white space devices will be everywhere in your audiences, my feeling is that they will consistently create intermodulation distortion products that may cause interference that’s harmful to wireless systems. Remember, these smartphones will be in the same radio spectrum as your wireless mics. It really seems to make sense then that any equipment placed in this band should be as tolerant to interference as possible. (Hello digital wireless mic systems!)
Planning For Success
Let’s bring things all together. Wireless audio system operation has gotten more and more difficult over the last few years. It will be even more difficult in the coming years.
Know where to operate for your best chance for success. Spread out wireless systems in frequency with proper radio band planning. Use VHF wherever possible for wireless IFB and intercom systems. Budget for digital wireless mics over the next few years.
Review the latest rules regarding where wireless audio devices and white space devices may operate. Know the two 6 MHz channels that are reserved for wireless audio in your city or cities of operation. Keep IEM transmitters in one of those 6 MHz chunks and wireless mics in the other to maximize signal-to-noise ratio, distortion, and audio quality.
And finally, stay informed about possible upcoming changes and new technologies. Stay tuned!
James Stoffo is the founder of Professional Wireless Systems and a founding member of Radio Active Designs. He continues to work as the RF technician and frequency coordinator on large-scale special events and installations.