Audio

Interesting Times (For Wireless Audio)

The incredible shrinking spectrum.

I’m going to shift my attention from all things display-centric back to a hot-button issue that’s going to affect a good part of our industry: the recent FCC spectrum auction.

Ever since Washington DC discovered in the 1990s that it could sidestep the intent of the Federal Communications Act of 1934 (“For the purpose of regulating interstate and foreign commerce in communication by wire and radio so as to make available…to all the people of the United States a rapid, efficient, nationwide and worldwide wire and radio communication service….”) and sell off chunks of spectrum for (initially) millions and, now, billions of dollars, the frequencies allocated to television broadcasting have become greatly truncated.

In the 1970s, I wrote my college thesis on the impact of the All-Channel Receiver Act of 1962 and how it finally made the section of the RF spectrum from 470MHz to 887MHz viable for TV broadcasting by ensuring someone could actually watch those broadcasts on new models of televisions. Back then, the UHF TV band was considered a wasteland, and anyone setting up shop there had a pretty good chance of losing lots of money trying to attract viewers.

However, that started to change in 1983, when channels 70 to 83 were taken away for other purposes and more UHF stations began to pop up. On the wireless audio side, most manufacturers were still cranking out gear for operation in the high-band VHF spectrum (175MHz to 220MHz), and there were plenty of issues with interference and overall reliability.

Some companies ventured higher. I remember staging an incentive meeting for Prudential Insurance in Anchorage AK in 1987, where the company’s internal AV support folks set up a brand-new UHF wireless mic system from Sony. Given that the venue (a Hilton) was the tallest building in town, the system was unexpectedly plagued by interference from two-way UHF radio transmissions from antennas on the roof. However, we’ve come a long way since then. Thirty years later, UHF is the preferred spectrum for wireless audio operation. The transmitters and receivers are reliable, and high-end models use digital operation, tone squelch and even encryption to provide secure, interference-free communication links. Unfortunately, the slice of spectrum they can use has gotten smaller, with the conversion to digital television having taken away TV channels 52 to 69 in 2009.

And, now, the footprint will get smaller still. The recent FCC spectrum auction that concluded in March will result in channels 38 (614MHz) and above becoming a “no man’s land” for wireless audio. Those channels will be repurposed for everything from Wi-Fi repeaters to mobile phone operations. Practically speaking, the upper boundary will be channel 37 (608MHz), as that has been set aside for some time for radio astronomy, and it’ll continue with that designation after the repacking of TV stations.

And there’s the rub: There are more TV stations on the air than can be accommodated by the remaining TV spectrum. That will force some TV stations back onto high-band VHF channels and, in some cases, low-band VHF (channels 2 to 6)—a largely undesirable part of the spectrum, due to man-made and natural noise and interference from unwanted signal propagation during warmer weather.

That, in turn, will ensure that UHF channels are jam-packed in metropolitan areas, thus leaving little room to shoehorn in wireless audio operations. Consider that, in New York City, where all Broadway productions make heavy use of wireless audio, as do sporting events, TV stations and corporate offices, there will be TV stations operating on every channel from 14 through 36.

So, what’s an audio guy to do? If you are in a major metropolitan area, you will have to get out your spectrum analyzer and look very carefully to find a channel that you can set up on without being clobbered by co-channel or adjacent-channel RFI. That will mean using directional antennas to null out unwanted signals, plus tone squelch and other tricks to “thread the needle.” And remember that your operations can’t interfere with reception of TV stations, as per the FCC’s Part 15 regulations.

Or, you could go higher in frequency. The 900MHz Industrial, Scientific and Medical (ISM) band permits wireless audio operation under Part 15. Because the wavelength of the signals is so small (33cm), it’s easy to design directional antennas with gain for short-range operation. Practically speaking, however, this part of the spectrum has not been popular for wireless audio, owing to in-band interference from a variety of restricted radiation devices.

Another option would be the 2.4GHz to 2.5GHz Wi-Fi band, using “white space” devices. Signal wavelengths are even shorter there, and the allowable power levels are adequate for wireless audio operation. Needless to say, all 2.4GHz wireless microphones use digital communications modes and a variety of tricks to ensure reliable reception (tone squelch, fast diversity reception). In addition, prices for 2.4GHz systems are reasonable. While writing this article, I found a basic (prosumer-grade) 2.4GHz wireless lavalier/handheld mic and receiver combo for $350.

There are drawbacks, however. Although 130MHz of spectrum will remain available for UHF wireless audio operation after repacking, only 83MHz is accessible at 2.4GHz. And that spectrum will be shared with numerous Wi-Fi access points and routers. Granted, 2.4GHz wireless mic systems can sniff out occupied channels and set up the most reliable TX-RX links; however, there’s no guarantee you won’t experience any interference.

Could we go even higher? Sure. The 5GHz Unlicensed National Information Infrastructure (U-NII) band has 24 channels available for white space operation. It’s becoming popular for distribution of high-bandwidth video (full HD and even 4K) through homes using one or more 20MHz channels (802.11ac), and it could easily work for wireless audio operation. Signals at 5GHz won’t penetrate certain materials, like reinforced concrete, so that could be a drawback. However, allowable power levels are much higher at the upper end of the band, reaching 1W. I don’t know of anyone making wireless audio systems for 5GHz operation at present; regardless, I’m sure they’re in R&D.

In the meantime, you have plenty of work to do to retune your existing UHF wireless systems to get them below 600MHz by the end of this year. In my RF workshops, I encourage attendees to use a spectrum analyzer to search for open channels. After all, databases can often be wrong. We’re seeing many low-power TV stations coming on air, even in major metro markets, and they are often missed in such compilations. If you do go that route, make sure you pick up a model that can also cover the 2.4GHz Wi-Fi band. They’re not all that expensive these days, and they can work double duty setting up and monitoring Wi-Fi links.

An old curse attributed (rightly or wrongly) to the Chinese states, “May you live in interesting times.” From the perspective of professionals who work with wireless audio, these are indeed interesting times…and they’re about to get a whole lot more interesting.

Previous ArticleNext Article

Send this to friend