Just about every live event uses wireless microphones. Whether it is musical theatre, a sporting event, a church sermon, or a corporate event, there is bound to be a wireless microphone somewhere. As sound professionals, we not only need to know what happens when we push the fader up, but we also have to be part Ham radio operator. We have to be well versed in sound theory and hold a proficiency in the dark art of RF or radio frequencies. RF is part science, part superstition, part math, part voodoo, and part folklore. You either have a story or know of a guy who had his whole RF rack scrambled with interference at show time because a recording truck or DTV station turned on. Every sound person develops a bag of tricks for dealing with wireless mics. This can include: combs, barrettes, toupee clips, surgical tubing, chicken blood, Purell, an intermodulation program, floral wire, and the phone number of at least one RF guru. When it comes to wireless mics, our job is not just to make them sound good but also to keep them from interfering with themselves and every other frequency floating around. And our job is about to get harder. Ladies and gentleman, meet white space devices and DTV.

As of this writing, all TV broadcasts will be digital in 383 days, three hours, and 19 minutes. In 1996, the US Congress authorized the distribution of an additional broadcast channel to each broadcast TV station, so that each could start a digital broadcast channel while simultaneously continuing the analog broadcast channel. We've been dealing with this issue ever since. You might set up for a gig and have no RF problems until 8pm, when the DTV station starts broadcasting, wiping out several of your RF channels. Later, Congress mandated that February 16, 2009 would be the last day for full-power television stations to broadcast in analog. For now, stations in all US markets are currently broadcasting in both analog and digital.

Currently, analog TV stations broadcast between 54MHz to 88MHz, 174MHz to 216MHz, and 470MHz to 806MHz. After the deadline next year, TV stations will no longer be allowed above 698MHz. One main reason for the switch to all-digital broadcasting is that it will free up parts of the valuable broadcast spectrum for public safety communications, such as police, fire departments, and rescue squads. Also, some of the spectrum will be auctioned to companies to provide consumers with more advanced wireless services, such as wireless broadband.

Consumers benefit because digital broadcasting allows stations to offer improved picture and sound quality, and digital is much more efficient than analog. For example, rather than being limited to providing one analog program, a broadcaster is able to offer a super sharp, high definition (HD) digital program or multiple standard definition (SD) digital programs simultaneously through a process called multicasting. Multicasting allows broadcast stations to offer several channels of digital programming at the same time, using the same amount of spectrum required for one analog program. For example, while a station broadcasting in analog on channel 7 is only able to offer viewers one program, a station broadcasting in digital on channel 7 can offer viewers one digital program on channel 7-1, a second digital program on channel 7-2, a third digital program on channel 7-3, and so on. DTV can also provide interactive video and data services that are not possible with analog technology.

To analyze how this affects our use of radio microphones, intercom, in-ear monitors, and IFBs, we need to look at a few issues. First is the broadcast spectrum. The FCC allocates the spectrum in which TV stations operate in this country, and each channel is allotted a 6MHz-wide band. Analog television broadcasts a video and audio signal within that band. For example, channel 20 is 506 to 512MHz. The video carrier frequency is 507.25, and the audio frequency is 511.75. Before DTV, we were able to squeeze a couple of frequencies in a TV channel's band if we had to, as long as we tuned our wireless device(s) to the “slot” between the video and audio carriers, or .500kHz below the upper channel edge, or even right on the channel edge, where the respective carriers' signal strengths are weakest.

DTV changes all of that. Henry Cohen, president of Production Radios explains, “DTV signal consumes just about all 6MHz of a channel. The adjacent channel, if unused, will also see some noise. DTV is one long, wide signal.” That means we cannot use anything within that 6MHz, which will make it more challenging to find open frequencies in heavily broadcasted cities. “The only option may be to use the channel edge itself if the two adjacent DTV signals are weak enough, as the actual DTV signal only encompasses about 5.6MHz half power (-3dB) bandwidth,” Cohen says.

More complex and consistent intermodulation distortion is another problem that DTV brings with it. Intermodulation is when two or more frequencies interact and create a third frequency. If that third frequency is too close to a frequency in your system, you might hear interference on that wireless channel. To better understand intermodulation, imagine dropping two stones in water. The ripples are the frequencies. When the two ripples intersect and create a third ripple, that's the intermodulated product. Of course, an intermodulated product is not as strong as its parent, so relative signal strength is also important in dealing with intermodulation. With analog TV, things were more straightforward. We had the potential to create intermodulation with the audio and video carrier frequencies. It was simple math, and we could predict and avoid it. But since DTV is broadcasting on essentially every frequency in a 6MHz band, there are an infinite number of frequencies to contribute to intermodulation. “You won't hear intermod problems,” says Cohen. “Instead, you'll have a higher noise floor, which will mean a noisier RF system.” Cohen goes on to explain that, since potential intermodulation problems could cause a higher noise floor, the RF systems may have a shorter operational range in order to bring the signal level up at the receiver and the noise floor back down and, thus, reduce the potential for dropouts during a show.

But, as Cohen points out, DTV is not the problem in and of itself. The issues created with DTV are palpable. The bigger scare is with white space devices. White space is what the FCC calls the unused space in the spectrum between active TV carriers.

Joe Ciaudelli of Sennheiser notes, “We are communicating with legislators, making them aware that the term ‘white space’ is a misnomer since broadcasters, film producers, and professional entertainers have been using licensed devices, such as wireless microphones and monitoring systems, in this spectrum for years. Therefore, major news, political, sports, and entertainment events would not be able to operate reliably if the spectrum was randomly flooded by new unlicensed devices.”

With this shift to DTV, the FCC plans to allow the use of a lot of this white space. There is even a White Spaces Coalition of eight large technology companies that want access to these “unused” spaces to deliver high-speed (broadband) Internet access to consumers. This space, however, is precisely where all wireless microphones are currently used. The Coalition claims that broadband access is expected to be available at speeds of 10Mbyte/s and above, and for white space short-range networking to reach 50 to 100Mbyte/s.

The Coalition includes Microsoft, Google, Dell, HP, Intel, Philips, EarthLink, and Samsung Electro-Mechanics. Karl Winkler, director of business development for Lectrosonics, Inc., explains, “The reason they want the 700 band is the same reason we want it: because it has excellent propagation which means better reception with smaller antennas.”

As a consumer, I am watering at the mouth over what the White Space Coalition is trying to do, but as a professional working monkey, I am terrified. The problem with white space devices is their reliability and use. These new devices will use a 6MHz band, the same as a TV channel, but it will use it like DTV and not analog. It will fully use the 6MHz stream for uploading and downloading from the Internet. There will be hundreds of millions of these devices, and they cannot each be assigned a portion of the spectrum. Instead, they will wander around and use available bands. Each device will be built with a spectrum sensor. All DTV channels will be very easy to see using a spectrum analyzer, because they will show a huge spike at a certain frequency. The devices will have a sensor to detect the signal and move to a band with no signal.

Chris Lyons, manager of educational and technical communications for Shure Incorporated, notes that, if these sensors work properly, the effects could be minor. “If the proposed ‘spectrum sensing’ technology that the white space devices are supposed to have really does detect and avoid wireless microphone signals, then the impact should be minimal,” he says. “Shure is actively working with the FCC Office of Engineering and Technology to make sure that spectrum sensing is proven to work under real-world conditions.” This would keep the devices from interfering with DTV stations or other broadcasts, but it might not be that easy for wireless mics. As Winkler adds, “Because wireless mics are so low-powered, they might just blend into the noise of the spectrum, and these white space devices may not pick up the wireless…Higher powered wireless is a good solution to this problem.”