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The Race for White Space

It always comes down to physics, just as our most fundamental understanding of the universe is in the creation, destruction, and movement of sub-atomic particles, physics explains the issues related to changing from analog to digital broadcast television in February 2009.

IT ALWAYS COMES DOWN TO PHYSICS, JUST AS our most fundamental understanding of the universe is in the creation, destruction, and movement of sub-atomic particles, physics explains the issues related to changing from analog to digital broadcast television in February 2009.

Next year, analog broadcast signals will be replaced by digital signals, all broadcasting below 698 MHz. The transition will create new opportunities. The first is what Dawn Birr, professional systems product manager for Sennheiser USA calls the “digital dividend.” This means there will be additional spectrum above 698 MHz available to a wide number of users, including those in the pro audio market. However it will also be available for other wireless radio services, including broadband and public safety uses, among others.

A lingering question for the audio industry is whether it will be illegal to make wireless systems (microphones, in-ear monitor systems, production intercoms, etc.) for spectrum above 698 MHz. But that's not the issue creating the most confusion.

That dubious honor is reserved for what known as the “white space” issue, and it stems from physics. For years, the Federal Communications Commission (FCC) has not allowed television broadcasts to occupy adjacent channels. There was always a guard band between broadcasters to ensure television receivers wouldn't pick up two broadcasts. This left space in the spectrum for other licensed transmitters and receivers to operate, including wireless microphones.

With the advent of digital transmission, those guard bands will disappear. This will increase the number of television channels below 698 MHz. But there will still be areas of open spectrum available, hence the second (perceived) opportunity.

COLLISIONS IN SPACE

Much of the spectrum in question is in the UHF frequency band (technically from 300 MHz to 3 GHz, but for the sake of this issue it extends down to 54 MHz, which is in the VHF band), which offers two inter-related advantages. The UHF band provides optimal wave propagation at low power levels. In other words, you can transmit a high-quality signal using less power. That's part of the reason wireless microphone manufacturers have used this frequency bandwidth for years. It's also why companies such as Motorola, Phillips, Microsoft, and Google (members of the Wireless Innovation Alliance and the White Spaces Coalition) want to use the spectrum to create nationwide wireless data transmission systems. And therein lies a conflict.

Shure, Sennheiser, Audio-Technica, and other wireless microphone manufacturers sell licensed devices for use in this spectrum and, as a result, have grandfather rights to the broadcast bandwidth. The Wireless Innovation Alliance respects this claim, but would like to use the spectrum when and where possible (such use of the spectrum is unlicensed and will remain so). The FCC is committed to protecting the licensed use of spectrum by insisting that potential unlicensed users utilize technology that checks to see if the spectrum is already in use. Such spectrum sniffing is currently done by the military, but now the onus is on members of the White Space Initiative to develop technology and present it to the FCC for testing.

The first round of testing was completed last summer and did not result in working prototypes that could be brought to manufacture. Microsoft and Phillips, among others, asked for an additional round of testing after they had an opportunity to make corrections and improve their prototypes. The FCC created an extensive set of lab and field tests that are necessary for these devices to pass before they are ready for prime time. As of now, the lab testing has been completed and field-testing is in progress. A final report and recommendation from the FCC is expected this summer.

The primary issue lies in the potential interference that unlicensed devices could inflict on licensed devices. The prototype technology would scan and listen for transmissions on frequencies in use and adjust to avoid those frequencies. However, many licensed devices are used only during certain days and times, like Sunday morning church services. Moreover, detection of a licensed device on a channel being used by an unlicensed device could create difficulties for both users.

The problems the FCC noted with the prototypes have been power supply failures, broken gear, and the inability to get the detection devices to work consistently. The first two issues are relatively easy to fix. That last one is a particular problem, especially when a device that may generate up to 4 watts of RF power has to detect and avoid a device that may generate anywhere from 10 to 100 milliwatts—a signal level that can be up to 26 dB lower.

Consistency is the primary concern of the National Association of Broadcasters, wireless system manufacturers, and user groups. Progress has been made since last year's test, but more work and testing remains to be done. As Bob Green, director of global product strategy for wireless at Audio-Technica says, “Design of high-quality wireless equipment is not an easy task. It is not something that can be done within the space of a couple of years. We've been at it for a couple of decades now.”

Wireless manufacturers have been preparing for a while now, eliminating models that transmit above 698MHz. None of the wireless microphone manufacturers expect there to be widespread problems come Feb. 17, 2009. There may be issues in specific markets on specific frequencies, but they expect those to be minor and quickly addressed.



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