Wirele Bodypack Transmitters: Constrained Spectrum Causes a Problem but May Spur Innovation

Broadcast-sports audiences love on-field audio, and the major leagues have shown increased willingness to give it to them. The looming problem is where to put it. Not in terms of where on the players; bodypack transmitters have gotten smaller over time, ultimately limited only by the size of the batteries needed to power them. But what's also shrinking is the RF spectrum around them, the oxygen needed by wireless audio systems to survive.

Spectrum reallocation by the federal government has substantially diminished the RF bandwidth once used liberally by broadcasters. In particular, the 600-700 MHz bands, which were part of the spectral sweet spot for both distance and sonic performance, will soon disappear as a result of FCC spectrum auctions scheduled for next year. They will join the 698-806 MHz range - aka White Spaces - that broadcasters were evicted from in 2010.

With wireless audio being pushed further down in the spectrum, manufacturers are working on new product lines for that ethereal territory, but it's far from the fertile fields of the lost bands.

Six hundred to 800 MHz was where we had optimal [sonic] performance and tenability, the ability to precisely shape RF filters for maximum range and best sound quality, says Karl Winkler, director of business development at Lectrosonics. As we move into lower frequencies, we have to develop better frequency filters. But it's still a part of the spectrum that's not as easy to work with as what we had before.

Quantum5X Systems, whose PlayerMic has been written into the most recent collective-bargaining agreement between the NBA and the players association, hopes to debut its newest NBA version during three games on Christmas Day. The new bodypack, which is undergoing trials with the Toronto Raptors, is 30% smaller than the previous iteration, an achievement company CEO Paul Johnson attributes to clever circuitry and more-efficient power allocation for its battery. However, it will be the first in its line to operate natively in the 500-600 MHz band. One collateral result of that is a longer antenna, necessitated by the lower-frequency range. Older models made for the 600-750 MHz range needed antennas that were about 3.5 in. long; a bodypack transmitter intended to work in the 500 MHz range needs one that's closer to 5.5 in. long. Taking that down to about 450 MHz - a destination that some manufacturers fear is the next step in the seemingly relentless spectrum sell-off - would require a 6.25-in. antenna.

The overall length of the transmitter is only 2 in., says Johnson. As the antenna gets bigger, it starts to look a bit foolish.

Higher frequencies may be an alternative, but they have their own problems, including occlusion, with the wearer's body blocking the radio waves as the athlete twists and turns on the court. This is pushing product design to focus more intensely on antenna development. But, compared with tweaking battery designs and power allocation, antenna innovation can be costly, given the low volume of manufacturing inherent in these types of products.

What it comes down to is that we have a growing market for wireless devices of all kinds but that's happening in an increasingly compressed spectrum, says Johnson. We're being pushed into looking at solutions that are not conducive to keeping the product as small as it needs to be.

Adversity Spurs Innovation

Spectrum issues have compelled wireless-systems manufacturers to reevaluate bodypack designs. One outcome of that is that digital audio signals are finding their way onto the playing field. Digital wireless had been historically avoided by field engineers concerned with the format's tendency to disconnect abruptly, versus an analog signal's more forgiving tendency to incrementally fade out as it reaches fringe coverage areas, offering engineers a kind of warning track before complete signal failure. According to Shure Senior Director of Marketing Stephen Kohler, spectrum pressures have made engineers take a second look at digital systems, such as Shure's ULXD and Axient system, which can offer the ability to compress more channels into less spectrum.

Digital does tend to act in a binary fashion; it can be either on or off, [with] little in between. Kohler acknowledges. But we've incorporated DSP that can address that to a significant degree by sensing potential interference and automatically switching channels on the bodypack.

Shure and others are also nudging the industry to accept rechargeable lithium batteries, which can be smaller than conventional disposable batteries, by incorporating better power management into the same DSP packages that are making digital wireless systems more attractive in this spectrum-constrained landscape. That, however, runs up against many field engineers' longstanding preference for carbon batteries that are changed before each use.

User-Cost Issues

In short, the looming issue of spectrum consolidation is compelling some fundamental and potentially radical technical changes in what had been a relatively mature pro-audio category. But it's accompanied by a battle on another front: after having had to purchase new wireless systems designed to work in the 600 MHz range after the big spectrum reallocation three years ago, wireless customers are now looking at having to do the same thing all over again, to have systems that will work the 500 MHz and lower ranges.

Sennheiser has publicly petitioned the FCC to create a compensation formula that will mitigate the cost of this transition for professional users. Other manufacturers have voiced support for that and for other ways to address customer concerns. For instance, in a statement filed with the FCC on Nov. 7, Audio-Technica stated, A-T agrees with Sennheiser that the commission has the authority to requi

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