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[펌] 무선시스템의 미래에 대해..

2008.08.24 02:18

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First, the perils. Yes, there are unsettling changes coming to the world of wireless, and the effects could be felt by early 2009. The Federal Communications Commission (FCC) plans to reallocate significant portions of “white space” frequencies currently used by wireless microphone and in-ear monitoring (IEM) systems. However, this impending action does not necessarily mean church wireless systems now using the endangered frequency bands will be shut down by the Feds or encounter severe interference. In many localities, the systems could function unhampered for years to come. Churches elsewhere, though, could encounter immediate problems. 

Therefore, the time to start planning for a future in flux is right now. Prudent church technical teams will want to take stock of all wireless gear, check frequencies against the FCC reallocation plans, and be prepared to replace some older and less frequency-agile systems just in case. 

To assess the issues facing wireless users, Church Production Magazine surveyed a sampling of manufacturers of wireless microphone and IEM systems. Following is their analysis of the current state of wireless, with suggestions as to how churches can sidestep potential problems while leveraging the performance advantages of newer technology. 

Collision Course?
From one point of view, it looks like we’re on a collision course as the increasing demand for high-performance wireless devices runs headlong into a limited resource: the available radio frequency (RF) spectrum. 

“A typical church that once used one or two wireless devices now may use more than a dozen,” observes Marketing Manager Steve Savanyu of Audio-Technica in Stow, Ohio, “and they may be using IEMs as well. Manufacturers have to find solutions that work in the crowded spectrum while providing users with audio quality approaching that of a wired microphone.” 

Complicating matters is the fact that the limited resource—RF spectrum—can be a political football, notes Mark Brunner, managing director with Shure Inc. in Niles, Illinois: “As much of the world transitions from analog to digital television, elected officials and some high-tech manufacturers are calling for new uses of the UHF TV spectrum, primarily for broadband service deployment. The drive has been fueled by a general misconception that the ‘white spaces’ between the TV channels are vacant, coupled with high consumer demand for wireless devices of all kinds.” 

According to Dawn Birr, product manager at Sennheiser USA in Old Lyme, Connecticut, there is a crunch coming as “the shrinking spectrum runs into customer desires to run more wireless channels. Manufacturers will have less space in the TV band to operate, so our challenge is to work smarter and more creatively.” 

Certainty and Uncertainty
Wireless audio devices using the UHF spectrum face two different issues. The outcome of one is certain; the other is still open to debate, compromises, and new technical solutions. 

“For some time we have known that the 700 MHz band—technically from 698 to 806 MHz—will be auctioned off for new services,” says Brunner. “Shure has stopped producing products for use in the United States in this part of the spectrum, as have most other manufacturers.” Users with existing devices in this frequency range, corresponding to TV channels 52–69 should be aware of an increased possibility of interference beginning in February of 2009. 

In the remaining TV spectrum (channels 2–51), we have both good news and bad news. The good news is that more “white spaces” will open up as analog TV transmitters go off the air when the transition to digital is complete. The bad news is that there may be increasing competition for use of these same open frequency bands. 

“The FCC is currently considering proposals that allow unlicensed devices in open channels,” observes Birr. “These include broadband services, PDAs, cell phones, and home networking devices. These devices will be potential sources of interference for wireless mics, IEMs, and intercoms.” Currently, legislation in Congress is pending that would require any such new devices to automatically detect other devices in range using the same frequency and shift to another frequency to avoid interference. Although preliminary tests of some early “frequency-sniffing” devices failed, legislation may require that frequency-avoidance technology actually work before the new devices can be sold. 

Accentuating the Positive
Despite the challenges, manufacturers are optimistic that the remaining UHF “white spaces” will accommodate all but the most severe demands. 

“There will be greater challenges in getting very large systems to work interference-free,” admits Mike Torlone, director of marketing services with AKG Acoustics in Northridge, California, “but we are confident that most current-generation systems will be usable long into the future.” 

“As technology continues to improve, we’re seeing new designs that can accommodate more channels,” says Karl Winkler, director of business development for Lectrosonics in Rio Rancho, New Mexico. “The wild card is the demand for more channels. Still, I find it amazing that, at least up to now, very large-scale systems with mixed types of signals have been coordinated to a high degree of performance. However, with parts of the spectrum gone, it will be interesting to see how things like the Super Bowl will be handled in the future.” 

To address the problem, manufacturers are applying more attention to technologies that can pack more channels into the same bandwidth. “Future needs will require a more sophisticated product with very tight upfront RF filtering and tuning agility,” says Birr. “For example, our new 3000 and 5000 series systems can be tuned within a 36 MHz window of spectrum, and then further tuned in 5 kHz steps. And if necessary, that 36 MHz window can be moved to a more user-friendly part of the spectrum.” 

Beyond Channel 69
One obvious solution is to rise above the crowded and contentious UHF spectrum and seek solutions on higher ground—up in the Gigahertz spectrums. Two manufacturers, Audio-Technica and Sabine, have moved in that direction. 

“By international agreement the 2.4 GHz band is available for low-power devices,” relates Rob Rothschild, sales and marketing director with Sabine in Alachua, Florida. “Using this band for wireless mics makes sense because one set of frequencies works anywhere in the world, and you don’t have to worry about existing for future TV channels. Only low-power devices are allowed in this band, so any interfering devices would have to be very close by in order to cause trouble.” 

As the only wireless microphone maker using the 2.4 GHz band, Sabine has carved out a niche market. But this “no hassle” spectrum does have its own limitations. “You do have to pay a bit more careful attention to line-of-sight issues,” admits Rothschild. “But extension antennas can be used if the receivers have to be locked away in a closet rack.” 

Audio-Technica uses the 6 GHz band for its new SpectraPulse systems, but so far the company is marketing the technology only for conference room applications, and not for performance systems. “With our license we are limited to a 75-foot transmission range,” explains Savanyu. 

Other manufacturers, at least for now, are focusing efforts on the remaining UHF spectrum, as they foresee problems with overcrowding in the Gigahertz ranges as well. “The tradeoff with 2.4 or 6 Gigahertz systems is in limited operating distance with practical antenna systems and current power limitations,” contends Winkler. “And if you go digital, then you have the latency issues tied to the A-D and D-A process. And competing signal issues may get worse as more and more devices utilize this spectrum.” 

Stay Agile, Informed, and Involved
All manufacturers surveyed agree that the best way to handle the coming changes is to stay informed and alert, and also to incorporate as much flexibility as possible into your microphone and monitoring systems. On one hand this can mean investing in advanced, frequency-agile wireless technology. It could also mean investing in new microphone cords. 

“As I always say in my church presentations, ‘If it doesn’t have to move, consider wiring it down,’” says Savanyu. “Avoid using wireless mics unless the performer needs to move freely around the stage.” 

But if you need to cut the cord, pay attention to basics and pay for what you need, advises Winkler: “Users should remember that thoughtful wireless system design and operation will trump just about anything else. It is very important for designers, installers, and end-users to understand the fundamentals of RF propagation, antenna system design, audio gain structure, and frequency coordination. Also, you generally get what you pay for in terms of equipment. There are no magic bullets, only tools of varying quality for specific applications.” 

Torlone points out that attention to “green” issues is important as well. “Battery life and charging solutions affect overall cost and user convenience. Also, coming ‘green’ legislation may make it illegal to throw batteries in the garbage, so rechargeable solutions make a lot of sense.” 

Brunner emphasizes that citizen involvement can contribute to long-term solutions: “UHF spectrum policy decisions are a work in progress, and commentary from all sides is welcome.” More information and links for sending comments to the FCC and legislators is available on the Shure and Sennheiser websites. 

Finally, Birr reminds us that, among the growing horde of low-power wireless devices, professional audio systems remain unique. “These systems need to be real time, full audio bandwidth, have no drop outs, and operate with widely fluctuating field strengths at the receiver. No cell phone does that.” 

Also in the Mix: DMX and Video
The focus of this discussion has been on audio applications as this is where the technical demands are high and changes are in the offing. But wireless applications are also expanding in church lighting systems and, to a limited extent, in video systems as well. 

In lighting, we’ve seen burgeoning growth in the availability of wireless DMX controllers. Obviously the greatest benefit is in temporary systems or in applications where lighting instruments are in far-flung locations. But certainly churches that do special events on a regular basis might consider some wireless DMX as well. Most systems use the 2.4 Gigahertz band, and operation is fairly immune to any kind of interference, largely because the data density required to control a lighting instrument is only a fraction of that needed to carry a full-bandwidth audio signal. Therefore, operation is quite robust, and costs relatively low. 

The term “wireless video” may seem redundant, and the most common video—broadcast TV—is inherently wireless. The new technology here is, of course, wireless streaming of a digital network carrying video signals. Again, most systems use the 2.4 or 5 GHz bands and can transmit up to 1,000 feet line of sight or up to 150 feet through typical walls. These systems are primarily for remote signal distribution (as in sending signal to the fellowship hall or cry room), as the inherent latency can be problematic in production applications.

Bruce Borgerson is sole proprietor of Wavelength Communications, a writing and consulting firm for the A/V industry. He is also volunteer technical systems supervisor at the First United Methodist Church in Medford, Oregon.