There are many issues when considering wireless systems.
Wireless is common for major events, but frequency limitations should not be taken lightly.
In a market oozing-with options for wireless microphone systems, it's relatively easy to find reliable tools you can recommend to clients who have fairly simple needs. Often, a church or a small band may only need two or three systems, and issues of interference will never surface. However, for large rental houses, whose clients might include the NFL, the NBA, reality TV shows, and major touring acts, issues of bandwidth management and frequency coordination are part of everyday life.
With the spread of digital television in the last couple of years, limitations on available bandwidth have made it tough for organizations that require 30 or more UHF wireless mic systems, particularly in markets where bandwidth is already scarce. With DTV's continued growth, it's even possible that simple systems will become compromised by limited bandwidth in the not-too-distant future.
At one time, several major manufacturers made wireless mics in fairly high areas of the band (above 750MHz), with a limited number of channels. With DTV emerging in major cities, those bands disappeared and companies had to change products to move into lower frequencies. Some, like Lectrosonics, already made several units that covered a broad range — from around 533MHz to, at one point, 802MHz.
The down side of all this, of course, is the cost of maintaining inventory for products that cover the range. If a customer calls needing units for a specific block — each block being a 25.6MHz chunk of bandwidth — and the rental company doesn't have block 24 in inventory, the customer might have to wait a month until the other blocks make it through the production line. That's an expensive prospect for any company that wants to have a broad customer base.
“You don't want to stock every conceivable frequency,” says Larry Fisher, president of Lectrosonics. “When someone calls up, you want to send them a generic unit.”
Another issue is that most wireless microphone users have never needed or wanted to license their products.
“About 99% percent of the wireless out there is not licensed,” says Fisher. “You won't find a group of musicians, who use the bulk of the wireless systems, bothering to license their units. It's a hassle — you're locked down to one location, and the FCC isn't enforcing it.”
Therefore, there could well be 100,000 wireless systems operating out there on a given Saturday, but not nearly that number of licenses. The FCC has reviewed this, and figures there is a relatively insignificant number of these users. For now, this isn't a problem because most of the equipment in the musical instrument world operates between 10mW to 25mW — so low-powered that systems generally won't interfere with other users.
In contrast, large Broadway productions and the broadcast world depend on having numerous wireless systems at their disposal. The big problem this sector runs into when it has 30 to 50 systems running is intermodulation between the different wireless systems. Each transmitter may be on a different frequency, but intermodulation between the transmitters generates additional frequencies that are weaker than the transmitter signals themselves, but still capable of causing interference. Two transmitters generate two additional signals through intermodulation, while 30 transmitters generate 870 signals in addition to the transmitter carriers themselves.
On Broadway, sound engineers get the frequencies that work for them and never deviate from those. No one will bring in other transmitters without intensive review to see if it will mess up what's already in place.
Often, people end up having to do frequency coordination using a computer program to find 12 or 15 frequencies that can operate in a 6MHz to 10MHz bandwidth. The program will go through all the possible iterations and come up with 12 frequencies that will operate. I asked Larry Fisher if intelligent systems could help do the calculations and avoid intermodulation issues.
“The problem is that an intelligent system would have to be tied to a central computer reviewing all the frequencies and doing trial-and-error,” Fisher explains. “That would be almost impossible without having a glitch or a delay. For big systems, it gets very complex, and requires someone at a computer doing all the coordination.”
Having a frequency coordinator is a necessity in the age of DTV. Judy Rice, rental manager at Systems Wireless in Herndon, Va., works with major clients like the NFL and the NBA, and says she has noticed a change in the past couple of years.
“Often we provide technicians onsite, and now, when we get asked to provide an aide, the client asks if it's someone who can do frequency coordination,” says Rice. “Just a few years ago, an event like that never had a coordinator. People could scroll through and know they could find an open channel. Now, it's certainly a lot more difficult than that. For example, I have a lot of wireless mics going to the (February) NBA All-Star game in Atlanta. They've hired a coordinator who only works as the RF clearinghouse to keep people out of TV channels, and from interfering with each other.”
The relatively short time it's taken for bandwidth to disappear has taken some clients by surprise.
“I think a lot of people were blissfully ignorant that their wireless would work,” says Rice. “They knew to get frequency-agile systems, especially if they were with an entertainer that went city-to-city. And they knew in some cities there was some RF present. They didn't realize it was because of sharing the airspace with TV. Now, as they travel and it gets harder and harder, they might call any of the major companies and say, ‘OK, why doesn't my gear work?’ And sometimes, the answer is, ‘In that particular market, there's nowhere you can go with that gear.’”
But in the Internet era, the struggle to find open frequencies in tough markets has become a little easier, thanks to websites like www.100000watts.com. Engineers maintain this site, and it is a regularly updated resource providing a comprehensive list of the latitude and longitude of radio and TV stations nationwide.
Bob Wigley, purchasing manager of Wexler Video in Burbank, Calif., recommends these websites, which have proved invaluable to his clients. He says such resources can help anyone grappling with frequency issues, whether they are working on a live event or a broadcast event.
“They can even show things like a tower location,” says Wigley. “For instance, in the Los Angeles area, where we can be on the other side of Mount Wilson going toward Palmdale, you can be on the same frequency as a digital TV station, but you won't get hit with it because of the positioning of the tower.”
Even with frequency-agile systems, the issue of shrinking bandwidth won't go away. Fisher says that in recent years, people have avoided some of these problems in exotic ways. In one example he cited, a company used a cavity resonator and found a frequency just below a sub-carrier on a TV station. It operated the wireless on that channel by taking advantage of the hole in the signal. The company couldn't change frequencies, but it worked. Of course, it was also illegal, and we're certainly not recommending that solution.
While it's not entirely clear what the next few years will bring as bandwidth gets swallowed up, it's fairly certain that the FCC won't be setting aside bandwidth for wireless use. For now, companies that cater to clients who use numerous systems in tandem will have to continue to develop and implement filtering systems to deal with frequency interference, as well as add circulators/isolators to transmitters to handle intermodulation between transmitters.
While this shouldn't dissuade anyone from going wireless, it's best to go in with your eyes open.
Alex Artaud is a writer and independent engineer living in Oakland, Calif. He can be reached at firstname.lastname@example.org