Wireless NationSeveral years ago, the introduction of digital television, along with the auction of bandwidth to digital cellular services, heralded major changes in the American RF environment. 1/01/2004 7:00 AM Eastern
Several years ago, the introduction of digital television, along with the auction of bandwidth to digital cellular services, heralded major changes in the American RF environment. Responsibility for more than a few channels of wireless in a U.S. metropolitan area is now a recipe for trouble, and the radio congestion continues to get worse.
The airwaves of New York and Los Angeles are determining factors in which wireless products will go on tour today, and as more digital TV stations come online during the next five years, the situation will become increasingly complex. Manufacturers such as Shure and Sennheiser have made a concerted effort to educate users about frequency coordination and potential problems, but the typical audio user of wireless mics can no longer simply turn them on and hope for the best.
The great majority of professional audio engineers remain incidental users of wireless technology, but there are a handful of wireless specialists to help guide our industry through the miasma of FCC regulations and potential hot spots. Mix caught up with ex-Navy ECM technician and RF guru James Stoffo in Florida. Stoffo takes RF very seriously, and he is known for leaving his cell phone turned off. He emphasizes that a few simple procedures can vastly improve your odds of wireless success.
He begins by reminding me that the main problem with RF is that our senses are not naturally tuned to it, as the radio band, between 10 kHz and 300 GHz) falls above sound waves and below light. “Often, your first indication of an RF problem is when your wireless mic clamors through the P.A. in a loud burst or goes mysteriously silent,” Stoffo says. “We never had big creatures that chased us and emitted RF before they pounced on us. You can't see it, can't hear it; you just have to know that it's there and understand how to manipulate it.”
Pro audio is really only concerned with a small slice of the radio band, and it's getting smaller every day. The UHF band, where nearly all professional wireless mics operate these days, starts at 470 MHz and goes up to 806 MHz (TV channels 14 through 69, with 37 saved for radio astronomy). Less than a decade ago, there were fewer than a dozen UHF stations in most areas, leaving more than 75 percent of the UHF band wide open.
Things started changing five years ago. The introduction of digital television meant the adoption of parallel transmissions, where the analog TV broadcasts continue and a second digital transmission is brought in on a new frequency. Every analog TV station will eventually be duplicated by its digital broadcast on a second frequency, though the FCC imagines that sometime later in this decade, we'll all have purchased DTV sets and the analog transmissions can then cease. In the meantime, there are more UHF TV broadcasts every month.
As of this writing, half of the TV stations have begun digital transmissions. The other 800 of the 1,600 total TV stations in the U.S. can be expected to begin digital transmission on new frequencies during the next couple of years. Future problems may arise, as the FCC's reallocation of the upper UHF band will force TV broadcasts in America below 700 MHz.
RF WAR GAMES
One of Stoffo's many high-profile gigs is RF coordinator at the Super Bowl, where last year there were more than 1,000 frequencies in use. And it wasn't just audio, but all sorts of wireless intercom, emergency, security, video and broadcast services. Long before kickoff, a day is set aside for what Stoffo calls “war games,” where users turn on all of the wireless equipment used by all of the departments to find out how it interacts.
“You don't need a fancy RF analyzer or even a scanner to find intermods,” Stoffo points out. “It's very easy: Turn all the transmitters on and then turn each transmitter off one at a time.” If the other transmitters are causing intermods on that one frequency, then they'll show up as an RF signal at the turned-off transmitter's receiver. This is the essence of RF war games — a real-world reality check to see which systems are interfering with others.
When there's enough interference on an RF system, and when that transmitter's signal fades or drops out, that interference can be hot enough to pass a burst of noise into the receiver and the sound system to which it's connected. This can happen in several ways: when someone touches the antenna, when the battery dies or when the transmitter enters a dropout location. And it doesn't have to be on a receiver's frequency, just nearby. Any RF that falls within a receiver's passband that's hot enough can open squelch and pass noise. The noise is typically 20 dB hotter than the signal it replaces, so it's usually a “showstopper.”
Even when these intermod products don't fall right on top of a receiver's frequency, they reduce the signal-to-noise ratio of a wireless system on a nearby frequency because that signal desensitizes the receiver's front end, reducing the sound quality of the wireless transmission.
Stoffo also points out that the likelihood of intermods is increased when two transmitters are in close proximity. “When two actors with body mics hug or kiss, the signal from RF number 1 gets into RF number 2, mixes with its signal, and retransmits not only an intermod, but also an intermod of an intermod, as the intermod itself gets back into the other transmitter,” he explains. “This is the RF equivalent of pointing your vocal mic right into the floor monitor.”
The worst-case situation for multi-unit systems is typical of how many engineers start up their wireless mics to check them: Turn them all on and set them down in a row on a section of the mixing board, which also happens to be near the receivers. When you turn on your mics, separate each transmitter from the next by a couple of feet, perhaps putting them on widely spaced stands, and keep them 15 to 20 feet from the rack of receivers or the antennae so that they don't overload the receiver's front end.
If it isn't already apparent, setting up a wireless system requires you to adhere to the “an ounce of prevention is worth a pound of cure” maxim. And as we head into the next five years, Stoffo says, the ABCs of wireless remind us that attention to the basics is the solution to many RF problems.
A IS FOR ANTENNA
Experienced engineers who just use the whip antennae supplied with each receiver are careful to arrange the two antennae of a diversity wireless system in a 90-degree V shape. Radio signals travel through space in a particular plane that is determined by the transmitter's antenna. “Imagine a rap artist who holds their handheld mic horizontal to the floor. That RF signal is traveling on a horizontal plane,” Stoffo describes. “We say that the electrical component of the electromagnetic wave is horizontally polarized.” He then refers to the concept of “aperture,” explaining that if an antenna on the receiver is parallel to the transmitter's antenna, then the transfer of energy from one to the other is maximized. If your diversity system only has whip antennae, keeping them perpendicular to each other maximizes their coverage of all possible angles of the transmitter's antenna.
Besides whips, there are directional antennae that have several important benefits: They can be located remotely from the receiver, they add gain and they favor signals coming from one direction. Unless you need to receive signals from all directions, whip antennae will put your wireless system at a disadvantage. Most pro audio action takes place onstage, which means the signals are in a particular direction. This directionality can be used to attenuate interference coming from elsewhere. By adding gain in one direction and reducing it in others, the use of directional antennae increases signal-to-noise to provide improved wireless transmission and, as a result, better sound. “The only time you'll see me using an omni antenna is when I'm doing a radio sweep with my RF analyzer,” Stoffo points out. “A directional antenna typically raises signal-to-noise by over a dozen decibels.”
One common type of directional antenna is the “log periodic dipole array,” or “log” for short. It's also sometimes called a “paddle” because it resembles a pingpong paddle. These typically have a 70-degree coverage pattern, add about 6 dB of gain and can cover hundreds of MHz of bandwidth — wide enough for many different RF systems.
Usually, paddle antennae are vertically oriented on mic stands. Occasionally, someone mistakenly uses a stereo mic bar to mount two antennae on a single stand. This not only places them in the same polarization, but also less than a wavelength apart. A better arrangement would be one that orients the two paddles perpendicular to each other (by putting at least one on a boom arm) and places them at least several feet apart.
Another less-common antenna type is the Yagi, which is highly directional, like a shotgun mic, and is tuned to a narrower bandwidth, usually that of a single RF system, about 30MHz wide. It also has almost twice the gain of a log. It looks like your grandma's old rooftop TV antenna, with a single long axis and several shorter crosspieces. In problem areas, these can be used for high focus and high rejection.
A third type is the helical antenna, which is a clear plastic tube with a ribbon of copper wound around its inside like a candy cane and a circular wire mesh at one end. It has about the same gain as a Yagi but covers all 360 degrees of RF polarization at once. Stoffo's company, Professional Wireless Systems, makes helical antennae, so naturally he uses only helical on the Super Bowl, where he can use anything he wants.
Under the rule of reciprocity, a transmit antenna can be used for the same frequencies as a receiver. This is why helical antennae have proven successful as transmit antennae for in-ear monitor systems. Their ability to modulate RF through all angles of polarization makes them less susceptible to dropouts than the usual paddles or whips because they permit 100 percent of possible energy transfer no matter what the orientation of the receiver's antenna — especially important as IEM beltpacks are not diversity systems.
B IS FOR BUNNY
“The last dozen problems I've had with RF all had to do with batteries,” Stoffo says, “so I've done a lot of testing on the different makes and models. We've used [Duracell] ProCells, and sometimes they've lasted longer, but often they don't. The Energizer is the most consistent. That way, we know how many hours of use to expect.”
Lithium batteries typically last twice as long as a Ni-Cad, but are only found at professional battery outlets. Most of us are going to get AA, 9-volt or Ni-Cads from an industrial battery supplier, not a local supermarket. Stoffo does warn that though rechargeable batteries are tempting for economic reasons, at best they're only good for half as long, which may not be enough for many shows. Also, if the batteries are regularly recharged before they're dead, memory effect kicks in and you'll no longer get full use.
Stoffo recommends to always meter batteries before putting them into play and using a proper battery meter, instead of an ohm meter as it puts a load on the battery to get a correct reading. The most cost-effective tool for your wireless system is a dedicated battery tester. You only have to suffer the embarrassment of putting a half-dead battery into play once before this small investment makes sense. Stoffo also points out that once you start using a battery, it begins the process of losing its charge so that it's no longer dependable for future use. Also, many newer transmitters use a DC regulator to get the most out of the battery, which means that when they die, they die fast. Also, the onboard fuel gauges are not necessarily accurate.
C IS FOR COORDINATION
Frequency coordination is more than simply fitting wireless systems in and around the list of known broadcasters in an area. The term intermodulation describes the interaction of any two nearby frequencies to create a third frequency. Stoffo uses a color metaphor to explain: “Red and yellow combine to make orange. If you have two wireless mics and they're on the red and yellow frequencies, you don't want to try to use a third on the orange frequencies because you're already generating signals there as a byproduct of the first two.” As the number of wireless frequencies increases, the number of intermod products increases exponentially. A typical rack of eight RF units produces an additional 27 third-order intermods. This is why manufacturers have preset groups of frequencies that are carefully designed to work when multiple systems are used together. Most of that information is available at the manufacturer's Website.
Mark Frink is Mix's sound reinforcement editor.