Wireless microphones are prone to interference, noise, drop-outs, and many other, and more severe, RF problems. These problems can be disastrous for both live productions and installed systems. Everyone remembers an embarrassing time when a wireless mic suffered harsh static or intermittent dropouts. Malfunctions lasting even a fraction of a second can destroy presentations and performances, while making everyone involved as crazed as a pack of feral hogs. Below are the three most common problems, and a few basic techniques to solve them.
1. Signal Dropouts and Multi-path Interference
Multi-path interference is caused by portions of RF energy arriving at the receiver's antenna at slightly different times. As a transmitted wave spreads, it encounters surfaces that reflect or absorb different parts of the wave. As these waves bounce off of and around surfaces, they arrive at the receiver at slightly different times and thus out of phase—creating dropouts and dead-spots for wireless microphones. This is a common problem for wireless microphone systems indoors, where there are a multitude of opportunities for RF to reflect off surfaces.
Multi-path is particularly problematic for microphones since they are usually constantly in motion and changing angles. So before showtime, diagnose your performance space for multi-path by simulating the motion of the talent, and walk around the entire area with a mic while monitoring RF signal level and audio at the receiver position.
Image courtesy of Lectrosonics, via Craig Smith
Curing multi-path interference is usually easy. The diversity microphone receiver was designed to reduce interference caused by multi-path, and virtually all professional mic systems use this scheme today. They use two antennas that have different perspectives, instead of one, and employ a switching function that discriminates between the relative strength of the two signals. The odds that a null develops at both antennas is much lower than with a single antenna.
However, the design of the diversity receiver can solve one problem while introducing another: a poor signal will often cause the system to rapidly switch back and forth between antennas, leading to quick dropouts and "swooshing" sounds caused by switching noise.
If your system encounters multi-path interference the first thing to do is to re-locate your receivers—or better yet, install a good remote antenna system. A polarization diversity antenna will reduce or entirely eliminate dropouts by providing the receiver with an improved signal on both branches of the diversity switching function. This causes receivers to maintain quality signal without all the switching and signal dropouts.
2. Noise Floor and Interference
It's important to understand that all environments are a soup of radio waves. Most of that energy is caused by other electronic devices emitting stray (unintended) RF—computers, hair dryers, pencil sharpeners, lighting, power supplies—just about any powered device. And even in the remotest desert, cosmic radiation and natural radiation generated by the earth causes noise.
An RF system—like a wireless microphone—needs a sufficient signal-to-noise ratio to stay above this ever-present ambient noise “floor.” Maintaining high signal-to-noise ratio for your wireless mics is aided by isolating the signal and directing it to only where it is needed. The closer wireless receivers are to wireless mics, the better. This gives your transmitter and receiver a shorter distance and stronger signal, which presents a higher signal over the noise floor at the receiver.
Since this is not always possible with stock antennas, selecting a local field antenna like the Spotlight antenna can isolate reception and decrease the amount of noise entering the signal chain, significantly opening up even the most crowded RF environments to accommodate your system.
3. Intermodulation Distortion and Frequency Coordination
Frequency coordination is essential in wireless systems to avoid not only third party transmitters but also the harmful effects of Intermodulation Distortion (also referred to as IMD or "intermod"). IMD is the result of two or more signals passing through a non-linear device, such as a diode or an amplifier. IMD manifests as ghost signals from wireless mic bodypacks or handheld transmitters. These appear at predictable frequencies in the RF spectrum. If these ghosts are too close to one of the frequencies used by your microphone, audible distortion can result.
By carefully selecting frequencies, you can force the distortion to occur only at frequencies that we don’t care about, and avoid those interference events. This is most commonly achieved through determining an "intermod-free" set of frequencies, which can be accomplished through various software tools like Shure's Wireless Workbench, IAS, and even onboard some receivers. Stay tuned for an interesting addition to those options from RF Venue in the near future. [That option has arrived: the RF Explorer RackPRO]
The wireless landscape continues to evolve as more and more devices pile into the spectrum. With each new development the challenges faced by audio professionals multiply. Thankfully, new methods and technologies are emerging to confront these problems and narrow the gap between the performance of wired and wireless systems. We’ll cover some of these in future posts.
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