Wireless Microphone Problems

WE'VE ALL FELT IT — THAT UGLY, sinking feeling you get when a simple AV job mutates into an unpredictable nightmare. For many systems integrators and technicians, the addition of wireless microphones to the list of AV gear causes just such a reaction.

Getting wireless mics to work reliably sometimes seems to require equal parts rocket science and black magic. One day you can pull off a trouble-free show or presentation with dozens of wireless mics; on another, you can't even get one bodypack to behave for a two-hour sales meeting. That's why knowing how these devices work is so critical to make them work.

Let's take a closer look at 10 of the most common problems that seem to pop up in most wireless mic applications — and what you can do to avoid them.

INCOMPATIBILITY

PROBLEM: When using multiple wireless microphones, interference between the systems themselves always seems to crop up. Even if each system is on its own frequency and spaced several megahertz apart, the mics can still interfere with each other through a phenomenon called intermodulation distortion (IMD), which occurs because radio transmitters interact with each other to create intermodulation signals.

If there is not enough space (in megahertz) between these intermodulation signals and the operating frequencies of the units themselves, the receiver has a hard time picking up the signal from its transmitter. Typical symptoms include crosstalk between systems, frequent signal dropouts, poor range, or excessive noise and distortion.

The minimum separation between frequencies depends on the design of the system's receiver. An entry-level receiver may require a 1 MHz interval between the nearest adjacent system or intermodulation frequency. A more expensive receiver typically has a narrower tuning “window,” allowing for closer spacing between each system or intermodulation frequency.

SOLUTION: To avoid intermodulation distortion, select only frequencies that have been calculated to be compatible with each other. Because it requires knowledge of the design characteristics of the transmitter and receiver, the wireless system manufacturers provide these calculations.

For example, when just eight wireless microphones are used together, thousands of calculations must be performed to ensure compatibility. As a result, most manufacturers publish lists of compatible frequencies for their systems. In addition, software is available that can help identify compatible frequencies in some cases.