Wireless microphones are ubiquitous. They’re used by broadcasters, corporate video producers, large- and small-venue sound-system owners, filmmakers and just about anyone who has a need to communicate or record audio from a microphone without a cable getting in the way. In the last 60 years, they have changed the way we look at audio challenges in field recording by allowing us to conceal microphones on actors or have a boom mic operator untethered to a camera. With high-end units, in the right conditions, wireless microphones can sound indistinguishable from their wired counterparts. But, like any technology, wireless microphone systems have their limits. And because their use is governed by the Federal Communications Commission, operators have to be aware of how to use these devices without breaking the law.
CHOOSING A WIRELESS MIC
Probably the first and most important consideration is the operating frequency of the system. Wireless microphones transmit and receive in a group of specific, allowed frequency bands of the VHF (Very High Frequency, 150 MHz to 216 MHz) and UHF (Ultra High Frequency, 470 MHz to 698 MHz) radio frequency spectrum. Systems operating in the UHF range are considered preferable for professional video, film and sound reinforcement use. Those allowed frequencies have changed over the years, as competition from wireless telephones and other consumer devices has increased exponentially. In years past, wireless microphone systems used frequencies that were in between frequencies occupied by broadcast television. However, when the transition to DTV was completed in 2009, frequencies above 698 MHz (megahertz) previously used by broadcasters were sold to commercial wireless users and allocated to public safety.
Since using unlicensed wireless systems above 698 MHz could potentially interfere with these services, it’s unlawful to transmit above 698 MHz in the United States. You may currently see new or used systems for sale that operate above 698 MHz, but those would be for use only outside the U.S. Most unlicensed wireless microphone systems that operate today use the current TV bands of Channel 2 to 51, except Channel 37, which is reserved for radio astronomy.
In May 2014, the FCC issued an "Incentive Auction Report and Order" that involved reorganizing and repurposing a portion of the existing 600 MHz UHF television band for new wireless broadband services, making some frequencies unavailable for use with wireless microphone systems. Exactly which frequencies are changing isn’t known as of this writing. Fortunately, the FCC has provided a transition period of 39 months from the issuance of the public notice of the channel reassignment, after which it will be illegal to use those affected frequencies in the 600 MHz band. So, basically, in a little over three years from that public notice, we may not be able to use unlicensed wireless systems that operate in the 600 MHz band, either. The logic is that the demand for consumer wireless devices such as phones and tablets is going to increase, and the FCC has to provide bandwidth to service those devices.
To future-proof your purchase, one recommendation is to purchase a system that operates below the 600 MHz band. Some new units provide a wider group of frequencies, some as much as three times those offered on previous models. These wide-band systems are the next step in wireless development. That should protect you from forced obsolescence for at least a few years.
A very important consideration when choosing a microphone system is frequency agility. A system that’s frequency-agile is one that can step through a range of operating frequencies within its assigned band. To further improve performance and signal reliability in the presence of interference, professional systems use a method called "diversity," where the receiver in the system is equipped with two antennas that minimize the effect of fading or reflective signal variations.