FCC approves Wireless Multichannel Audio Systems for the US

The Federal Communication Commission (FCC) recently approved a change in its regulation that now permits operation of Wireless Multichannel Audio Systems (WMAS) in the United States. WMAS is a wireless microphone technology said to offer offers higher spectral efficiency – the ability to transmit more audio channels within a limited block of radio frequency (RF) spectrum – as well as greater interference protection to other wireless services that share the same spectrum.
In her statement during the Open Commission Meeting, Chairwoman Jessica Rosenworcel pointed out, “Unless you’re in video and audio production, the odds are you haven’t thought much about wireless microphones. But they’re everywhere. […] So when a new technology comes along with the potential to improve the efficiency of wireless microphone operations, we’re going to take a second look. That’s why a few years ago we issued a rulemaking to explore this new type of technology known as Multichannel Wireless Audio Systems or WMAS. […] they can significantly improve the efficiency of wireless microphone operations […] that’s a spectrum win-win.” (Watch the meeting here, WMAS from 27:50)

Sennheiser started development of its WMAS over ten years ago. In August 2018, Sennheiser filed a Petition for Rulemaking with the FCC requesting a modification in the bandwidth limit specified for wireless microphones by the Commission. At that time, Sennheiser demonstrated a prototype system to the Commission’s Office of Engineering & Technology (OET). During the past five and a half years, the FCC monitored further developments of WMAS technology and concluded that it justified the requested rule change.

Dr. Andreas Wilzeck, Head of Spectrum Policy and Standards, added: “We applaud the FCC for its adoption of WMAS technology. In particular, the alignment with EN 300 422, which was developed by ETSI, is a milestone for manufacturers and users of wireless microphones and in-ear monitors (IEMs). It is an important signal for the harmonization of regulatory conditions for these important global applications.”

Today, conventional wireless microphones are narrowband, i.e. limited to an RF bandwidth of 200 kHz. Each audio channel uses a dedicated, mono-directional transmitter-receiver pair tuned to a unique RF carrier. Each transmitter continuously outputs power, typically 50 mW, which raises the RF noise floor with each channel that’s added to the configuration. These multiple carriers are also prone to generating non-harmonic distortion products, known as intermodulation (IM), which can interfere with wanted signals.

The benefits of WMAS technology are accomplished by multiplexing audio channels onto a wideband RF channel, which is 6 MHz for the US in the case of Sennheiser’s WMAS technology. In this space, portable bidirectional transceivers will be able to communicate with a base station at an extremely low power spectral density. This will reportedly provide superior protection to nearby operators of other wireless devices.

Since WMAS uses a single RF carrier, the possibility of IM products is eliminated. In-ear monitor channels will be able to use, in parallel, the same block of RF spectrum as wireless microphones, further enhancing spectral efficiency. “For years customers have been asking for a single pack that is both a transmitter and an in-ear monitor receiver,” states Ciaudelli. “This will finally become possible with Sennheiser’s WMAS.”

At a request from Shure, the FCC also increased the output power to 100 mW for unlicensed WMAS systems.