In February, the Federal Communications Commission adopted new rules permitting the operation of Wireless Multichannel Audio Systems (WMAS) in the United States. According to the FCC, "WMAS will enable more wireless microphones to operate in the spectrum available … which provides additional options when more microphones are needed. The rules will permit WMAS to operate on a licensed or unlicensed basis, while preserving the existing spectrum rights of others that share those frequencies."
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WMAS is an advanced microphone technology designed to meet the challenges of highly demanding live performance and studio applications. Compared to conventional wireless microphone systems, WMAS offers higher spectral efficiency—the ability to transmit more audio channels within a limited block of RF spectrum—as well as greater interference protection to other wireless services that share the same spectrum.
Sennheiser, which applauded the FCC's new rules, started development of its WMAS more than 10 years ago. In August 2018, the company filed a Petition for Rulemaking with the FCC requesting a modification in the bandwidth limit specified for wireless microphones. At that time, Sennheiser demonstrated a prototype system to the Commission’s Office of Engineering & Technology (OET).
Since then, the FCC monitored further developments of WMAS technology and concluded that it justified the requested rule change. “We especially thank OET, who carefully studied our claims and supporting evidence,” said Joe Ciaudelli, director of spectrum and innovation for Sennheiser. “WMAS will revolutionize the professional audio industry as well as fulfil the Commission’s goals of greater spectrum efficiency and sharing compatibility.”
Today, conventional wireless microphones are narrowband, 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 United States (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, which will provide superior protection to nearby operators of other wireless devices.
Since WMAS uses a single RF carrier, the possibility of IM products is eliminated. As a result, 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,” Ciaudelli noted. “This will finally become possible with Sennheiser’s WMAS.”
