14Wireless mics and the fight for stage spectrum

The short version: In the US, most wireless microphones and in-ear monitors live in the gaps between TV stations, as second-class tenants who must protect broadcasters. Licensed users get more power under FCC Part 74; everyone else is held to 50 mW in the TV bands. Twice, starting in 2010, the FCC has sold chunks of that TV spectrum to mobile carriers, first the 700 MHz band and then most of the 600 MHz band, and mics built for those ranges are now illegal to operate in the US. At a Super Bowl, roughly a thousand frequencies are reported in use, and coordinators plan them as a set.

Who is allowed to put a microphone on the air?

What you see: A belt-pack transmitter clipped to a presenter, a handheld mic with no cable, a performer with a molded earpiece.

A wireless mic is a small radio transmitter, and in the US it falls into one of two buckets. The licensed bucket is FCC Part 74 Subpart H, which calls these devices low power auxiliary stations (LPAS). Eligible users can get LPAS licenses for unused TV channels, but even licensees "only have secondary status" and must protect primary broadcast operations.²⁵⁰

Eligibility used to be narrow: broadcasters and film and TV producers. In 2014 (FCC 14-62) the FCC added "large venue owner or operators" and "professional sound companies" that routinely use 50 or more LPAS devices at major events.²⁵⁰ A church, a school theater or a wedding band will usually be in the other bucket.

That other bucket is unlicensed operation under Part 15 of the FCC rules (section 15.236), the same framework covered in unlicensed. Unlicensed mics run at lower power, may not cause harmful interference, and must accept interference from others.²⁵⁰ An in-ear monitor (IEM), the wireless earpiece that sends a performer their own mix, follows the same rules; it is a transmitter too, pointed the other way.

The practical difference is power and standing. A licensed user has some recourse if someone stomps on their frequency. An unlicensed user has none.

What is the 50 mW rule?

What you see: "50 mW" printed on a spec sheet or in a manual's regulatory notice.

Power here is measured as EIRP (effective isotropic radiated power): the transmitter's output including antenna gain, counted as if it radiated equally in every direction. The FCC's Office of Engineering and Technology laid out the limits in a November 2017 presentation:²⁵¹

Fifty milliwatts is 0.05 watts. The licensed tier's 250 mW in the UHF TV band is five times that, which is one reason large productions bother with the license.

The sources disagree slightly on when unlicensed mics arrived. Wikipedia says a January 15, 2010 order let mics in the core TV band operate "up to 50 mW without a license" under a waiver of Part 15.²⁵² The FCC's own topic page says unlicensed mics have been allowed in the TV bands under rules adopted in 2015.²⁵⁰ Both can be true - a waiver first, formal rules later - but the FCC page is the one to trust on the current rules.

What frequencies can a wireless mic use in the US today?

What you see: A frequency readout on a receiver rack, often a number between 470 and 608 MHz.

According to the FCC's wireless microphone page, usable spectrum is:²⁵⁰

• TV channels 2-36, everything below 608 MHz, on channels no local station is using
• 614-616 MHz, unlicensed (part of the 600 MHz guard band, a buffer slice that keeps neighboring services apart)
• 653-657 MHz, licensed, and 657-663 MHz, unlicensed (the duplex gap, the unpaired slice between mobile uplink and downlink)
• Licensed bands outside TV: 169-172 MHz, parts of 900 MHz, 1435-1525 MHz, and 6875-7125 MHz
• Unlicensed options outside TV: 902-928 MHz, 1920-1930 MHz, and parts of 2.4 GHz and 5 GHz

The TV-band list moves from city to city, because which channels are "unused" depends on which stations broadcast nearby. A mic that works cleanly in one town can land on top of a TV station in the next.

What happened to 700 MHz wireless microphones?

What you see: An older mic system with a label in the 698-806 MHz range, often sold cheap secondhand.

The first eviction came in 2010. The FCC prohibited wireless mics in 698-806 MHz under FCC 10-16, "Revised Rules for LPAS stations operating in the 698-806 MHz Band."²⁵⁰ The Report and Order was issued January 15, 2010, and "no longer allows Broadcast Auxiliary Service devices to operate in the 698-806 MHz portion."²⁵² The band went to mobile broadband and public safety.

The FCC set June 12, 2010 as the deadline: users of 700 MHz mics had to "retune or replace their equipment" and "cease operations" by that date.²⁵³

Is my 600 MHz wireless mic illegal?

What you see: A receiver that tunes somewhere between 617 and 698 MHz.

Probably, in the US. The second eviction followed the FCC's broadcast incentive auction, which ended April 13, 2017 and started a transition period of up to 39 months that ended July 13, 2020.²⁵⁴ Mics on 617-652 MHz and 663-698 MHz had to stop "no later than July 13, 2020," and making, importing and selling such mics is now prohibited.²⁵⁴ In places where a 600 MHz wireless carrier switched on earlier, mics had to stop earlier, "in any areas where a 600 MHz wireless licensee has commenced their operation."²⁵⁵

The FCC phased it in. From January 13, 2018, no new 600 MHz-capable LPAS devices could be certified; from October 13, 2018, legacy 600 MHz-capable Part 74 devices could no longer be sold.²⁵¹

The industry pushed back on the details. Sennheiser filed a petition for reconsideration in December 2015.²⁵⁶ In a 2017 Order on Reconsideration, the FCC let legacy 600 MHz gear be modified rather than scrapped and adopted an ETSI-based emission standard; Sennheiser called it "a victory for customers" (July 14, 2017).²⁵⁶ Joe Ciaudelli, Sennheiser's Director of U.S. Spectrum Affairs, called it "a very positive outcome for our customers."²⁵⁶ The same order optimized 169-172 MHz for more mics and clarified that the 30 MHz limit in 1435-1525 MHz applies per operator.²⁵⁶

The surviving slivers inside the old 600 MHz band, the guard band and duplex gap, are where the 20 mW limits in the table above come from.

Who coordinates frequencies at the Super Bowl?

What you see: A broadcast compound full of trucks, referees with mics, coaches with headsets, and a person with a spectrum analyzer and a very long spreadsheet.

At a big event, a frequency coordinator gives every wireless mic, IEM, intercom and radio its own frequency so nothing collides. The reason is not only crowding. When two or more transmitters mix, they create unwanted extra signals called intermodulation products, so frequencies have to be chosen as a set, not one at a time.

For football, this work is done through the Society of Broadcast Engineers (SBE) and the NFL's Game-Day Frequency Coordination program, which began with the Super Bowl: before Super Bowl XXX, NFL Films asked SBE coordinator Karl Voss to take part.²⁵⁷ Each NFL venue gets a volunteer coordinator, and a game-day database "can contain hundreds of frequencies," covering referee mics, IFBs (cueing earpieces for on-air talent), coach-to-quarterback links and security radios.²⁵⁷ Enforcement rests on voluntary cooperation backed by the threat of pulling credentials; SBE's formal authority covers only Part 74 broadcast frequencies.²⁵⁷

The numbers keep climbing:

Large events can ask for more room than the standard rules allow. Sennheiser's coverage of the 2017 order notes that operators needing more than 30 MHz should seek a special temporary authority (STA), and venues wanting 941.5-944 MHz should consult the local SBE coordinator first.²⁵⁶

What about wireless lighting control?

What you see: A small box with a stubby antenna on a lighting truss, or a battery-powered uplight with no cable running to it.

That is usually wireless DMX, the lighting-control signal sent by radio, and it commonly rides the unlicensed 2.4 GHz band alongside Wi-Fi and Bluetooth. The FCC also lists parts of 2.4 GHz among the unlicensed options for wireless mics, which says something about how many things share that band.²⁵⁰ The details of lighting control and show networks belong to the sister guide: The Show File: show networks.

Where do the mic makers come in?

What you see: A frequency chart in a mic's manual, or a web tool asking for your ZIP code.

The big manufacturers publish band charts by country because mic bands differ by jurisdiction, and both Sennheiser and Shure were active at the FCC during the 600 MHz transition. Sennheiser's petition and its reaction to the 2017 order are covered above.²⁵⁶

Shure's own guidance explains that two carriers generate third-order intermodulation products at (2 x f1) - f2 and (2 x f2) - f1, and notes that "most manufacturers recommend a minimum margin of 250kHz" between any such product and a tuned carrier.²⁶¹ Shure tells users to scan for open frequencies with the receiver, and calls coordination software such as its free Wireless Workbench "by far the best method, especially when using multiple wireless systems."²⁶² It also warns that "the environment changes constantly," so a fixed installation still needs rechecking.²⁶²

In Boston

The FCC's rule that mics may use only TV channels no local station occupies means the usable list in Boston is set by Boston's TV stations, not by the national table.²⁵⁰ Anyone running a show in the city works from that local list, the 600 MHz slivers, and the bands outside TV.

On the football side, the SBE/NFL model assigns a volunteer coordinator to each NFL venue.²⁵⁷

Elsewhere in the world

The US model is not universal. According to Wikipedia's survey of wireless mic licensing:

• UK: Ofcom licenses wireless mics as PMSE (programme making and special events). Channel 38 (606-614 MHz) is licensed "on a shared basis," and users coordinate among themselves.²⁵²
• Australia: a class license from ACMA allows mics up to 100 mW EIRP in 520-694 MHz on unused TV channels, with no individual license needed.²⁵²
• Europe: licensing is coordinated through CEPT and its Electronic Communications Committee (ECC), and "in many other countries" mics require a license.²⁵²

A US-market mic carried abroad may be illegal there, and the reverse is also true. The UK and EU have had their own digital-dividend clearances on their own timetables, and ISED in Canada and MIC in Japan set their own rules, so check the national regulator before taking a system abroad.

Report an error in this section