The UHF Spectrum Problem
The UHF Spectrum Problem
By July of 2020 most of the 600 MHz band will be off limits causing problems for wireless mic deployment. Alteros has been working to address this problem with the GTX Series Ultra Wide Band wireless mic system.
The UHF TV band spectrum was home to professional wireless microphones for several decades. These mics operated in the unlicensed “white spaces” between active UHF and VHF TV stations. Most markets had plenty of white space and it was no problem to find a frequency to operate a microphone. But that is no longer the case.
Modern productions now demand significantly more spectrum than they did even 10 years ago. In addition to the mics there are wireless instruments packs, in-ear monitors, lighting controls, crew communications and more.
The last 10 years has also seen the UHF TV spectrum shrink. The FCC has repurposed 100s of megahertz for cell phones, broadband services and public safety communications. RF coordination was once only required for the largest productions. Now it is becoming mandatory for much smaller productions as well. Furthermore, it takes more skill to coordinate frequencies than it used to.
On July 13th 2020 all legacy 600MHz wireless microphones will become ILLEGAL to operate
A Background in the UHF Spectrum
Commercially available wireless mics have been around since the 1950s. Early models operated in the FM radio and VHF spectrum because of the technology limitations of the time. The practicalities of the early systems slowed their adoption. When the FCC cleared almost 100 MHz of the UHF band for cellular service in 1983, the audio industry barely noticed. UHF mics were still relatively uncommon.
The FCC repurposed another 100+ MHz of the UHF spectrum in 2010 for smart phones and emergency services communications. This had significant impact on the audio industry as UHF mic usage had become quite prevalent. Users were forced replace their 700 MHz systems. Failure to do so could result in a $10,000 fine per day per frequency. Despite this, the UHF wireless market continued to grow.
The UHF band is contracting once again and by July of 2020 most of the 600 MHz band will be off limits. The changes have already occurred in some markets and cell towers in those frequencies have already popped up. Frequency coordination, which was already becoming a challenge for large productions is becoming nearly impossible.
This trend looks like it will continue. Although controversial, the FCC is hoping to auction off the “T-band” (470-512 MHz) in 2021 with current users vacating it by 2023. Wireless mic users already avoid t-band frequencies in major cities, but another auction would make coordination even more difficult. Making matters worse, the FCC is allowing more unlicensed devices to share the remaining UHF spectrum. Consumer oriented white-space devices (WSDs) from Microsoft are now entering the market to offer broadband services. These compete with wireless mics for the little spectrum that is left. It is clear the industry needs to expand beyond the UHF band.
Other Frequency Bands
Wireless mic vendors have been producing mics for the license-free ISM (Industrial, Scientific & Medical) bands for quite some time. These bands (particularly, 900 MHz, 1.9 GHz, 2.4 GHz and 5.8 GHz) are shared with numerous consumer devices including cordless phones, DECT, Bluetooth devices, Wi-Fi and various short-range devices (SRDs). These bands tend to be relatively narrow, supporting limited channel counts, and they are already very crowded. In other words, they don’t replace the spectrum lost from the UHF TV band.
The FCC opened up the 6-10 GHz band for low powered unlicensed operation in 2002. The requirement is that the average power be below -41.3 dBm/MHz. That is less than 1/10000th of a milliwatt! That power is way too low to operate a traditional wireless mic that employs a modulated RF carrier signal. However, pulsed RF systems based on Ultra-Wideband (UWB) technology can meet this average power limitation while still providing reliable coverage. Alteros, Inc is currently the only manufacturer offering a UWB based microphone system.
Licensed Frequency Bands
Other frequency bands are available for wireless mics for users that obtain an FCC Part 74 license. There are narrow slivers of spectrum at 900 MHz, 1.4 GHz and 7 GHz for this. These licenses were once limited to broadcasters but now they are available to anyone that regularly uses more than 50 frequencies (or 100 in some cases).
A license grants one the ability to register in a database for protection against unlicensed device operation, and permits operation at higher power levels. However, the FCC doesn’t guarantee the spectrum is clear. It is up to the end user to scan the local spectrum. In fact, to use a licensed part 74 mic on co-channel TV band, users must verify the local TV signals are below -84dBm in level at the use location.
Making Wireless Spectrally Efficient
The FCC regulations limit traditional wireless microphone occupied bandwidth to 200kHz whether analog or digital. To coordinate multiple channels, each microphone may need between 300-400kHz of RF spectrum to avoid intermodulation interference. Digital modulation technologies can offer greater spectral efficiency but there is a cost of added latency.
Further efficiency can be gained by trading off quality. For instance, lowering the audio bit depth and sample rate reduces the amount of data being transmitted. This is commonly done for IFBs and crew comms but it is not popular for mics and IEMs where low latency and high quality are paramount.
The Microwave Spectrum
Microwave frequencies above 2.4 GHz are not being auctioned by the FCC and there are some traditional modulated carrier mics to be found here. However, the higher frequencies are sometimes maligned because they have higher path loss and are believed to require line-of-sight links. Wi-Fi signals operating at these frequencies, as well as mobile phones operating even as high as 28GHz get around this using mesh network topologies. Alteros’ GTX Series is the only wireless mic system currently on the market that employs mesh technology.
The ability to contain microwave signals offers some important advantages over UHF signals. If a transmission can be contained it can be reused repeatedly. More devices operating in more spaces is far more spectrally efficient. Multiple studios or performance spaces within a building or campus can reuse a frequency without much concern about potential interference. The only potential interference sources one must concern themselves with are those that originate within the same space. This is makes it an easily controllable situation when many wireless channels and locations are needed.
Alteros GTX Local Area Wireless Mic Network (L.A.W.N.)
In 2016 Alteros, Inc spun off from Audio-Technica in order to focus on the development of a 6.5 GHz wireless mic system based on UWB technology. The early adopters were primarily broadcasters desperate to find a wireless solution to cope with the shrinking UHF spectrum. The GTX Series supports up to 24 channels in each location, replacing approximately 12 MHz of UHF spectrum, before system reuse is factored in.
One broadcast campus installation has 8 systems in use within a few hundred yards of each other with just walls separating the systems. This is equivalent to having ~100 MHz of UHF spectrum for wireless mic use. It is an impressive feat and one that is now getting noticed by other markets that include corporate AV and houses of worship.