by Ryan Pedersen
The last two decades have represented a seismic shift for our RF environment. In 2008, the FCC sold off the 700MHz frequency band to the highest bidder with only a few small parts reserved for emergency services. Then again in 2017, they sold another large chunk of spectrum. TV stations and all other existing UHF users were relocated to the lower end of the band. Another auction saw the 616MHz to 698MHz bands sold, with a majority of it being bought by T-Mobile for cellular telephone expansion. Wireless microphone users were relegated to the gaps between TV stations and the very few other open frequencies of the spectrum. Our current bands to operate in with either no license or a Part 74 license are as follows:
VHF 174-230MHz
UHF 470-608MHz
UHF Guard Band 614-616MHz
UHF Duplex Gap 653-663MHz
STL Band 940-960MHz
Most of the UHF band is clogged full of TV stations, especially in major metro areas such as Los Angeles, New York, and Chicago. A few major cities also lose spectrum to the T Band, which is reserved for emergency services. In Los Angeles, that covers 470MHz-488MHz and 506MHz-512MHz. This link provides a list of all cities with T Band deployed frequencies and what those frequencies are: https://wiki.radioreference.com/index.php/UHF-T_Band.
So, for production sound specialists, the question becomes how do we fit higher channel counts into this ever-shrinking RF world even with the more spectrally efficient digital systems?
One way is to lobby the FCC for other areas of the spectrum. This is currently happening, with companies like Shure being a leading voice for the wireless community.
The other way is to use the spectrum more efficiently through new advances in RF technology. I was afforded the opportunity to sit down with Jason Waufle from Shure to discuss this new technology and hear where it will take us in the near future.
Wireless Multichannel Audio Systems (WMAS) is a technology neutral approach to broadband digital transmission defined by the European Telecommunications Standards Institute (ETSI) to employ new wideband modulation techniques to support the transmission of multiple audio links in one single wideband radio channel. In other words, it’s the next evolution in wireless transmission that can allow higher channel counts in the same amount of spectrum than we can currently achieve using narrowband transmission. It is essentially, a multichannel carrier system using different codecs or algorithms to decode the individual channels. Even though this technique is new for wireless audio transmission, it is not a new idea. This wideband transmission scheme has been used for many years for Wi-Fi, cellular, and wireless comms in the 1.9 GHz DECT and 2.4 GHz bands.
Before we get into WMAS digital systems, let’s cover what a traditional narrowband digital microphone system entails because the technology behind WMAS is all based on digital transmission schemes. Digital wireless systems convert the analog audio to a digital signal that modulates a radio carrier in discrete steps (ones and zeros). Unlike companded analog radio signals, a digital signal reaches the receiver unaffected by the radio link. This allows digital wireless systems to operate with less noise, closer to the noise floor, and with few to no artifacts. WMAS takes advantage of all these benefits and expands it to a new regulatory framework.
Narrowband transmission uses a single radio carrier per channel that is no more than 200KHz wide. WMAS would allow a wider carrier up to 6MHz wide in the TV bands in the United States, but requires that wider carriers transmit no less than three channels per 1MHz or eighteen channels per 6MHz. WMAS is not a technology, but a regulatory framework that manufacturers must adhere to when developing technology and products that use WMAS transmission. It defines the spectrum emission mask and minimum system performance requirements.
Many of the questions you might have about WMAS fall to the manufacturers’ implementation of WMAS and not to the regulatory framework itself. For example, you might need to know how many channels can fit into one carrier? What happens if there is interference to the wideband channel? Would I lose all of my channels or just a few if there was interference? Do I have to use an entire 6MHz TV channel for each WMAS deployment or can I use a smaller section of spectrum? What is the audio quality of each channel? How much delay or latency can I expect? How much range does the system have? The answers to all of these questions would be dependent on how the framework is deployed by a given manufacturer.
However, the WMAS standard can answer what frequencies can be used and maximum output power specifications. WMAS will still operate in the current available spectrum bands, including the AFTRCC 1.4 GHz band. One limit to WMAS frequency use is that each radio must only occupy one TV station (6MHz) and cannot overlap to adjoining TV channels. For example, if you have the upper 3MHz clean in TV channel 21 and need 6MHz total, you cannot use the clean, lower 3MHz of TV channel 22. In the AFTRCC 1.4 GHz, where up to 30MHz is permitted, users could operate one 20MHz WMAS system and one 10MHz WMAS system, or two 15MHz WMAS systems, etc. Unlicensed users are allowed a WMAS max system power of 50mW up to 1MHz wide and 100mW when your carrier is 1-6MHz wide. Part 74 licensed users are allowed the same power levels as narrowband carriers, typically 250 MW. Different manufacturers will implement this power in different ways.
As with any new technology, there are pros and cons to the WMAS framework. On the positive side, WMAS has the potential to offer much better spectral efficiency than we currently have with narrowband carriers. It also offers the opportunity for physical hardware efficiency as well. The ability to do more with less hardware can reduce the need for large racks of equipment, saving both space and weight for audio departments on production, on tour, and on live events. The hardware designs will be manufacturer dependent, but WMAS offers the potential for a major shift in the need for rack space.
A major advantage is the ability to deploy a high channel count. Large events such as major sporting events, large conventions, live theater, and reality TV—which all typically need hundreds and hundreds of wireless channels—will benefit greatly from the increased spectral efficiency that WMAS offers. When using traditional narrowband digital wireless in higher efficiency modes with sub 3ms delay, it’s possible to reach up to forty-seven channels in 6MHz. WMAS can offer even higher channel counts in that same space. Channels can be as wide as 6MHz in the U.S. and 8MHz in Europe. A system is not required to use a 6MHz wide carrier, meaning a manufacturer could design a system that uses only a 2MHz or 3MHz wide RF carrier as long as it adhered to the minimum channel count of three channels per 1MHz. It may even be the case in the future that a WMAS system is manufactured that could be scalable based on the needs of the user.
Another benefit that WMAS offers is bi-directional communication. While this is also going to depend on each manufacturer’s implementation, a system could transmit IEM’s and also transmit the talent microphones all within the same 6MHz carrier without any frequency separation typically created by a frequency coordinator. Frequency coordination is simplified because there is only one carrier to coordinate. While this doesn’t eliminate the need for frequency coordinators or even properly choosing a clean frequency, it puts the coordination of each individual channel in the hands of the hardware. I know a lot of us don’t like frequency coordination and it can be difficult in congested places like Los Angeles, so letting the box handle all of that will definitely make your RF life easier.
Everyone who has coordinated wireless microphone systems is familiar with intermodulation or intermod. All wireless microphone systems, whether they are analog or digital, create intermodulation. Digital microphone systems have extremely linear components in addition to transmitting at lower power outputs. Thus intermod is kept to a minimum. Intermod created by digital wireless microphone systems is typically below the noise floor. WMAS systems allow engineers to stack the transmitted signals in a system with improved emission masking and filtering, and place the intermod on the ends of the wideband carrier. Intermodulation is therefore not a factor to the end user of a WMAS system.
All these benefits sound excellent, so let’s explore some of the limitations to a WMAS system and explore why a user still might consider using a narrowband system. Due to the wideband carrier of WMAS, you will need to have a fairly clean area of spectrum to place your wideband carrier. It could require a carrier up to 6MHz wide. This may be challenging based on location and spectrum availability. You can’t place this carrier, for example, in the very small sections between digital TV stations or, if needing a full 6MHz, in the guard band. A full 6MHz wide system will be difficult to deploy in most major markets in the U.S. unless using it in the duplex gap or in the STL band or having the ability to use smaller wideband carriers.
Another limitation to WMAS is the redundancy factor. These devices will typically be built with high channel counts into one physical box. Certain things like power and the audio interface are easy to make redundant, but if the entire box went down, it would affect all of your wireless channels. Users will need to ask themselves, “How many eggs should I put in one basket?” Manufacturers will need to develop extremely robust systems, as well as offer redundancy in their ecosystems, especially for mission critical applications.
“A major advantage is the ability to deploy a high channel count. Large events such as major sporting events, large conventions, live theater, and reality TV—which all typically need hundreds and hundreds of wireless channels—will benefit greatly from the increased spectral efficiency that WMAS offers.”
An area of WMAS systems that could both be a benefit as well as a limitation is the lower RF power. Lower RF power is beneficial because it allows more systems to be physically closer together utilizing the same TV channel than a higher power system, but it probably won’t be able to have the reach that a higher power narrowband system could offer. If your circumstance requires a longer distance transmission, then a narrowband system may still offer a greater benefit than a WMAS system.
One area of development that I believe to be important is how the system deals with RF interference. If you experience interference on a narrowband system, it would typically only affect one or two channels at a time. A WMAS system has all channels sharing a single RF carrier, so the concern for interference is much greater. A potential user of WMAS will need to understand how each manufacturer’s system deals with interference. The need for a system to be able to deal with RF interference will be an important factor in the use of a WMAS system. How many interference events a system can take, how it heals itself, and how resilient is it to RF noise will all be important factors in a WMAS deployment.
Manufacturers have already started to design, test, market, and release WMAS systems. Each system will have its own design, specifications, and features. Unlike Wi-Fi products that are all compatible with each other, WMAS systems will likely not be compatible across manufacturers. Each manufacturer will have their own proprietary system based on their interpretation of WMAS and based on their own codecs and algorithms.
Shure recently announced their first WMAS product, the Axient Digital PSM, will available in January 2025. I’ve had the opportunity to explore what the WMAS deployment entails in this new product and it’s very exciting. Axient Digital PSM is a single-space rack mount digital IEM system that has both four stereo narrowband carriers, as well as four WMAS wideband carriers in the unit. Each of the four WMAS carriers is 800KHz wide with four stereo channels in each carrier. This allows the unit to have up to sixteen individual stereo IEM mixes in one RU. In combination with an AD600 spectrum manager and AD610 Showlink access point, the unit will be able to manage frequency deployment and interference events. Within each 800KHz carrier are four 200KHz wide subcarriers. If an individual subcarrier experiences interference, the other three subcarriers will not be affected by that interference. For more information, you can visit Shure’s website.
There is still a lot to learn about WMAS. It offers so many possibilities for the future of RF systems and will only continue to develop as technology advances. Each manufacturer will have their own approach, but that will just provide more tools in the toolbox to choose from. We live in challenging times for wireless microphone systems but one thing that this framework encourages is that we should all be good RF neighbors.