Fundamentals of RF Coordination for Live Sound Part 1 – The RF Environment

As live sound audio engineers, we are often required to deploy multiple channels of wireless audio equipment to satisfy the ever-growing trend of eliminating visible cables from performance areas. The job falls into our laps simply because it’s audio gear, and operating it perfectly “should” be within the parameters of our professional expertise. It seems that few people in positions to make demands for a cable-free stage fully realize how rapidly the complexity of the task increases as more wireless channels are added, how specialized a skill set the job requires, and that the time required to execute the process is dependent on the number of wireless channels required and how congested the RF environment is at the performance location, which varies wildly from location to location.

Professional audio engineers have many tools available to aid in operating their wireless equipment, including hardware and software packages. Some are very expensive; some are free of charge; some are strictly dedicated to specific brands of wireless gear. All require technical skill and deep understanding of wireless equipment behavior for maximum benefit. This becomes more evident as a user increases the number of wireless systems operating simultaneously at a given location.

Multiple wireless systems not only compete for limited interference-free frequencies in a given RF environment, but also interact with each other in ways that can cause them to generate unintended transmissions and interfere with each other. This is known as intermodulation distortion, which we have all heard so much about. It is where the real complexity in RF coordination becomes evident. Most of our better software tools are designed to calculate intermodulation products and account for their behavior in our systems. We will explore intermodulation in a future chapter because its effects can be detrimental if ignored or improperly calculated, but we must understand that it is secondary to the foundation that all of our work must be built upon: the RF environment at our performance location.

Software is only as good as the data entered into it.

Software-only solutions are convenient and can yield reliable results, but very often they are hampered by their use of incorrect or incomplete data as the basis for all of their calculations. Accurate information about the RF environment at the user’s location is absolutely required to generate useful results. Most often, the user defines his or her location by ZIP code or geographic coordinates, and relevant data is acquired through the Internet. This information is usually based on data published by the FCC for the United States as a publicly accessible database. Unfortunately, that source of information is often incorrect, incomplete, and/or inadequately utilized for our purpose.

The example here is a musical concert event at the Georgia Dome in downtown Atlanta, Georgia, in early 2012. The event required the use of 84 channels of wireless audio. Figure 1 shows a graph of the wireless spectrum to be used by wireless UHF audio equipment, which is shared with broadcast television. Frequency is plotted along the horizontal axis. Vertical violet and turquoise bars indicate television channels listed in the FCC TV database as licensed broadcast television transmitters operating within a 40 mile radius around the venue. The software assumes that black bars are channels that are free of licensed television signals and available for professional audio equipment.

Figure 1

This makes the assignment of frequencies for our audio equipment appear to be simple, and the software should be able to easily generate a clean channel plan by using only frequencies within the black bars. Unfortunately, that would be disastrous, because the information is incomplete and thus incorrect.

What?! This is one of the things that we all pay the FCC for, right?

You can rely on Uncle Sam, but only partly.

The science of radio frequency propagation is complex and relies on myriad variables to make predictions. Geographic topography and atmospheric conditions make each location at each moment in time unique. Multi path interactions with natural and manmade objects can significantly vary the RF environment on scales of a few feet. License-based predictions cannot anticipate nearby transient transmissions from unlicensed wireless equipment. It would take a massive national investment to create a new infrastructure and deploy state-of-the-art remote RF spectrum sensing hardware to operate around the clock in all corners of the country to make such complete and accurate information available to us.

When accessing the FCC TV license database through RF coordination software, it is usually up to the user to define a radius of distance within which TV channels will be selected. If we select too small a radius, transmissions from outside it that reach our location will be omitted from the data and cause interference with our gear. If we select too large a radius, unoccupied TV channels at our location will be included in the data, erroneously preventing us from using clear parts of the spectrum for our gear. It is also up to the user to determine whether to include or exclude TV transmitters within the defined radius, based on the maximum power levels that their individual licenses authorize them to transmit. Furthermore, there are many types of transmissions that occur within the UHF TV band that are never included in the TV database. These include municipal public safety licensees, Broadcast Auxiliary Service licensees under part 74 of the FCC rules, and all types of unlicensed equipment allowed by part 15 of the FCC rules. Soon they will also include TVBD’s, which is a whole other can of worms that can be the subject of an entire article. An expert user must know which FCC ULS databases to search, how to effectively conduct those searches, and how to utilize the returned data.

The most reliable way to deal with this today is to supplement database search results with on-site radio frequency spectrum analysis. This uses precision instruments to collect sets of data that accurately represent the RF environment. It’s one reason why when you work a gig that will be widely broadcast to millions of TV viewers, you should not be surprised to see a professional RF Coordinator working with a $15,000.00 Agilent RF Spectrum Analyzer (or equivalent) at the backstage RF bench. Professionals know how critically important reliable, precise, and accurate RF spectrum analysis is to successful RF coordination. Of course, we can also use this expensive lab equipment for other related tasks like fine tuning adjustments and transmission line & antenna analysis, which are a bit beyond what most audio engineers would normally perform in the field. There are many examples of equipment for RF measurement that satisfy our lesser requirements without costing nearly that much, but the prevalent use of such pricey hardware does illustrate how valuable RF measurement data is to the consummate professional.

Figure 2 shows the previous graph overlaid with RF spectrum analysis data collected at the Georgia Dome on gig day. Amplitude is plotted vertically. You can see correlations between violet bars and peaks in the yellow spectrum trace, but there are also many peaks covering black bars. Most of these represent RF signals that are absent from our FCC data and will cause audible interference with any audio equipment attempting to operate on those channels.

Figure 2

This new spectrum analysis information reveals that far less of the wireless spectrum will be usable by our equipment than the FCC TV data implies.

Note: Experienced RF Techs might assume that those peaks spanning across channels 23 and 24, channels 36 and 37, and part of channel 38, are artifacts created by our measurement hardware (known as images) because they do not conform to FCC rules for TV channel allocation, yet exhibit typical DTV signatures. They could easily be dismissed as intermodulation products generated in the front end of our spectrum analyzer. However, tuning our wireless microphone and IEM receivers to those frequencies revealed significant interference and verified that those off-band signals are real enough to be treated as such for our purposes.

Figure 3 shows that we have set a threshold, represented by a horizontal red line. Every place where the yellow trace rises above the line indicates a signal which would cause interference with equipment tuned to that frequency. White vertical bars are areas of the spectrum that are truly open for reliable operation of our gear. You can see that the white spaces available to pro audio users in Atlanta are severely limited in comparison to the initial assumptions made by our wireless coordination software.

Figure 3

Now that we have determined the real UHF white space available for practical use, we have a good baseline for the rest of the RF coordination process.

It is worth noting that when we perform a band scan with our wireless microphone or IEM receiver hardware, we are effectively performing a rudimentary form of spectrum analysis and signal-threshold-based channel assignment that is very similar to the above example. Even better, when we perform a group/channel scan, the results from most of the higher-end hardware will be intermodulation coordinated within that group. The main drawback of relying on those results is that each band of equipment will not be intermodulation coordinated with all of the other brands, bands, and types of equipment that might be in use within the performance area. Even a single uncoordinated frequency can severely compromise the entire frequency plan.

But wait, there’s more!

An important thing to keep in mind is that any time wireless interference occurs, it almost always affects all parties involved in a negative way. Whenever our equipment experiences detrimental interference caused by another transmitter, it may also cause interference to the offending counterpart. Our gear can also cause interference to other RF equipment without exhibiting any malfunction of its own. A well executed RF coordination can avoid these problems.

In setting up a comprehensive RF coordination, the best results are obtained when combining spectrum analysis, FCC database search results, and research about the venue. There are practical, legal, and ethical reasons for this.

In practical use, RF spectrum analysis can give us information about the environment at the specific point in time that the measurement was taken. However, the RF environment is dynamic. A perfect example is that changing atmospheric conditions can slightly change the boundaries of TV channel reception areas. If our performance area is on the fringe of a licensed broadcast or perhaps at low altitude down in a valley, it is better to be safe than sorry and avoid tuning our equipment to frequencies within that TV channel, even if our spectrum analysis shows low signal levels on that channel. As a very general rule of thumb, if we are located within 30 to 40 miles of a Class A DTV tower and our analyzer is showing negligible signal levels, we are probably within a fringe area. TV database search results will serve us well in making the determination.

Another aspect of fringe areas is that they can put us in situations where we inadvertently violate the FCC rules under part 15. In the aforementioned example where our performance area is located low in a valley, we may see low signal levels on a channel used by a distant TV transmitter and have beautifully clear performance on a mic tuned within the channel. Unfortunately for us, a mansion at the top of the hill on the edge of the valley uses a high-gain antenna to pick up that distant station. The owner just missed the final touchdown of the big game because we turned our mic on. Now he’s on the phone with his congressman’s office, and his complaint is far more serious than that our PA is rattling his windows! In this case we are in the wrong, legally and ethically.

The FCC ULS municipal public safety license database should ALWAYS be consulted and utilized, because human lives could be endangered by our equipment causing interference to licensees. These licensees are typically police, fire, EMS, and other emergency services. It is prohibited to operate wireless audio equipment on any licensee’s frequencies or cause interference on those frequencies within an exclusion zone defined as a 130KM (80.77 mi) radius around each licensee’s transmitter location. Since public safety radio systems can be intermittent, two-way communications, they often fail to appear in spectrum analysis data until a licensee keys their mic to transmit. One manufacturer’s newest RF coordination software version conveniently and automatically adds a public safety license database search to its geographic TV search, and excludes each entire public safety occupied TV channel from coordinations. This is very effective, but can be inaccurate in some cases and overkill in others. An example of inaccuracy is Detroit, where channels 15 and 16 are reserved for public safety licenses, yet none have actually been granted due to the city’s proximity to Windsor, Ontario and the likelihood that they would interfere with Windsor’s TV stations across the river. An overkill example might be a location in the far suburbs of Chicago, where public safety licensees use TV channels 14 and 15. If your location is near the edges of Chicago exclusion zones, some zones may cover your location and others may not. Armed with the right information, a skilled RF coordinator could utilize thin slices of bandwidth within channels 14 and 15 without compromising public safety licensees. However, it is better to err on the side of caution if you are unsure and avoid those TV channels altogether.

Ethics isn’t what country your ancestors are from.

I’d like to mention a little more about the ethics of RF coordination because everyone benefits from ethical behavior. It contributes to an orderly RF environment that we all must share. Many conscientious users don’t apply good practices because they simply lack experience or knowledge, and unfortunately some people just don’t care. As audio professionals we should aspire to not belong to either group.

Ethical behavior always starts with you– doing research about the location, including the oft-mentioned database searches, performing spectrum analysis site surveys, and contacting the venue and all acts on the playbill in advance to inquire about any frequencies that they rely on for internal communications, house microphones, IEM’s, etc. Venue staff truly appreciate this, and your job on gig day will be much easier if you do it. They will almost always have helpful suggestions about their experiences with wireless in their house and will remember your professionalism. I can personally cite the Hollywood Bowl and the Wolf Trap Amphitheater as having wonderfully capable and cooperative staff who will help you to help them.

Ethical behavior also includes reaching out to media outlets and inquiring about their plans to visit your event with a remote broadcast. Many of us have experienced that awful dropout or blast of static, followed by the Clearcom beacon blinking, followed by the squawk on your production radio that someone on your crew just spotted a local reporter blabbing away into a wireless microphone at dimmer beach, monitor world, or front of house, utterly clueless about the havoc that he’s causing. He possibly won’t realize until later that his own mic is also affected and the take is ruined. This cannot always be totally prevented, but it can be mitigated by a little advance work, participating in meetings with security staff before the show, and by posting prominent signs at venue entrances notifying all users of wireless to check in at the production office and ask for the RF Coordinator before operating. It is important to know that in the case of media entities that possess Broadcast Auxiliary licenses, they are entitled to use their frequencies under part 74 of the FCC rules and it is actually the unlicensed user who is required to cease operating. Sometimes, we have contractual protections with larger events and touring acts that allow the venue to toss unauthorized wireless users off the premises. Whether or not those agreements trump FCC rules is debatable. Implementing them certainly requires a delicate touch, because we should all be keenly aware that few performers, artists, and powers-that-be would approve of anything that might cause disdain from a media entity. Most often, a spirit of friendly cooperation will win a bloodless battle and save the day.

Help can be a phone call away.

The Society of Broadcast Engineers (SBE) has certified RF Coordinators serving many major metropolitan areas in the USA. Although their primary focus and area of expertise is licensed RF communications for the broadcast and telecommunications industries, SBE certification requires that their coordinators respond in a prompt, unbiased, and professional manner to all reasonable requests for information from the SBE RF coordination database or access to it, and they must make every reasonable effort to accommodate all parties desiring to coordinate in good faith.

There are also a handful of businesses dedicated to RF coordination services specifically for the professional audio industry, and most of these also offer sales, rental, and lease of wireless audio equipment. Over the years, they have been very helpful when I have called and asked for free advice. When you’re operating in an urban area with a congested RF environment, the larger companies will almost certainly have experience working in the area. If the performance of your wireless gear is crucial to a successful event and you feel like you’re in over your head, seeking them out and hiring them is time and money well spent.

Don’t forget that one of the most valuable sources of help is the manufacturer of your wireless gear. The R&D that they conduct in the process of designing and marketing their equipment encompasses just about every RF related problem we will ever have in the field. Their online user forums are invaluable sources of exhaustive information, and personalized help is usually just an email or phone call away.

© 2012 Jason G. Glass and Clean Wireless Audio LLC