Audio

Digital Wireless Mic Frequency Coordination Part 1: I should have known there was an app for that.

In the AV industry, the end users are typically represented by two separate yet equally important groups: the designers who specify the systems, and the integrators who install them. My company acts as a third party to commission these systems. These are our stories.

Checklist Items Under Test: 6.35: For wireless microphone systems, with all wireless microphones turned on, confirm that, throughout the specified operating area for the transmitter, there are no dropouts, or RF-caused artifacts, or intermodulation interaction between wireless systems. Also confirm that there is little or no RF activity on a receiver’s “S” meter when the designated microphone transmitter is off.

Reasoning: It is fairly obvious when there is a problem with analog signals. If audio is distorting, it sounds squared. If wireless microphones are interacting with other transmitters, the intermodulation sounds like a feedback squeal. If video signals are too strong or too weak, they appear bloomed or dim, respectively. It’s easy to interpret problems in the analog realm. Not so in the digital world: When dealing with digital wireless microphone frequency coordination, nailing down whether issues are shortcomings in reception, interference or frequency interaction gets difficult. Luckily, there are tools out there to help us.

The Story: Divisible Multi-Purpose Rooms (MPRs) are the banes of my existence. I shouldn’t complain too much because they also create some of the most interesting AV challenges. But still…a room that can morph into many different shapes and change its functionality several times in a day can rub you the wrong way. Especially the big ones. Especially the big ones with digital wireless microphones. Especially the big ones with digital wireless microphones that have to operate in low-power mode because so many channels are required. Those really get to me.

We were hired to improve the reception of wireless microphones in a divisible MPR. There were two sets of microphones: Room A Mics and Room B Mics. Each set was fed by its own antenna distribution system. So, the Room A Mics worked great in Room A. The Room B Mics worked great in Room B. However, if someone took a Room A Mic into the nether-regions of Room B, or vice versa, the reception cut out. They were simply too far from the antennas of their designated room. The solution was easy enough. Both sets of microphones just needed a common deployment of antennas that covered the entire divisible space, not just half of it. We provided the solution, it was implemented and we were ready to test the system.

We had dropouts. Not only did we have dropouts, but we had weird dropouts. Transmitters 8m away from the antennas weren’t putting sound into the room reliably. They could spit on the antennas. They were that close. It didn’t make sense. We did the calculations. We measured the reception with spectrum analyzers. This solution should work. I was baffled. And then one of our new specialists unknowingly found the issue.

She: “What does ‘Overload’ mean?”

Me: “It usually means that the RF solution is unstable and the transmitter is clobbering the receiver with radio energy. Why?”

She: “Well, the Overload LED keeps on lighting up on the problem mics.”

Me: “That doesn’t make sense. That would mean intermod and we’d hear a terrible squeal-like feedback. We’re looking for dropouts. Quit playing around back there, we have work to d…You’re a genius!”

As much as I like to think of myself as still being young, I am constantly reminded that I am not a digital native. I have too much experience in analog devices for my own good. In the analog world, if there were two transmitters that did not have their frequencies coordinated (which involves verifying that transmitters do not share harmonics of the frequencies of other transmitters in the system), they might interact with one another, causing spikes in the RF energy. These spikes would produce a loud squeal in the room that sounds a lot like feedback, and you knew to check your PAG-NAGs for feedback or check your frequency coordination for intermodulation issues. This is no longer the case in the digital world.

If a digital receiver doesn’t receive a strong, clean signal, it simply drops out the audio. You won’t hear distorted audio due to poor reception. Similarly, if digital receivers are getting overloaded RF signals, it simply drops out: no more intermod squeal. By pointing out the Overload LED on the receivers, my new specialist had reminded me that we are in a digital age. Many system problems sound differently in the room, if they sound like anything at all.

These digital microphones are sneaky in that they play by different rules than their analog brethren. To protect the audience from poor-quality audio or squeals, they simply pull the virtual plug. The audiences are none the wiser, but we have to look in different places to find clues about what the issues are. However, solving the problems caused by these new rules can sometimes be simple. This particular installation was a Shure ULXD deployment with Wireless WorkBench (WWB).

(I love this software. We didn’t even have to be in the equipment room to see the indicators on the receivers. We could have done it from the iPad app. Not only that, the software can use one of the receivers to perform an RF sweep of the space to look for any new TV stations in the neighborhood and make sure the microphones aren’t anywhere near them, or perform a live scan during an event to make sure everything is peachy. But I digress.)

The receivers were on the AV Network and could be controlled by WWB. The inventory of receivers was already loaded into the system and, sure enough, several transmitters came up as problems because they were set to potentially interacting frequencies. A new frequency solution was calculated by the software and pushed to the receivers. The solution took five minutes to implement. It took a little bit longer to find.

I should have known there was an app for that.

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