Jason Glass Talks Wireless for the 2015 CMA Music Festival

This year’s four day CMA Music Festival shattered attendance records and featured country music’s biggest stars—far too many to list.

The show’s RF Designer and Technician, Jason Glass, owner of Clean Wireless Audio, in conjunction with veteran touring audio provider Sound Image, used a fleet of ten RF Venue collapsible CP Beam helical antennas for all performance RF audio used at both of the Titan’s LP Field Stadium’s two stages, the main stage, and “B” stage.

Jason’s design was unconventional and clever. It solved a lot of the RF challenges that come with large-scale festivals, and he was kind enough to speak with us about how it worked at length.

“This is my third year doing the LP field show,” he says.
“The lighting rig encompasses the stage, it’s a veritable faraday cage. We deploy a number of antennas up in the trussing. This configuration is designed to give the best possible line-of-sight link between the performers on stage and the antennas.”

Jason and Sound Image wanted to give guest artists and their engineers the ability to use their own wireless equipment (which artists are much more comfortable doing), but also recognized that, from a wireless perspective, the positions of monitor world and FOH at the venue were less than ideal for reception. Each act would have struggled with antenna placement.

So instead, he flew seven CP Beam helicals above the main stage, in an ideal, lofted position in the trussing, and three antennas in a great spot in front of the B stage. Artists used all their own equipment, but simply patched into Jason’s antenna system for their transmit and receive links. Jason even deployed a toggle switch of his own design to flip audio from one stage to the other.

“Your ultimate goal is to have a reliable link,” Jason waxes. “You have to have a certain amount of signal-to-noise ratio to open squelches and to provide adequate signal for FM capture. I do path loss calculations in my imagined worst case scenarios. With my artists furthest away from the receiver. I account for losses in free space. I account for gain or loss in each component from the antenna through the cable, adapters, splitters, connectors, to assure that I have a margin of signal strength in my worst case scenario.”

Because Jason so scrupulously calculates all of his variables, to obtain as good an SNR as possible, no RF amplifiers or active stages were used, even though the antennas were remoted with long lengths of coaxial cable. Instead, low loss coax was used to complement the inherent 9 dBd of gain found in the CP Beams.

“Even as well-behaved as our video walls are on this show they still generated a measurable amount of noise. When you have active amplification it exacerbates that noise problem. If I avoid the amplification the noise floor is so low it is almost inconsequential. That’s why I used higher gain directional antennas feeding passive splitters, which in turn fed active multicouplers.”

But why the CP Beams, specifically?

“Well, it seems like over the past year or so more and more of my clients own RF Venue equipment that they employ me to operate,” he explains. “I tested a CP Beam with my VNA tester and I saw that it was very close to the other popular helical antennas that I was using over the years, and when you combine that with how easy it is to pack these things, that they are collapsible, it didn’t take long to put two and two together to see that this was a much more convenient way to achieve the same ends.”

In fact, Jason has been using CP Beams for some time now, and has discovered the performance is superior to helicals he used previously.

“I didn’t risk my reputation with my biggest client at my biggest show solely because I measured the antenna and it looked good. It was because the more I used the CP Beam, in every case I was ending up with very reliable performance. All arrows pointed towards making this a standard, so we did.”

Photographs by David Bean/The Visual Reserve. Illustrations by Clean Wireless Audio.

Want better wireless? Download our eBook on three essential concepts for correctly deploying and maintaining interference-free wireless audio systems.

FCC Further Clarifies Wireless Microphone Rules for New 600 MHz band, Guard Bands, Duplex Gap

We still don’t have the final, final rules for the Incentive Auctions, but we are getting damn close.

The purpose of the hair-splitting document, “Incentive Auction Second Order on Reconsideration” is to address the petitions for reconsideration that have trickled in over various proposed Incentive Auction rules, and more importantly, “to provide certainty for prospective bidders and other interested parties in advance of the incentive auction.”

The Commission wants to give broadcasters and bidders as much information as possible about how the auction will work. Broadcaster participation will be more robust if they believe there is a higher likelihood of receiving a big payout, and advance information allows bidders to formulate a more coherent (and larger) bidding strategy. That’s why we can be much more sure of the discussions around these rules for wireless microphones than we have been in past documents.

Here is a summary of pending or passed regulations that benefit wireless audio users. I’ve sorted them by confidence: regulations/changes we either know we have (or are very confident we will get), and regulations/changes that will probably come to pass. More detailed discussion follows the summary.

High Confidence

• Expanded eligibility for Part 74 licensure.
• One reserve channel, nationwide, shared by both unlicensed wireless microphones and white space devices.
• Operation permitted in the guard bands and duplex gap.
• Unlicensed and licensed microphones may operate closer to active TV stations.

Moderate Confidence

• Ability for licensed Part 74 users to reserve white space channel with priority over TVBDs.
• 4 MHz of spectrum within the duplex gap for exclusive use of licensed wireless microphones.
• Exploration of additional frequency bands outside of UHF for use by wireless microphones after auction.
• Licensed wireless microphones operating inside the contours of active TV stations.
• Improved speed and reliability of WSD database.

Reserve Channel and Additional Spectrum

There will be a single reserve channel in UHF in all markets that will be shared by wireless microphones and white space devices.

More specifically, The Commission denied petitions to deny the creation of a single reserve TV channel in all markets, which means they can now move forward with proposals to finalize its creation.

This received its own hair-splitting NPRM, which I think is the fruit of a proposal that was being circulated to the Commissioners internally for some time, because it isn’t listed on the circulation page anymore. It mostly discusses the challenges of repacking TV stations into remaining spectrum without eliminating the reserve channel.

The Commission is considering giving licensed (Part 74) wireless microphones the ability to reserve the reserve channel with priority over white space devices.

Guard Bands and Duplex Gap

The duplex gap will remain the size that has been previously discussed. 11 MHz. And unlicensed wireless devices will be allowed to operate there. We don’t yet know what size the guard band (located between the downlink portion of reallocated spectrum and the UHF band) will be, but we do know that wireless microphones will be allowed to operate there.

Licensed wireless microphones may be given 4 MHz inside the duplex gap for exclusive use (no unlicensed TVBD/WSDs), but the Commission has also recently said that where necessary they will place a TV channel in the duplex gap in crowded markets, which puts the usefulness of these 4 MHz in doubt.

As I wrote a few weeks ago, LPTV advocates are concerned that allowing unlicensed devices to operate anywhere in UHF where licensed devices are not is inappropriate and possibly illegal.

Last week the commission pointed back to the Spectrum Act which, indeed, authorizes both the size of guard bands and their use for unlicensed, and said, “The fact that the Spectrum Act allows us to make guard bands available for unlicensed use does not mean that we are reallocating spectrum from licensed services to unlicensed use.”

I don’t think it is too much to say that LPTVs are getting thrown under the bus, and my heart goes out to the millions of rural and underserved populations and that rely on LPTVs for information and entertainment in the absence of access to other forms of media and connectivity, even though wireless microphones received some accommodations here that LPTV advocates fought against.

The ATBA’s thoughts on this matter are here.

There are related concerns for non-commercial Class A stations that wish to remain on the air.

Vacating 600 MHz

There wasn’t much on this. In part because while this document was being written, the NAB/Sinclair challenge was still being debated in a D.C. court of appeals. Therefore the Commission declined to consider requests relating to the 39 month post-auction transition period.

In between when this document was finalized and when it was released, the FCC triumphed in the appeals court and the challenge from the NAB was, at least for the moment, defeated. I assume if they do not appeal yet again and take it around to the rest of the D.C. Circuit and potentially Supreme Court, we will hear more details about the 39 month transition period soon. In fact we will hear either way because the NAB litigation is not the only one in the pipes.

There was also no discussion on the definition of when a cellular company is said to “commence operations” in purchased spectrum.

Compensation for Displaced Wireless Microphone Users

As was voraciously reported in the trade journals, Sennheiser filed a petition arguing that the forward auction winners should be compelled to compensate wireless microphone end-users for the expense of replacing 600 MHz equipment. Specifically:

“It challenges the Commission’s decision not to require reimbursement on the grounds that wireless microphone operations are “secondary,” and asserts that the Commission has statutory authority outside of the Spectrum Act to require such reimbursement.”

While Sennheiser cleverly discovered that the FCC does have the authority to compel primary incumbent winners to compensate mic users, such an event would be completely unprecedented, and the FCC has no intention of using the loophole.

Although compensation would be a welcome turn of events for our industry, it’s very unlikely and this most recent document puts another nail in the coffin to that end.

“Contrary to Sennheiser’s arguments, our rules and policies are clear that licensed wireless microphone operations are secondary, and not primary, in those portions of the current TV bands that will be reallocated for wireless services following the incentive auction. The Commission has never required that primary licensees (here, the 600 MHz Band wireless licensees) moving into a band reimburse users that have been operating on a secondary basis in that band. We also decline to require reimbursement of unlicensed wireless microphone users that currently are operating pursuant to a limited waiver under certain Part 15 rules; unlicensed users as a general matter do not have vested or cognizable rights to their continued operations in the reallocated TV bands.”

Want better wireless? Download our eBook on three essential concepts for correctly deploying and maintaining interference-free wireless audio systems.

Leading image courtesy Dwayne Bent.

Video: How to War-Game Transmitters

//fast.wistia.net/embed/iframe/xhbyfg0ffg//fast.wistia.net/assets/external/E-v1.js

[Note: a little confusion on my part in my use of the terms “harmonic” and “intermod.” Will update ASAP. The procedure itself though is performed correctly.]

War-gaming is a useful, hands-on procedure that makes coordinated frequencies as reliable as they can possibly be.

Professional frequency coordinators have used war-gaming for decades at things like the Olympics, the Super Bowl, major concerts and other big events, but, there’s no reason audio professionals everywhere can’t use it, to.

Aside from a software program that can calculate intermodulation, you don’t need any special equipment to war-game, so long as your receivers have good RF level meters.

We learned about war-gaming from James Stoffo, currently the CTO at Radio Active Designs, which is fitting, because he pretty much invented it. At the very least, he coined the term “RF war game,” so we need to give credit where credit is due.

We also need to thank Talamas in Newton, MA, for helping us out with some of the equipment (they are the go-to for audio and video rental in the Boston and greater New England areas) as well as Babson College, which lent us the use of their beautiful space.

This video could have been longer. Much longer.

Let’s take a closer look at that scan we blazed past in the beginning.

The two spikes in the middle are our two carrier frequencies, from transmitter A and transmitter B. Transmitter A is tuned to 555 MHz and B to 556 MHz. On either side of our two carriers we see a skirt of odd-ordered intermodulation products.

We expect to see a third-order product at two times fundamental frequency A minus fundamental B, or:

2A – B

And another one at:

2B – A

If you look at the scan you can see that, since our two fundamental frequencies are at 555 MHz and 556 MHz, our third order products are at 554 MHz and 557 MHz.

Third and sometimes fifth order intermods are the most troublesome because they contain the most energy. There are other products, all over the place, in fact they theoretically extend out infinitely on either side of two mixed signals, but they may be too quiet to be of practical significance, or too far away from our carriers to bother with.

Third and fifth order IMDs may be beneath the noise floor until the transmitters themselves are very close to one another, which is one reason why unscrupulous people who use lots of channels and don’t coordinate can sometimes get away with a show, or many shows, without any IMD related dropouts.

Sooner or later, fate will catch up to these people.

By now you probably realize that when three or more transmitters are in use, and because intermodulation products can mix to form yet more intermods, finding frequencies that do not contain an intermod product is challenging indeed.

Software like IAS, Clear Waves, WWB, etc, does most of the heavy lifting for us, yet all of them still make compromises in their algorithms. If they didn’t, even the fastest computer would take thousands of years to compute all possible scenarios (or longer) and of course they don’t take into account what the system is actually producing in the real world.

War-gaming let’s you iron out the limitations of the software and test calculated frequencies in the real world.

There is another lesson to be learned here. Do not evenly space frequencies!

If you were to space your frequencies evenly, like every 1 MHz or 2 MHz, you are placing each of your center frequencies on the odd order intermodulation products of the other previous and following devices in the chain. Not good.

Intermodulation itself, what causes it, where it comes from, etc, is a big topic, and one that I’m not equipped to fully answer.

In the context of wireless audio devices, the harmful type of intermodulation usually happens when radio waves from one transmitter leak into another, and the two carrier frequencies “mix” and spit out other, spurious, radio waves at the predictable frequencies we talked about above.

Intermodulation can happen elsewhere, like at the receiver and in physical connections (passive intermodulation), and of course intermodulation is a good thing when it is used to create modulated RF signal in a frequency modulated translator.

Want better wireless? Download our eBook on three essential concepts for correctly deploying and maintaining interference-free wireless audio systems.