RF over Fiber

[Jason M Wagner]

I have a theatre that produces an outdoor production every summer and we have had RF drop issues for years. The mic system should probably be updated, but with non-profit funds, not so likely. I've recently discovered these:

RF Optix Fiber Optic Conversion System

and

DVB-T & Audio Link | Optical Zonu Corporation

These devices allow long runs of fiber compared to coax so as to remote antennas anywhere within 1 kilometer. Has anyone experienced, used, researched, etc these devices?

Wags

 

[FMEng]

They might be great, but I am skeptical. They add several potential points of failure to the system. They do not have a selective front end and are not perfectly linear (no active RF device is), so they can allow intermod and overload problems to occur. I could envision someone talking on their cell phone very close to an antenna, connected to a fiber converter, and sending the system into orbit. Ruggedized fiber optic cable is probably very expensive.

They have their place, where distances between antennas and receivers are so long that coax losses would be excessive. In most venues, there are better solutions. Locating the receivers near the stage, high gain antennas, fat coax, receivers with network GUIs are all potentially helpful in places where these might be applied.

I have used very similar devices for shipping satellite IF signals several thousand feet. In that case, there really were no good alternatives. Cable loss at 1400 Mhz is extreme after a few hundred feet.

Put in the effort to understand why your system has drop outs before you throw money at something like this.

 

[Jason M Wagner]

The setup that is in place is 24 Shure ULX-S4 units with body packs at FOH with Performance Wireless helical antennas. I believe the coax cabling is less than 50ft long on both antennas connected straight to the distributors. So signal loss through cabling isn't an issue, but to extend the cabling, it could become an issue. The theatre is an outdoor 1500 seat amphitheater on water. I don't know why the original design did this, but I think for keeping an eye on the Rx's and because the system has to be cased and put away daily. Having it in one location was easiest. They've had issues with drop out for years. Every time the system is set up, it seems we find all good frequencies, then a day later they're terrible. Even from morning to night, but the Shure system in place isn't exactly a quick setup type system for on-the-fly work. Especially without network capability.

I would love to get their antennas and Rx's on the stage, but they don't have a way of getting the signal back to FOH. They would need another long run analog snake.

So, I understand why the system has dropouts I was the head engineer in 2010 with the exact same system. It needs updating, but non-profit theatre... The Shure system is old and doesn't seem to work well enough in the space, so I'm trying to come up with options to make it better. Your thoughts?

Thanks,

Wags

 

[venuetech]

My thought would be; it may be best to invest in a copper solution to get the Rx's on stage. I have not priced any of the optical RF items, but they appear to be a stadium solution and likely come at a stadium price.

 

[FMEng]

With that many units, intermod is a huge factor. Randomly picking frequencies is a recipe for problems. Another factor is that you are in a major metropolitan area, with all of the RF problems that come with it. Shure says the maximum number of systems for ULX-S is 20, so you are pushing the receiver design.

The first thing I would do is choose frequencies with Shure's Wireless Workbench software (it is free). The second thing I would do is ensure the antennas are spaced for maximum advantage from diversity. The next thing I would do is make sure to use RG-8 cable, on the long runs, to reduce loss to the absolute minimum. A 50 foot run of RG-58 loses 5 dB at 500 MHz. That isn't trivial if you already have big distance losses. In addition, how are you doing the splitting?

 

[Jason M Wagner]

My thought would be; it may be best to invest in a copper solution to get the Rx's on stage. I have not priced any of the optical RF items, but they appear to be a stadium solution and likely come at a stadium price.

I tend to agree with that as well, but in trying to keep their setup as designed, the Rx's would stay at FOH. I do love that option and it makes the most sense to me, but this entire thing is about convincing the people who need convincing what is the best and most plausible solution, monetarily and logistically. This is an outdoor venue, and the three racks that are the sound system have to be stored every night. Knowing the staging, they would have to undergo some major changes to store the Rx's at night on stage and keep out of the rain during shows. Also, there is no good way to roll a rack from FOH to backstage every night. That would do more damage than good. Again, a snake is still an option, just looking for all viable ones.

FMEng, I have done all those things you mentioned. Multiple times in order to get the best setup possible, but in my experience working with that system, the problem is trying to convince those with the money to invest in some changes. The first thing I wanted was better quality mice with the benefit of better frequency coordination. The antennas have good spacing, and the cable runs are shorter than 50 ft at the moment. The splitting is being done by the active Shure splitters in the Rx rack. One thing I actually didn't know was the max number of ULX-S units suggested. That makes me question the company that did the design. I feel that when the system was designed, it was a cost cutting measure to choose those, which is a bad decision.

I'm looking into low loss cable that could be used for over 100ft runs to remote mount the antenna closer to the stage without altering the physical design. Maybe RG8/U, RG213, or LMR400.

Wags

 

[Jay Ashworth]

"at a stadium price".

That reminds me of my favorite observation about the prices of things.

The five most expensive adjectives in the English language, in ascending order, are

Marine, Aviation, Mil-Spec, Aerospace, and Man-Rated.

 

[lakota651]

Hi Wags,

Have you also taken into consideration where your interfering signals are coming from? You might find you actually have your helicals aimed at those sources. Here in Phoenix it's fairly safe practice to have them placed between your stage and South Mountain, with the greatest point of rejection facing the mountain, since nearly all HDTV transmissions come from there.

http://www.antennapoint.com

Cheers!

Lakota

 

[Carl75]

I just looked up the Shure ULX Frequency Compatibility Chart.

VERBATIM: To maximize the number of compatible wireless systems in a given area, eliminate the frequencies that are
occupied by local TV Channels, and then use the group with the most remaining available (open) frequencies.

So it may say you can run 20 at once but, what that really means is that their are 20 channels to choose from BEFORE you eliminate TV channels that may interfere. So depending on your exact location you may really only be able to find 10 clear frequencies. You can have the best equipment in the world but if your overlapping onto an occupied frequency things just aren't going to work.

Good Luck out there!

 

[Jason M Wagner]

We need to have a proper RF scan done in the theatre at multiple points during the day and night. It seems so much changes from day to night. And again, the ULX-S system needs to be upgraded to something that can properly handle more channels at one time and can frequency scan and monitor. I'm hoping they will see something more drastic needs to be done, but if not, I need to provide some solutions for the current system.

 

[lakota651]

Wags,

What bands are your units in? Also, are you aware of the advance tuning function/mode for the ULX systems? It's bit of pain because you need the supplemental guides to decipher the alphanumeric display, but it will allow you to maximize the use of your current setup, for what it's worth.

~ Lakota

 

[Jason M Wagner]

Wags,

What bands are your units in? Also, are you aware of the advance tuning function/mode for the ULX systems? It's bit of pain because you need the supplemental guides to decipher the alphanumeric display, but it will allow you to maximize the use of your current setup, for what it's worth.

~ Lakota

Oh boy, do I. I've been through the master lists plenty of times.

 

[mbenonis]

Hi Wags,

A few questions that I don't think I saw asked/answered above:

- Where exactly is the venue? You say it is over water...is it on the gulf coast? Is it possible that the antennas are pointed toward the water, and perhaps toward a distant city? This could account for the diurnal changes in interference.
- How far are the antennas from the stage?
- How many antenna splitters are cascaded? Are there any booster amplifiers, or are the antennas active (powered)? Each active device cascaded adds noise and the potential for mixing (intermod).
- What other RF is in use at the venue? Two-way, IFB, Hearing Assist, etc? All RF sources should be accounted for if possible (and included in the Wireless Workbench analysis).
- Is this an urban, suburban, or rural environment?
- No chance any mics operate above 698 MHz, right?

I agree with FMeng--the RF-over-Fiber solutions are probably not appropriate for this case. You really need a trained RF engineer with the right tools (a spectrum analyzer, for one) to ensure that it is working correctly and not causing more issues than it is solving.