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posted by martyb on Monday August 13 2018, @12:47PM   Printer-friendly
from the project-your-voice dept.

The email blast from the head of my son and daughter's theater group relayed a frantic plea: "We need to raise $16,000 before the upcoming spring performances," Anya Wallach, the executive director of Random Farms Kids' Theater, in Westchester, New York, wrote in late May. If the money didn't materialize in time, she warned, there could be a serious problem with the shows: nobody would hear the actors.

Random Farms, and tens of thousands of other theater companies, schools, churches, broadcasters, and myriad other interests across the country, need to buy new wireless microphones. The majority of professional wireless audio gear in America is about to become obsolete, and illegal to operate. The story of how we got to this strange point involves politics, business, science, and, of course, money.

Story: https://www.wired.com/story/wireless-mics-radio-frequencies-fcc-saga/


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  • (Score: 3, Interesting) by RS3 on Monday August 13 2018, @06:42PM (9 children)

    by RS3 (6367) on Monday August 13 2018, @06:42PM (#721088)

    No disrespect intended, but your "I'd be..." is very typical of online speculation. That is to say, reality is often very different from philosophy / theory.

    I have never personally had any kind of repeater cause a problem. These systems are extremely frequency selective. RF interference certainly can be a problem. Most better wireless systems allow you to select the frequency within a band range, and sometimes 2 or more bands. With the better systems you start by asking the system to scan for existing RF sources. They can come from almost anywhere and anything- light dimmers, your IoT botnet, etc. It's not like companies actually adhere to FCC RFI rules. Anyway, as you add mics (transmitters), you create your own very complex RF interference field. In the old days you had to use a calculation system to determine the next usable frequency. Again, now the better mic systems will do that for you and assign the next available RF carrier. I've seen up to 32 wireless mics successfully used. I think the most I had going at once was 16, and that was partly just dumb luck. I started with the ones that can't be assigned, then added the auto-scanned ones. Then even added 2 more and it all worked somehow. The worst is when the room fills with 2,000 people and you're expecting all those cell phones and whatever else to cause problems but it all works.

    In most "big" shows the receivers will be onstage with highly directional antennas. The disadvantage is that I can't monitor the receiver status displays. Sometimes I use 2 directional antennas on poles up over the crowd. Some really fancy systems will let you remote program the mic pack's frequency. Say you've started a live show and you see interference on a channel- you tell it to search for an open frequency and it tells the transmitter to change and the receiver of course does too and nobody knows. Theoretically you could have this happen automagically, and maybe the newest systems do, but you would risk a dropout while someone's speaking or singing, so it's best done under an attentive engineer's control.

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  • (Score: 2) by All Your Lawn Are Belong To Us on Monday August 13 2018, @09:59PM (7 children)

    by All Your Lawn Are Belong To Us (6553) on Monday August 13 2018, @09:59PM (#721141) Journal

    I think my point was a lot simpler than that. RF Interference from the new system into the receiver of the microphone directly on the mic's freuqenc(ies), thus rendering the microphone useless. I rather doubt you've had transmitters of whatever the new frequency licensee is, operating at the microphone frequencies, as the interference source.

    Thus you might (and yes, that is "might") need either a Faraday cage or to change your equipment. The point of the Wired article, no?

    (Although sure, you can disagree with that point and you might well have the experience to be right.)

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    • (Score: 2) by RS3 on Monday August 13 2018, @11:47PM (6 children)

      by RS3 (6367) on Monday August 13 2018, @11:47PM (#721172)

      I'm sorry, I admit I'm not following you, but I'd like to and share my experience and knowledge. I've used all types of wireless audio mic systems- VHF and UHF from the old now illegal 700s, to 600s, to 500s now. I'm a long-time electronics (and mechanics) hobbyist, dabbled in ARRL (HAM), electronics tech, and BSEE (and some MS courses in signal analysis and processing- sampling, DSP, etc.).

      When you say "RF Interference from the new system" - do you mean the new wireless mic system? Or some other system?

      I'm just a little confused. Since this is not very interactive, I'll write a guess and please ignore / excuse me if it's tangential. By definition, RF receivers ignore everything except the frequency they're tuned to listen to- old, new, AM, FM, VHF, UHF, GHz, millimeter, etc. If you get a powerful transmitter close enough to the receiver's antenna, you could overload the RF input stage, cause it to distort, and make frequency "splatter" which would cause interference. But otherwise the receiver's job is to ignore everything except that one desired tuned-to frequency. We call that: "receiver selectivity".

      You may not be aware, but many RF systems, such as cell phones, WiFi, Bluetooth, etc., use overlapping frequencies and still function using spread-spectrum, frequency-hopping, etc. Of course they're digital systems and buffering and retransmitting after errors is part of the whole system. You can and we do use those digital systems for audio. The only problem: buffering, by definition, causes a delay and at some point of too much buffering the delays become noticeable, objectionable, and unusable.

      Sorry if I've overexplained it again. Blame it on my learning to touch-type too fast...

      • (Score: 2) by kazzie on Tuesday August 14 2018, @06:35AM (4 children)

        by kazzie (5309) Subscriber Badge on Tuesday August 14 2018, @06:35AM (#721258)

        I believe the OP is concerned with interference from the new radio-based internet & cellular networks using the reauctioned bandwidth. TFA raises some concerns along this line.

        • (Score: 2) by RS3 on Tuesday August 14 2018, @02:59PM (3 children)

          by RS3 (6367) on Tuesday August 14 2018, @02:59PM (#721390)

          Yes, thank you, I finally understood his point and commented further down in this discussion.

          I missed that point initially because, again as I commented further down, due to RF power, distances and inverse square law, etc., I can't imagine it to be a problem. Additionally, most if not all newer (even 20 years old) UHF systems allow you to program the RF frequency within a band, so you could avoid a local transmitter if receiver interference was an issue.

          But also, and I know this directly, FCC isn't worried about my receiver getting interference, they're worried that my mic will interfere with "emergency services" or whatever the bandwidth has been sold to. FCC puts people in prison.

          What's bugging me: as a kid I messed with "walkie-talkies". Maybe the rules have changed, or maybe I misunderstood at the time, but I remember believing that FCC allowed low-power unlicensed RF transmission provided it only carries for very short distances, like 1/4 mile or something. Of course you'd have to define the actual power level at the 1/4 mile point, etc. My point is: if my wireless mic is low power enough that you need specialized highly sensitive equipment to receive it more than say 100m away, then it should be a non-issue. People should be allowed to continue using their $5,000 systems, unless someone can prove said system is causing measurable provable interference.

          That FCC sold the frequencies and told everyone "too bad for you" is outrageous. The companies who bought the frequency rights should have to replace all now unusable wireless systems.

          Footnote: this whole thing happened 8 or so years ago.

          • (Score: 2) by All Your Lawn Are Belong To Us on Tuesday August 14 2018, @05:04PM (2 children)

            by All Your Lawn Are Belong To Us (6553) on Tuesday August 14 2018, @05:04PM (#721424) Journal

            I believe that the rule was actually if it was low power to a certain milliwatt ERP specification (which commonly came out to less than 1/4 mile), you accept any interference to your end, and does not cause interference to any other user you can utilize any frequency you wish... and I think separately the broadcasting rules had provisions that still limit a transmitter from making a broadcast to the public without at least a community broadcaster license (very generally - there were exceptions). And I really could be wrong but I thought those rules were still in effect. It's not "no harm, no foul" but it is close. But any interference caused if you're unlicensed/out of licensed band is still automatically your fault, and interfering with public safety comms is a criminal offense even if you were acting in good faith under that rule.

            The current low power rules are here [fcc.gov], which actually specify 200 feet.

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            • (Score: 2) by RS3 on Tuesday August 14 2018, @05:35PM (1 child)

              by RS3 (6367) on Tuesday August 14 2018, @05:35PM (#721441)

              Yes, I remember 100mW being the spec, but memory can be flawed. I intentionally omitted it because 100mW at 100KHz will carry much farther than 100mW at 32GHz, with similar antenna gain of course.

              But even with a 200 foot rule, there's still some power at 200 feet, so how do you decide that it's low enough so as not to interfere with an average receiver?

              My point is that in practice, the 700 MHz band wireless stuff should not be a problem and people should be allowed to continue using it until someone can measure a problem, which is pretty easy to do with a simple field strength meter. FCC worry that it will interfere with emergency services, but I'm skeptical.

              Typical UHF wireless microphone systems are 50 mW https://en-us.sennheiser.com/wireless-microphone-live-monitoring-system-ew-300-iem-g3 [sennheiser.com]

              • (Score: 3, Informative) by All Your Lawn Are Belong To Us on Tuesday August 14 2018, @10:40PM

                by All Your Lawn Are Belong To Us (6553) on Tuesday August 14 2018, @10:40PM (#721577) Journal

                Very true, all.

                I did just a little more digging on that page and it said that the 1991 technical notice is still actually in effect and had a link to it. The notice (which is probably what we remember) also specifies: AM 100mW to final RF and .05W ERP and FM .01uW ERP with actual spec of 250uV/M measured at 3 meters, in addition to the 200 foot maximum coverage radius and that reads more like it's cannot be picked up by any receiver at 200 feet away, period. (Also says TV is not allowed at any time).

                And aside from that, it is Part 15.5 that additionally imposes the requirement that the device cause no harmful interference and that the device is disabled upon receiving a complaint from the FCC that it is causing interference, and that's aside from all the other rules.

                So I guess all the old rules still apply as well.

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      • (Score: 2) by All Your Lawn Are Belong To Us on Tuesday August 14 2018, @04:55PM

        by All Your Lawn Are Belong To Us (6553) on Tuesday August 14 2018, @04:55PM (#721419) Journal

        No problem, and I'm the same way.

        It's not like it hasn't happened before (as you point out) and won't again.

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  • (Score: 2) by VLM on Tuesday August 14 2018, @12:16PM

    by VLM (445) Subscriber Badge on Tuesday August 14 2018, @12:16PM (#721335)

    Sometimes I use 2 directional antennas on poles up over the crowd.

    My guess is even if there's a cell transmitter down the street, if you can get the stage hands to put your antenna in the set itself (taped underneath a table, perhaps) then you'd still win.

    Of course the wireless mics on stage might interfere with the cell phone service in the first couple rows of the theater which is the legal problem.