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Cell Phones.

Discussion in 'Safety' started by Rhonda, Feb 22, 2007.

  1. Rhonda

    Rhonda Member

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    Last week my high school had put on A Midsummer Night's Dream, and I was lucky enough to be doing my first sound designing job for a mainstage show.

    Our thursday show had went awesome, even our mic girl didn't mess up once.
    But then came the Friday show.

    First, it should be said that although I'm normally very good at troubleshooting, I definitely still don't know a lot of the little rules that come along with sound.

    Like, for instance, that all the recievers should be on the same group.
    (I think you can all see where this is going.)

    So, on Friday, apparently everyone in the audience thought it would be cool to leave their cell phones on, even though our director came over the mains before the show started, and told everyone to turn their cell phones off, since they do interfere with the microphone system.

    Next thing I know, a mic is cutting in and out.
    Luckily, it's one of the recievers that we have up in the booth, so I look at it and see that it's not getting any reception.
    We figure, "hey, let's try just holding it out the window."
    Still no luck.

    Then, we see that the other mic that's on the same channel as the one that's freaking out on us (hereafter known as mic 3) isn't picking up anything either.
    During a scene when that mic (4) and 3 aren't being used, we quickly change the channel on 4, give whoever uses 3 a different mic, and just pray to god that the bit players who now have that mic are able to project, in case it doesn't work. (the actual mic display is broken, so we can't ever change that frequency.)

    The problem with frequencies seem to be over, mic 3 ends up magically picking up again, and 4 goes off without a hitch.

    After the show, i'm turning off the recievers in the pit (we have mic outputs there, i don't know how any other theatre does it) when i hear someone talkign about how they heard people that weren't onstage during the play.

    Uh oh.

    Later, we learn that because of interference from cell phones and the mics all being on different groups, some of them were cancelling each other out or turning them on, even when we didn't touch them on the board.
    Apparently, at one point someone's mic who was in the drama room (where, truthfully, mics shouldn't even be picked up at ALL) was turned on, and they were cursing too.

    At this point you'd blame me for not being able to hear that, but even the people in the audience that heard it said that it was very, very low, and it was a techie alumni who told me anyway.

    Needless to say, the next day all the recievers were magically put on the same group, and we never had interference ever again.

    Moral of the story: Don't be stupid like me and not know that mics like to play with each other. and tell everyone you know to turn their cell phones off during shows.
     
  2. Kelite

    Kelite Apollo Staff Premium Member

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    <apparently everyone in the audience thought it would be cool to leave their cell phones on, even though our director came over the mains before the show started, and told everyone to turn their cell phones off, since they do interfere with the microphone system.>


    Because everyone in the audience is SO [email protected]#n important, they need to be available during a live performance for a call. Geeez....

    If we had less "all about me" attitude and a little more time to pay attention to our kids, perhaps the kids would be a little more respectful to their elders. Just a thought.


    (Stepping off the soap box now....)
     
  3. Eboy87

    Eboy87 Well-Known Member

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    Cell phones are the devil. A quick question, did you have mics sharing the same frequency? If that's the case, you might have had some intermodulation between them that caused your dropouts. I've had that problem before.
    EDIT: Check over at Shure's website, there's a frequency tracker there, here's the link http://www.shure.com/ProAudio/TechLibrary/WirelessFrequencyFinder/index.htm. Check and make sure you're not sharing a frequency with a local TV station. Though if you're getting interference from the venue's HVAC, then you're screwed.
     
    Last edited: Feb 23, 2007
  4. steine

    steine Member

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    We use to tell people to turn off their cellphones, and silent is NOT off.

    I remember we did Grease some years ago, I did the sound, suddenly around "Summertime" I pick up a ligt from my left side. 2 girls sitting and sending each other sms's, yes they were sitting next to each other.
    I dropped a line in the intercom, the next I see is the followspot in that side vanishing from stage and fixing straight on the girls.

    They turned off the phones and watched the show, magically almost half the audience did the same as well.
     
  5. Van

    Van CBMod CB Mods Premium Member

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    I wonder which was more distracting ? The girls IMing each other or the followspot leaving stage and showing up in the middle of the Audience?

    When stuff like that happens it's the FOH crews responsibility to deal with it. In this case FOH refers to the House Manager and Ushers.
     
  6. propmonkey

    propmonkey Well-Known Member

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    ive read on here before that cellphones dont really interfere with wireless mics. i remember it says it can sometimes cause some background noise. cellular phones in the US operate on 900/1800 mhz.
     
  7. fairjon

    fairjon Member

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    how old are the mics. cause for the cell phones to be interfireing they would have to be pritty old. i use wireless on a regular basis with my theatre and i have my phone on all the time with it on silent of course. and i never get any interferance.
     
  8. Chris15

    Chris15 CBMod CB Mods Premium Member

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    See while we don't have conclusive proof one way or the other as to whether mobile phones interfere with radio mic transmissions, I think we have all seen or rather heard mobile phones interfere with audio lines as they change cells or are about to ring... So as far as I'm concerned. Off, not silent, off.
     
  9. audioslavematt

    audioslavematt Active Member

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    The only times I've had trouble with cell phone noise, it's been in system with extremely long unbalanced runs (>100 ft/30m). In a balanced line with good gear, you shouldn't have problems. In balanced lines the shield is not part of the signal and drains RFI into the ground. Any remaining RFI is cancelled out by having a dedicated hot and neutral.
     
  10. Chris15

    Chris15 CBMod CB Mods Premium Member

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    This is true. Though I've seen it happen in all properly balanced systems. In the end, speaker cable isn't balanced, could that be the culprit? And in most systems, there is at least SOME unbalanced cable. I'm not saying it happens all the time, but it does happen and so I do everything I can to stop it.
     
  11. Edrick

    Edrick Well-Known Member Premium Member

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    With our system we've never had problems thus far with cell phones. Shure SLX1 Mic System. But I always thought that you shouldn't have Mics on the same "channel" as it sounds you have them? I noticed you said channel and then group. But the problem I would see and have seen with mics on the same channel is a) you don't have good control over the audio b) You'll get cut off sometimes.

    Now If you had them all on the same group but different channel then that's different. We have all our mics on one group but different channels each mic with it's own reciever.
     
  12. mackoid101

    mackoid101 Member

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    I find that cell phones really don't do anything to mics. But remember never to put your phone on the mixer or near a rack because you'll get that beeping sound every time data is sent/received. If you don't know what I'm talking about, put your cell phone right next to your computer speakers and call it from another phone. Imagine that in a venue after amplification...
     
  13. Edrick

    Edrick Well-Known Member Premium Member

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    I was doing a PSA announcement video for our theater, and we were only doing practice runs while waiting for someone and anytime id get a text message you'd hear the dadada dada sound, text message coming! Now if you were down on stage you'd think i was magical or something :p
     
  14. audioslavematt

    audioslavematt Active Member

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    It depends on how the gear is constructed. As soon as I sit down behind the Venice, the first thing I do is rest my phone above the FX return faders. It sits there the entire rehearsal as the official clock and it has never caused any noise. I wouldn't try that with a Mackie though.
     
  15. Radman

    Radman Well-Known Member

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    I'm pretty sure a cell phone wouldn't noticeably affect speaker runs, I would think that it would be more of a problem in signals BEFORE the amplifier.
     
  16. gafftaper

    gafftaper Senior Team Senior Team Fight Leukemia

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    Do you use walkie talkies for communications during the show? I've had problems with walkie talkies bleeding into a system before. Never figured out exactly how it was happening but it did a couple of times. It's been years since I've seen a cell phone cause a system to have issues. It's pretty much near impossible with the newer cell technology and a decent modern wireless mic.
     
  17. Chris15

    Chris15 CBMod CB Mods Premium Member

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    Hmm. Had a thought. Most mixers and for that matter, other gear, runs unbalanced internally. So if the shielding on the equipment is a bit how's your father, then you could run into problems... Another good reason not to EVER disconnect an earth connection, along with the safety issues of course. I've also read stories of keyboards (the noise making type) picking up radio transmissions because there was a conductor the right length to act as an aerial and pick it up. Turn the thing 90 degrees and it all goes away. I imagine the same applies to walkie talkies... Mobile phones I would not expect this, given the whole digital thing... needs processing to the get audio much more than radios.
     
  18. theatermngr

    theatermngr Member

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    This is the first time that I have ever heard of cell phones causing interference in regards to wireless microphones. Now, granted there are a number of things out there that can create interference.

    Looking into this a little deeper, it seems that cell phones operate around 824 MHz to 894 MHz. Found this on a website:

    850 MHz (824.2 - 848.8 MHz Tx; 869.2 - 893.8 MHz Rx)
    900 MHz (880-2 - 914.8 MHz Tx; 925.2 - 959.8 MHz Rx)
    1800 MHz (1710.2 - 1784.8 MHz Tx; 1805.2 - 1879.8 MHz Rx)
    1900 MHz (1850.2 - 1909.8 MHz Tx; 1930.2 - 1989.8 MHz Rx)

    Newer phones are start to use the 850 MHz area. So, I guess that if you have wirelss microphones in that range, then you would have a problem with interference, and possibly bleed. But for the most part, manufacturers are staying away from those frequencies in the equipment that they are selling these days.

    In regards to cell phones on equipment, I don't think that you have a problem based on the phone's frequency, but rather the magnetic field created by the phone itself.
     
  19. Eboy87

    Eboy87 Well-Known Member

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    I've not heard that expression before sir. I may have to borrow that one. And yes, if you run anything unbalanced, you will get cellphone interference on that line. Amazing what one learns in two, three years.
     
  20. mbenonis

    mbenonis Wireless Guy Administrator

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    A few notes on interference in audio systems and in wireless systems.

    First, to address Rhonda's original question (I know the show is long over but she might have another show coming up), what model systems were you using? It sounds to me like someone had not coordinated the frequencies your system used properly.

    Second, cell phones DO NOT DIRECTLY INTERFERE with wireless mic systems. They operate in the 800-900 MHz and 1.8-1.9 GHz bands, and no wireless system I know of in the United States uses these bands (There are systems available for the european market which go up into the 800 MHz range, however). They CAN, however, cause intermodulation interference and couple interference into the audio-frequency side of things at any number of stages. The same applies for two-way radio systems - the ones you're using most likely operate in the 450-470 MHz band, and thus also do not directly interfere with wireless mic systems.

    As far as intermodulation interference goes (which occurs on the RF side of things), the important thing is filtering and shielding. Make sure you buy real wireless systems with metal housing (preferably all-metal including the front, but any metal is good metal). Also buy systems with good front-end filtering. Many manufacturers do not list what kind of filtering they use, but the reality is the more expensive your system, the better the filtering. Avoid active antenna splitters with wideband reception. If you need one of these, place filtering in line with the antenna inputs to the splitter. Finally, be sure your frequencies are coordinated and will not cause intermodulation. Remember that intermodulation doesn't just "occur," but rather is caused when two signals mix in a nonlinear circuit such as an amplifier or antenna splitter. If you can filter your undesired signals out before the splitter or amplifier, then you're much less likely to have intermod.
     
    Last edited: Apr 25, 2007

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