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Wireless Problem

Discussion in 'Question of the Day' started by mbenonis, Dec 21, 2007.

  1. mbenonis

    mbenonis Wireless Guy Administrator

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    It's been a while since I posted a question, so I thought I'd put one up. The following hypothetical scenario takes place in/near Washington, DC (important).

    A theater has four UHF wireless mic systems, operating on 543.250, 551.450, 562.000, and 562.075 MHz. The make/model of the systems is not relevant. When any one system is on, and right next to the receiver, it works fine. However, when on stage (~75' from the receivers), they don't work very well - lots of cutting in and out, and occasionally the sound from one mic appears on another channel. The receivers each have two antennas attached to them directly. Assume batteries are fresh, everything is connected, and that the fault lies in the wireless mic system (i.e., not in the console or anything else)

    Identify the various problems here, and how to fix them. Bonus points for identifying what problems will be associated with what mic(s).

    HINT: You may need to use reference sources, such as the FCC's website, Shure/Sennheiser/A-T's wireless tools, etc.
    http://www.fcc.gov/mb/video/tvq.html
     
    Last edited: Dec 21, 2007
  2. Hughesie

    Hughesie Well-Known Member

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    well im the first person, so im going to offer the simplest answer crosstalk, and not knowing the US radio frequencies i would say you might be picking up a radio station tv station and the receiver doesn't know how to interpret the new signal, the reason it works close to the receiver is the mic's signal is stronger than all over sources until you move away when other signals become stronger
     
  3. mbenonis

    mbenonis Wireless Guy Administrator

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    Yes! That is definitely one problem with the scenario. Bonus points for you, Alex, if you go to the US FCC's website (above) and figure out which mic has this problem.

    There also a number of other issues with this setup (not all related to US frequency allocations)...
     
  4. mbenonis

    mbenonis Wireless Guy Administrator

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    Anyone else want to take a stab at this problem?
     
  5. n1ist

    n1ist Well-Known Member

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    Those last two seem too close in frequency to keep out of each other. I haven't checked what's the deviation on these mikes, but 75KHz separation is probably not enough.
     
  6. Stoldal

    Stoldal Active Member

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    Hmm One problem that i have at the place i help out at, normal Cell phone some times interfere with our wireless mics.

    It could also be ask what kinda building is this production in. I also did network admin at the same place i do lights at. In one building we where trying to put wireless in, we could not get the signal to go across the room. the room was make be 60-70' wide. this was before we had started to do productions in the area.

    When we started to do productions in there we had the same problem with the signal. SO what we had to do what put the wireless mic receivers next to the same, and snake them back to the mixers.
     
  7. Stoldal

    Stoldal Active Member

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    I found this FCC Chart It is a Large PDF
     
  8. mbenonis

    mbenonis Wireless Guy Administrator

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    Yeah, that's one of the problems. Mics are definitely wideband, and should probably get 200-400 kHz between channels (if anything to keep one channel from desensitizing the other receiver).

    73's de KI4RIX :)
     
  9. mbenonis

    mbenonis Wireless Guy Administrator

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    Cell phones operate at 800 MHz and 900 MHz, as well as 1.8-1.9 GHz so they won't interfere directly with the RF side of the wireless mic (one would hope the filters on the front end of the receivers is tight enough tocut them out anyway).

    That said, they can inject undesired noise into the audio path before the transmitter and after the receiver. I suppose it would also e possible to get intermodulation if the cell signal were really strong (although the chances of IM products landing on frequencies in use is very unlikely).

    The type of building is definitely relevant, or more specifically the construction. If the building tends to let lots of RF through at UHF, then noise from the outside (such as from TV stations) may cause problems with the mics. If it is fairly shielded, though, these problems may be reduced or eliminated altogether.

    I'd encourage everyone to take a close look at the TV stations in the Washington, DC area... See the link I posted above.
     
  10. Stoldal

    Stoldal Active Member

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    Well i think that channel 20 is in that frequency range.
     
  11. mbenonis

    mbenonis Wireless Guy Administrator

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    Yes, exactly. And the astute reader will notice that one of the mics is sitting on top of the luminescence carrier of Channel 20.
     
  12. Chris15

    Chris15 CBMod CB Mods Premium Member

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    As others have noted, there is a potential for problem with mics 3 & 4 - you would have to have pretty darn good filters to pull them apart from each other...

    One thing I did note Mike mentioned, each of these mics has their own set of antennae, now the done thing would normally be some form of RF distribution. This makes things less cluttered and allows you to use external antennae with directionality and hence passive if not active gain. Something in the back of my mind has a vague recollection of reading somewhere sometime that antennae in close proximity to each other interfere with each other and it makes sense even if my engineering mind can't quite remember the details of why at the moment...

    Or am I confuzzled?
     
  13. mbenonis

    mbenonis Wireless Guy Administrator

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    Using antennas connected directly to the receivers can cause a few problems. However, surprisingly the biggest problem isn't interference but simply lack of diversity. Simply put, the farther apart the pair of antennas is from each other, the better the probability that the wireless system will always have a good signal.

    That said, having multiple receiver antennas in close proximity can cause direct interference, if the receiver's output stage is not properly designed. These antennas can actually radiate as well as receive, and in some cases this radiated signal can cause interference on other units. Good multicouplers/distribution amplifiers isolate the receiver ports so that input signals do not make it from one receiver to another.
     
  14. superdoo

    superdoo Member

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    I strongly recommend antennae distros. I work for an outdoor theatre company in Bismarck, ND. Needless to say interference on the open prairie is a little more than a problem!
    Last year we got 8 new mics complete with antenna distribution. We went from saying "maybe the mics will work okay at best?" To holy crap! All 8 aren't even cutting when the actors are in the building behind the stage!

    My nights got so much easier...:mrgreen:
     
  15. Charc

    Charc Well-Known Member

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    What does that do? / What is it?
     
  16. avkid

    avkid Not a New User Fight Leukemia

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  17. mbenonis

    mbenonis Wireless Guy Administrator

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    A multicoupler, or active splitter as it is sometimes called, is a device that takes an RF signal from an antenna and splits to a number of separate outputs. It also amplifies the signal slightly to compensate for the loss that occurs when the signal is split.

    Multicouplers often act as a power supply for receivers to minimize the number of power cables needed inside the rack. Additionally, they have filters that prevent spurious signals from being coupled from one receiver unit to another. Finally, they usually apply a band-pass filter to the input (the cheap ones usually have a broadband filter that passes 470-806 MHz, while the better ones have a tighter window on the order of 50 MHz).
     
  18. Charc

    Charc Well-Known Member

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  19. Chris15

    Chris15 CBMod CB Mods Premium Member

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    See when you mentioned a multi coupler, my mind immediately went to an active combiner, quite a different beast. Combiners take RF signals from either multiple antenna or multiple transmitters - normally IEM, and combines them to a single RF output to go to either a transmit antenna or to the input of a splitter, active or passive.

    What's the rule of thumb? Only one passive split and the rest need to be active? And on the flip side, only one active combiner and the rest must be passive...
     
  20. mbenonis

    mbenonis Wireless Guy Administrator

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    There really isn't a rule of thumb here. You need to know how strong your signal is at the input of your receivers with various levels of splitting. If you have a very strong signal, you can certainly use two or three passive splits. However, if you have a weaker signal, you should probably use an active splitter with a narrow bandwidth. The second part is very important, as you absolutely do not want undesired signals, such as adjacent TV stations, amplified as well - they'll only serve to desensitize your receivers (especially if they're lower end models with poor filtering inside).
     

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