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What do you professionals do to balance multiple microphones?

Discussion in 'Sound, Music, and Intercom' started by tenor_singer, Mar 31, 2009.

  1. tenor_singer

    tenor_singer Active Member

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    Location:
    Orwell, Ohio
    Hi,

    I just closed our first weekend of our spring musical and I had the most awful experience with our microphones Friday.

    We are currently using 22 microphones in our production:

    9 AT 1400 series (pre-set single frequency)
    5 AT 3000 series (can be set to dozens of different frequencies)
    8 Telex FMR-500 series (similar to teh AT 3000's with a scanning feature to find the best frequency).

    For the life of me I cannot stop microphones from interfering with each other. I've changed frequencies on the microphones that I can change, I've researched area television stations to be sure that I haven't set a microphone to their frequency per the instructions, I've had sound checks where I have multiple microphones on at all times to make sure that I create a good gain structure... all to no avail. I have microphones cutting out, I have mics not amplifying the actor and yet feeding back when I try to volume it up, I have mics making a zinging noise while the people are speaking, they constantly feed back, several times through the productions their receivers showed no signal coming in only to have it abruptly come in and then go out... it was maddening.

    So... what do you professionals out there do to get a good sound mix with multiple microphones? What advice can you give to a theater person who's ready to just toss it all in and go camping instead?

    I'm frustrated! I've worked for 16 weeks on this show at an average of 85 hours a week only to have my students' hard work ruined by these frigging microphones. I'm to the point where I'm going to try to hire a professional theater person (except I simply don't have the budget for one... small school and all).

    AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH!
     
  2. museav

    museav CBMod CB Mods Departed Member

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    Occupation:
    Acoustical, audio and audiovisual consultant
    Location:
    Marietta, GA
    Unfortunately, you seem to be encountering an all too familiar situation of big production use with numerous wireless mics coupled with limited financial and technical resources. The obvious solution is to try to limit the production to better fit your resources, such as fewer wireless mics, but that is often easier said than done.

    From a technical perspective, there could be several issues involved. To get one out of the way, you need to look at replacing the AT 1400 units as they operate at 732.660MHz (Channel 57) to 745.200MHz (Channel 59). That is in the 700MHz spectrum that he FCC auctioned off for commercial use and it is almost certain that operation in that spectrum by any other users will soon be prohibited by the FCC, it probably would have already been prohibited if not for the delay in the final DTV transition date. Because this spectrum was auctioned, there may already be usage of that spectrum in your area by the new licensed users, which are not TV stations, and those particular wireless system offer little option to address that, especially with nine systems in use.

    Another potential factor is frequency allocation of that many mics operating simultaneously. The issue is not just local broadcasters, but also interactions such as IM between the systems, so it may take more than just looking at the local broadcast frequencies in use. This is a common issue when dealing with that many wireless mics operating simultaneously in one venue.

    Also, with the delay in the DTV transition we are in an extended period where there are many stations broadcasting both analog and digital signals with some of those ceasing analog operations at various times, thus in some areas the UHF broadcast spectrum use is greater than it will be and changing (many broadcasters have one analog frequency, one temporary digital frequency and one permanent digital frequency, while in most cases this means two different frequencies, in some cases these are actually three different frequencies; two during the transition and a third different one when they transition) . Once the transition is complete and all analog transmitters are shut down there may be more spectrum available.

    Another possible factor is your antenna scheme. What are you doing for antennas and antenna distribution?

    It also sounds like you may have some gain and squelch setting issues. In many cases both the transmitter and receiver have level controls and you may need to adjust the transmitter so that it is getting a good signal to the receiver then adjust the receiver audio level. And, of course, make sure you have good batteries in all the transmitters.

    Sorry that there is no one simple answer other than reducing the number of wireless systems.
     
  3. hsaunier

    hsaunier Active Member

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    Location:
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    Brad is correct,

    Large production shows use systems with lots of frequency agility. The more agility the more the cost. You may find that you will be far ahead to use mics only on those with lead roles and then place overhead mics to cover the chorus. Have camio roles change mics back stage to cut down even more. I did "Hello Dolly" with 12 body pacs, and covered every line but 2. I did have a Sennheiser evolution G2 system to work with. Long run, sounds like you have too much RF in the air and you need to start turning some of them off.
     
  4. waynehoskins

    waynehoskins Active Member

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    Location:
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    Yeah, there's no simple answer. You get that many in-band radio transmitters all in each others' nearfield and they all mix together and make sum and difference signals, some of which stomp on other link radio sets.

    You can also be dealing with front-end overload, where a transmitter that's right near the rack of receivers will overload the front-end of all of the receivers. There's also receiver desense that can happen when you tight-pack a bunch of receivers, where the local oscillators in the receivers all desensitize the others in the rack because they're all in-band.

    If you even have a dozen link radio systems, especially a dozen in-band, you need a radio engineer. You the schoolteacher are being asked, increasingly, to also be an RF specialist.

    Another possibility that comes to mind: how closely spaced in frequency do you have your link radios? If you have one at 650.100, where's the next one up from there? If it's any closer than 650.300, you quite probably have them too close, and you're making co-channel interference for yourself.
     
  5. jowens

    jowens Member

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    Location:
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    There's a lot more than just putting frequencies "far away" from other busy frequencies, it's quite an intense algorithm... Go to Sennheiserusa.com find their SIFM program, and it will help you pick frequencies for your area.

    Additionally, I agree with aforementioned gain structuring. With all the different places to adjust "volumes" in the signal stream, this is something that you should spend a lot of time on.

    also, imho, 6 or 7 properly placed good hanging mics (obvi. depending on stage size) will be far better (and cheaper) than 22 poorly functioning wireless. Noise from malfunctioning wireless is abhorrently distracting.
     
  6. waynehoskins

    waynehoskins Active Member

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    In looking back at the original post and something Brad wrote, it seemed good to point out again what's likely part of your problem. If you researched the television stations in your area, at this point in time like Brad said, each station will have one or two (or possibly three) channel assignments. That means that effectively the number of available unoccupied 6-meg channels are half what they used to be, half what you'd think, because each major analog has a corresponding digital that's either temporary or permanent. In our area, for example, the ABC affiliate has always had their analog on Channel 8. What nobody tells you, though, is that their digital is on the air now on channel 9, so what you would otherwise think to be an unoccupied channel is not.

    There are also low power, Class A, and translator stations that aren't highly publicized. As stations have the option to complete the Transition early, you may find that a full-power analog and a temporary digital go off the air overnight while their permanent full-power digital fires up on another channel. This frees up the other two channel assignments for stations who might have their Post-Transition assignment there. In other words, it's a mess. It's a helluvamess.

    In Dallas, I'd be hard pressed to find spectrum to put 22 link radio systems in TV Band.
     
  7. mbenonis

    mbenonis Wireless Guy Administrator

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    Occupation:
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    It sounds like there are a number of issues going on here. First, use a program like SIFM or IAS (see the FAQ) to coordinate your frequencies. Frequencies should be separated by at least 300 kHz, probably more. These programs will make sure that oddball frequency combinations are less likely to cause interference issues.

    Once this is sorted out, now check all of your mic elements to make sure they are fully functional, not filled with gunk, etc - these things will affect the mics' frequency response and thus where they are likely to feed back.

    Next, stick a graphic eq in between the console and the amplifiers. Put an actor on stage and go through the frequencies, looking for ones that cause easy feedback and notch them out as best you can.

    Once you do this, it's all about controlling and balancing the volume. But this should get you off to a better start then you're at right now. :)
     
  8. aemeeich

    aemeeich Member

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    I've been battling this issue for the past 12 years at the high school where I work. I started with 4 VHF Shure SC systems when I was a student there, now I have 10 VHF Shure, 6 AT 3000 D band, 7 AT 3000 C band, and 1 Shure SLX system. The plan is to get 3 (maybe 7) more AT 3000 systems, and 4 Sennheiser 100 G2 systems. This would give me a total of 21 or 25 UHF systems. The VHF systems are slowly starting to die (they are my oldest ones from 1993 and 1998). Shure no longer supports them so they get removed when they fail. This is somewhat unfortunate because I like the idea of spreading they frequencies as wide as you can get them in order to minimize interference.

    I have spent lots of time working out frequency coordination. It took some careful planning, but I managed to fit them all in. I have a whole packet assembled that lives next to the mixer/receivers that has all the possible frequencies for each system, with a complete listing of all the TV stations in use in the area - both analog and digital.

    For the most part I have everything working very well now. I had lots of drop out issues when I first got the 3000 series, but that was before I knew to avoid using frequencies that were taken by TV stations. Now that I avoid them, it's much better. However, I still sporadically have interference that crops up and I need to change a frequency. It gets really frustrating since it crops up so randomly. I've had frequencies that are perfect during the day, but in the evenings are unusable.


    Could you post a list of all your current frequencies? I'd be able to look through them and see if there are any conflicts.

    Michael
     
  9. Chris Chapman

    Chris Chapman Active Member

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    Occupation:
    Technical Director
    Location:
    Greenville, Michigan, United States
    I agree with all above about frequency selection. The original post also talks a lot about feedback. I'd also look at mic head placement on the performers and make sure they have good placement.

    We had some Shure Headset Mics (Oldstyle Madonna Big Mics) that had an annoying problem with the mic head rotating 90 degreesaway from the mouth.
     

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