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I'm shooting footage on a college campus using a wired external shotgun. In certain places on campus, there's an occasional buzz--like someone using a sped-up morse code--followed by a few seconds of thumping. It doesn't happen all over campus, and it's not constant. Since the environment is a college campus, there's a ton of stuff "in the air": phones, internet, the campus network, security guards on radios... First, could these be the causes of the interference, and if so, is there something to do about it?

Thanks for any advice.

  • FYI, interference sounding a bit like morse code could be GSM cell phones — the radio protocol was defined such that it has audio-frequency components. – Kevin Reid Oct 7 '11 at 12:33
  • What kind of shotgun microphone is it? – leftaroundabout Oct 7 '11 at 21:14
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Those sorts of things could definitely be causes - but you probably can't do much about telling them to stop, so you should concentrate on what you can improve:

The wire between the mic and the camera is going to be the biggest problem - it is acting as an antenna, and as microphone signals are very low you need to improve your signal to noise ratio.

Options:

  • Use a shielded or balanced cable - probably your easiest option. You can get balanced cables for mics that are very resistant to interference but this will depend on what your camera supports
  • Use a pre-amp at the mic to increase the signal - this reduces the effect interference will have
  • Record audio at the mic (thus shortening the wire) and add it to the video later - you will need to look at synchronisation, etc
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GSM cell phone communication. Try routing the cable where no audience member sits or sets down their bag (and crew members carrying live GSM phones are to be treated accordingly, and switching the ring tone off is not enough). Longer connections should always be done with a balanced connection. If that's not the case, using an appropriate mic preamp with balanced output and placing it close to the mic might be an option. And don't use high impedance (or "instrument") inputs at either mic or line level. When you are already using a balanced connection, a better cable (additional shielding, more robust mechanics, more dependable inner geometry, refusing to bend sharply) may decrease the susceptibility to such noise.

Now you state "in certain places on campus" and GSM phones are not usually installed at fixed points. That makes it somewhat likely that you are carrying the culprit around in your team, and sometimes a person or bag with a cell phone on it gets placed close to the susceptible connection whenever you are at such a "certain place". If you suspect someone specific, send him a text message (SMS) or give him a ring. That's pretty certain to trigger the interference, immediately before the phone announces the reception of the message/call.

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