Senn G3, interference, florescent ballast? at

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Old April 22nd, 2010, 09:26 AM   #1
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Senn G3, interference, florescent ballast?

I was trying to record a class recently. I tested the G3 the instructor was going to wear to be sure the frequency was clear. And where I was at the back of the class it worked flawlessly. When I put the mic on the instructor, and the instructor was in position, I got bursts of static -- sounded like bursts of loud white noise. I changed frequencies four times to no avail. I pulled my backup mic (older Sennheiser wireless) which uses a different block of frequencies and tried that -- same problems. Switched out the lavs, same problems.

The only thing I could figure was that the interference was coming from a florescent light ballast -- they had a bunch of 30+ yo fixtures and whenever the instructor walked under the one on her left I'd get the buzz. Unfortunately these were the only light source and it was a night class, so they had to stay on.

So... How can I figure out the real cause (the G3 worked flawlessly both before and after this class in different locations in the same general area of the city)? And more importantly, how can I address it in the field?

I've never had this problem before, but that doesn't mean I won't have it again (and no doubt all sorts of other wireless weirdness).
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Old April 22nd, 2010, 10:11 AM   #2
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I've never experienced any florescent light interference AFAIK, nor have I ever heard of that being an issue and I've used lots of systems in some pretty old buildings with many florescents. I have been in places with very strong electrical fields that caused problems, for instance a high-voltage power transformer in a factory.
A cell phone in close proximity to a transmitter can reek havoc, vibrate or silent mode does not matter, as long as the device is turned on.
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Old April 22nd, 2010, 10:44 AM   #3
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Seems more likely that it was cell phone RFI. It is a growing problem with wireless mics. The instructor (or someone in the class) almost certainly had a cell phone. They "talk" to the cell tower even when they are in "silent" mode and you aren't making or receiving a call. That is why they demand that you turn them OFF when you are in an airplane.

I have had to abandon use of wireless mics in some venues because the cell phone RFI is just unmanagable. you can ask people to turn them off (not just on silent) and some (perhaps even most) of the people will comply. But it is NEVER 100%. Some people just don't think it is important. Some people think their phone is more important than your video, and some people just forget (or don't realize that their 2-way gadget, pager, Kindle, whatever, is transmiting) Even though they are illegal, I can see why some people use cell-phone jammers.
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Old April 22nd, 2010, 11:37 AM   #4
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wireless mics always work great when they are right next to each other, even on a channel that isn't clear. as you experienced, separating the units made the interference more obvious. the "real" test should have the transmitter on the stage **where it will be used** and the receiver on your camera **where it will be used**, the only good test IMO.

when you changed channels, did you arbitrarily pick a channel, or check the channel for use first? you only tried four out of 1440 available to you. don't panic, sometimes it takes me a while to find a good one. the best way to find a good channel is with your transmitter turned off. cycle the receiver through the channels and find one with ZERO bars on the rf meter, and the rf light should be out also. if these two conditions are not met, there is SOMETHING on that channel and will probably cause you trouble-I wouldn't try to use it move along to another channel. after you find a clean one, power up the mic and see how well it works/sounds.

all that being said, the G2/G3 analog units don't always show you activity when it comes to digital TV stations. I would also use the sennheiser frequency finder site (www dot senheiserusa dot com slash findfrequency) to make sure you are not using channels occupied by a broadcast station in your area.

i don't think the florescent lights are your problem, i've never had them be a problem. if it was, the receiver is more likely to be affected by the distance to problem source, not the transmitter. if you heard a buzz near the light it is likely that the mic element was picking up stray magnetic field-this is not rf interference and has to be dealt with differently.

cell phones...where do i start. again, most times cell phone noises (blip blip blip buzzzzzzzz) are picked up by the mic element or poorly shielded mic cables. i don't have issues with them disrupting the wireless transmissions unless the phone is within a few feet of the receiver. the frequencies that most cells use (800-1200 MHz) don't usually interfere with most wireless mics (VHF 160-170 MHz, UHF 500-600 MHz). to verify this, disconnect your mic from the transmitter when you hear this noise-if it goes away it is the mic or cable picking up the problem...

one last point...always make sure that the antennas can "see" each other. any obstruction-human body, clothes, walls, light stands, etc.-will reduce the amount of signal and potentially degrade performance. sometimes the easy fix is to move the units closer together and use a cable to make up the difference.

good luck!
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Old April 22nd, 2010, 09:44 PM   #5
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Good thought about the cell phones. That one never occurred to me, although I don't know why. And good advice on how to approach the problem. Now I have a basis to go forward.

Thanks for sharing your hard won knowledge.
Bruce Watson is offline   Reply With Quote
Old April 23rd, 2010, 07:17 PM   #6
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Always confiscate cell phones from people holding/wearing the microphone(s). Turn them OFF (remove the battery if necessary) and hold them safe until you are all done and then return them. If the people talking are THAT important, assign somebody to go at least 500 ft away and provide an "answering service" with the presenter's phone and take messages or explain why they aren't available. The presenter's own phone is the most likely to cause problems simply because of very close proximity to the microphone.

The audience is really a sticky problem. We had to stop an event TWICE (because of strong cell phone RFI "hits") and STILL had problems with somebody's two-way pager. It had NO off switch at all. We had to remove the battery. As I said, in some venues, the problem is so bad that we have given up using wireless mics on the presenters. We have switched back to old-fashioned wires. This problem will continue to grow as more people go wireless and even have multiple devices. Between this problem, and loss of the 700MHz band, it feels like wireless mics are becoming an endangered species.

In more controlled conditions (like on a film set), there are legends of famous directors/producers bringing a hammer and nails. If anyone's phone goes off while shooting, the producer confiscates the offending device and nails it to the nearest door frame or fence post.
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