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Old August 24th, 2005, 01:18 AM   #1
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Weird noises coming through!

I did a search and couldn't find anything about this online, thought I'd bring it to you guys.

The setup:

Audio Technica 3031 mic
Behringer 802 mixer (tone controls and supplies Phantom Power)
Sony FX1 camera
5' XLR male to female (mic to mixer)
1/4" L & R to 1/8" stereo mini (mixer to camera)
Bose Triport Headphones
Mic stand

The recording is taking place in a corporate board room. One air vent, no computers (there was a land line phone on the desk and a couple of cell phones in the room).

The issue:

Every 5 minutes or so, I get a sound during recording that sounds like a computer making calculations. A few clicks, beeps and hums and then it goes away, only to come back randomly.

Troubleshooting:

I think I isolated the problem to the Behringer, but I've never had this issue before so it seems weird.

Here's what I did:
1. Changed the XLR cable for another one.
2. Double checked the connection between the mixer and the camera.
3. Unplugged the mixer from the camera and the noise went away (camera switched to built in mic - no noise, but no good for sound!)
4. Turning the volume down on the mixer made the sound go away (which, with #4, makes me think the issue is in the mixer).
5. Moved the Behringer wall plug to another outlet.

None of this did any good.

Other stuff:

On the mic the the high-pass filter and 10 db pad were ON.
The gain and master volume were up *slightly.*
The camera input was set to "Line."
Recording in "DV 16bit" Mode on the camera.
Audio on the FX1 was set to Manual with the dial at about 6.
This was the first time I've used the 802, FX1 and AT3031 together for longer than a few minutes of testing.

What do you guys think?

If it is in the mixer is there a way to work around this?
Could it be the mic?
Could it be the camera's mic input?
Interference from the building's AC outlets?

Thanks for any help!

EDIT: Changed "Low-Pass" to "High-Pass." oops!

Last edited by Patrick Swinnea; August 24th, 2005 at 08:17 AM.
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Old August 24th, 2005, 05:33 AM   #2
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What you describe sounds like either a cell phone or perhaps a nearby pager transmitter. You mentioned there were several cell phones in the room. Cells phones, even when not in use, transmit their identification periodically. They also transmit a signal whenver they ring even if the ringer itself is turned off - I've got a neet toy my wife gave me a few years ago, a pen with an LED on the cap that blinks whenever a nearby cell phone receives a call. So I'd remove the cell phones from the vicinity or make sure they're completely powered off and see if that helps.
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Old August 24th, 2005, 07:02 AM   #3
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I agree with Steve about a phone or pager causing noise in the mixer. Small, inexpensive mixers like the 802 have their place but this is not one of them.
Many have a pin one problem where the shield is tied to the circuit board ground and not the case putting noise on the signal ground.

Look for a mixer with a metal case from a pro manufacturer. Transformer inputs like Beachtech uses helps also.

Sam
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Old August 24th, 2005, 07:42 AM   #4
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Ditto on the cell phones.

The periodic noise is your cell phone checking for text messages and voice mail. Turn it off.
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Old August 24th, 2005, 08:20 AM   #5
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Eliminating the cell phone signals is primary, but another detail is of secondary interest. The reason you're not protected by the XLR run is that the unbalanced run from the mixer to the cam is long enough to be an atenna at the high frequency/short wavelength of cell signals (note the length of a typical cell phone antenna). There probably isn't much you can do about that, but if the mixer has a ground lift switch it might be worth a shot.

MORE IMPORTANT: Where can I get one of those pens, or a similar indicator? As a high school teacher I'd love to know when a student is being summoned "on vibrate" to exit the classroom, which happens a lot these days :>)
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Old August 24th, 2005, 08:29 AM   #6
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Wow. I had no idea!

Either way I figured I'd have to replace a piece in the chain. I'm mostly glad to hear it's not the camera.

No ground lift switch either.

I do have a Beachtek adaptor but it's the DXA-2 and doesn't supply Phantom Power. I originally bought the Behringer as a way to get voice recording into the computer. I orginally bought the DXA-2 to use with an AT897 (running on batteries) into a small Sony TRV-18 camera.

With the 3031 mic it requires Phantom Power so I grabbed the closest thing I had that would do that.

I could sell the DXA-2 and Behringer on Ebay and buy a DXA-6. Would the DXA-6 be suitable for this or should I still look for a pro mixer like you mentioned?

Thanks!
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Old August 24th, 2005, 08:43 AM   #7
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Another idea.

Is there an inexpensive filter I can add into the mix to eliminate the interference signals from the mixer (I know I can't do much about the unbalanced signal into the camera)?

Is there a way to solder in a ground to the Behringer? The case itself is metal.
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Old August 24th, 2005, 09:00 AM   #8
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One thing that may help is turning the -10db pad on the mic OFF. If the cellphone signal is leaking into the Behringer circuitry, the extra gain you're applying in the mixer to make up for the mic pad is making things worse.
Since the camera is set to line level, I would think it would be too hard for the signal to be coming solely through the unbalanced cable between the mixer and the camera. Did you listen with headphones to the mixer or only at the camera? Doing both would tell you if the problem is occuring at or before the mixer, or whether it's the output cable or camera input jack.
At close range, a mic capsule can pick up cellphone interference, although I've never had any specific problem with an AT3031. Even using all balanced equipment, I still have everyone on the set power down all phones and pagers. Of if they are just too important to do that... It's placed in another room.
Before you buy any major equipment, I'd make absolutely certain where the interference is entering your chain. I would invest in StarQuad type mic cables with known good wiring and connectors. They will help with interference problems in harsh environments and aren't that much more expensive.
I'd also try headphones that are designed specifically for field recording.
During your testing you could also set up a second mic, like your AT897, split to the other channel and see if that makes any difference.
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Old August 24th, 2005, 09:24 AM   #9
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Thanks for the ideas. I never did listen at the mixer level, only at the camera. Great suggestions. I think I'll do some testing at home, with the pad and phones off. I have one more recording session at the same place next week, so changing any equipment may be difficult before then anyways.
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Old August 24th, 2005, 09:32 AM   #10
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It may be possible to modify the Behringer mixer but it is not practicle unless you are skilled at soldering and circuit board repair.

The DXA-6 should work well but I have not tried it. Shure and Sound Devices also make small mixers that should work well. Remember to keep the unbalanced cable from the mixer to the camera as short as possible.

In a corporate setting turning all cell phones off may not be possible but it is possible to pick equipment that will work in this environment.

Sam
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Old August 24th, 2005, 09:41 AM   #11
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Steve, I want one of those pens too!

Ditto on what everybody else has said. GSM/GPRS phones seem to be the worst (this technology is also used by blackberries and the like). TDMA phones are less potent, but they can still cause problems. While we're on the topic, I've heard that some WiFi networks can create some issues as well (though I have not experienced this one yet).

I have a recording of some GSM noise that I could email to you if you'd like to compare it to what you're hearing.
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Old August 24th, 2005, 09:42 AM   #12
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Quote:
Originally Posted by Patrick Swinnea
Wow. I had no idea!

...

I could sell the DXA-2 and Behringer on Ebay and buy a DXA-6. Would the DXA-6 be suitable for this or should I still look for a pro mixer like you mentioned?

Thanks!
You might take a look at the SoundDesign's MixPre. An excellent professional grade field preamp and it does supply phantom power. About $700 from B&H. Or for more bucks, SD's 302 or 442 have excellent reputations, with corresponding stratospheric pricing of course. Also Shure makes some field mixer/preamps to consider.

Last edited by Steve House; August 24th, 2005 at 10:28 AM.
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Old August 24th, 2005, 09:55 AM   #13
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Quote:
Originally Posted by Patrick Swinnea
Another idea.

Is there an inexpensive filter I can add into the mix to eliminate the interference signals from the mixer (I know I can't do much about the unbalanced signal into the camera)?

Is there a way to solder in a ground to the Behringer? The case itself is metal.
The mixer's metal case, a Faraday shield, diverts my suspicion away from the mixer. Meanwhile, I think that the strength of the cell signals due to close proximity together with the magic of resonance can result in respectabe line level interference from the wire. So grounding the mixer box if you have time to play would still be interesting for its effect on resonance.
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Old August 24th, 2005, 09:58 AM   #14
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Quote:
Originally Posted by Jeremy Davidson
Steve, I want one of those pens too!...
Darn it, and both of you have ignored my request to tell me where to get one. Where's the love?

[Edit: Sorry, Jeremy-- At first glance I thought it read, "I have one of those pens too."]
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Old August 24th, 2005, 10:26 AM   #15
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I'll have to look it up this evening when I get home. Got it from a web source in Quebec but don't remember the name of the top of my head <grin>.
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