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!

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.

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

Ditto on the cell phones.

The periodic noise is your cell phone checking for text messages and voice mail. Turn it off.

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 :>)

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!

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.

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.

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.

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

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.

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.

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.

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."]

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>.

Jeremy, I'd love to get an mp3 of those sounds, though I'm sure that's what they are.

The unbalanced 2 x 1/4" output to 1/8" stereo input (mixer to camera) is about about 5 feet long. (I know. I know. This is way too long). It's the cable that came with the mixer. I usually use this to plug into an 1/8" on my computer, where 5 feet is a good length. Sounds like this is a serious mistake.

I'll try to contact Behriger to see what they have to say about it. Fred, you mentioned the mixer may not be the culprit due to the metal case. If it is possible simply use a shorter cable from the mixer to the camera that might hold me over until I can determine it is *for sure* the mixer.

The board room we recorded in is about the size of a large office. So even if the three phones in the room are off, there could be one on the other side of the wall that could still interfere (is this right?)

Next week I think we'll move it into a larger board room and shut down all phones, and go with a shorter cable. This would only be a temporary fix for next week's recording. Plus, I really like the sound of the 3031 Cardioid better than the 897 Shotgun for indoor recording. But I could pull out the 897 in an emergency.

And ditto on that pen. I want one too.

Jeremy, my email is cheezorg@gmail.com

Thanks again everyone.

I just got off the phone with Behringer and the guy I spoke with suggested better shielded cables. He also reminded me that the main out is a balanced output, and that it might be possible to find a Balanced cable that would run from the 2 x 1/4" jacks to the 1/8" stereo. He wasn't sure about this, though. He did tell me the question is extremely rare and that they don't get a lot of video related questions. Do you think it's possible to find a cable like this? I found several Balanced RCA to 1/8" and single 1/4" to 1/8" but nothing with *Dual* 1/4." Also, would it make a difference using balanced or unbalanced cables running into an unbalanced input like that on the FX1?

Could you run two balanced cables from the mixer to the Beachtek box, and then to the camera?

The stereo 1/8" input is not balanced, so "balanced" cables will not help there. The Beachtek, however, accepts balanced signals and could convert and send them to the unbalanced input on the camera.

I'll send that MP3 over to you as soon as I can get at my audio workstation. I'm a little surprised that the tech had never encountered this before, as it's not a problem limited to video applications.

Fred -- no worries!

If the mixer circuitry is completely enclosed in a metal housing, which I, and probably Jay, did not realize, I'd use stronger language and say it can't possibly be the source of the problem. The camera itself, if it has plastic case, can't be ruled out. A printed circuit trace as short as 1/4 wavelength, or about 1.5 inches, can resonate. But I think that the smart money is still on the unbalanced run. The five foot length of it is not an issue for this particular source of interference. Shortening it may not help Any length that is electrically a multiple of a quarter wavelength could resonate. That's why I thought that gounding might work. Infinite length.

I was about to mention the possibility of a shielded run or a balanced run up to the camera. But I see you've posted again as I was writing this.

To answer your last question first, any run that's unbalanced at either end is unbalanced throughout.

I need to go an have a look at that unit. The need for two 1/4" jacks for a balanced main out doesn't make sense, unless you're trying to take stereo.

Jeremy is right (and Fred too), there's no way to go into the 1/8-inch stereo jack that won't be unbalanced for at least some short length. Here are two ways to accomplish this:
The BeachTek is one good way. I often put a DXA-6 in between the mixer and the camera when using my Sound Devices MixPre with a friend's XL-1.
However you said you have the DXA-2, which only has one XLR jack. Are you using more mics than just the AT3031 that are panned apart or are you just sending the single mic to both outputs of the mixer in order to work with the cable you had? If you're keeping mics panned apart, then you'd need a two-XLR BeachTek. If it's just the one mic, then use one cable between the mixer and BeachTek and set the Beach for MONO. Depending on the design of the Behringer pan pots, you may get more level by panning to one side versus centered. For example Mackie mixers add 6db to the signal when you pan it hard over.
In addition, the BeachTeks are designed for MIC level camera inputs. Since your camera has the ability to handle LINE level inputs you may loose some noise advantage by using the BeachTek to knock it back down to MIC level.
However, has anyone experimented with keeping the BeachTek set at mic level and letting a line level signal go through unpadded into a line input? Would that produce distortion since the Beach is just a passive device and the camera can handle the hotter signal? It would depend on the Beach transformers I suppose.
Another method is to use a two-channel Ebtech Hum Eliminator. It's inexpensive and in addition to balancing or unbalancing any combo it will isolate you from ground loop hums (but wouldn't specifically filter your interference except perhaps by shortening your unbalanced cable). You would need to buy two balanced TRS 1/4-inch to TRS 1/4-inch cables to go between your mixer and the Ebtech. Then use a shorter, better shielded version of your Y cable into the camera. The Ebtech doesn't change the levels of the signals.
Fred, the Behringer has two main outputs that are each impedence balanced TRS jacks, although I'm pretty sure they cheap out and only send the full signal on the tip rather than sending a positive and negative half on the tip and ring. It's still balanced as far as noise is concerned, but less expensive to do it this way.
As a side note, I only meant to use the AT897 in testing, not that it would be appropriate for this recording.

Ah ha! So if I understand you are sugggesting to run the balanced connection from the Behringer through the Beachtek. That would keep the connection balanced up until the Beachtek, and the the connector from the Beachtek to the camera is just a few inches... Much less prone to interference.

The main out on the Behringer is a dual 1/4". So I would need a dual 1/4" to XLR to go from the Behringer to the Beachtek.

You would need one balanced TRS 1/4-inch to one XLR male cable to carry a single balanced signal from the mixer to your single-XLR BeachTek.
You'd need two of these cables and a two-XLR BeachTek (or Ebtech Hum Eliminator with TRS to TRS cables) to carry two mono signals or a stereo pair of signals.

Well. the both the spec sheet and the user manual contradict the phone rep. The main outs are unbalanced. [However, the spec sheet says that they are TRS jacks which seems self-contradictory. Probably a misprint.]

But now I see what you were talking about. You don't have a mono out. The first thing I'd do is pan the audio to one side and work with that. You're using a mono mic anyway.

You can convert the output to balanced and back to unbalanced with inline transformers, but you'd still be fighting to keep any unbalanced ends from peeking out.

Anyway, you can quell the cable's contribution cell phone interference with a properly shieded unbalanced cable, which you can easily make out of XLR. You can ground the shield to the ground lug of any AC outlet through a very small capacitor which would avoid the potential new problem of creating a a ground loop Let me know if you want details.

[Edit: Again I see there were several posts while I wrote this one and cooked some Kraft Mac&Cheese]

Although having wire runs that are a multiple of the wavelength would help induce interference, in short proximity situations anything can become an antenna due to the strength of the radiated rf field. I can get cell phone interference in the car stereo by having it too close to one of the speakers or the radio itself.

The room you are using may have weak reception for the phones which will have them searching more intently for a tower site.

If you could convince the meeting attendees to shut off their phones, it would be great for your audio track.

-gb-

This is fast moving thread. We must LIKE this subject.

BTW, update on subtext of this thread. A google search on "cell phone detector" turns up several devices, including the pen. Check it out. I'd say more, but I really don't want to distract.

Wow. You guys are awesome. There is almost too much information here to form a response!

I'm only using this one mic. The session itself is voice over for a food training video. We've been shooting the food portion in a noisy test kitchen, then walking down the hall to an empty conference room to record line spec voice overs. So only one mic. And I can definitelty connect it through the Beachtek and set it to mono. I'll play around with setting the Beachtek at MIC level and and the camera to LINE, just to see what kind of signal comes through...

I haven't had any problems with ground hums in the setup. Fred, I'd love to hear more about this -

"Anyway, you can quell the cable's contribution cell phone interference with a properly shieded unbalanced cable, which you can easily make out of XLR. You can ground the shield to the ground lug of any AC outlet through a very small capacitor which would avoid the potential new problem of creating a a ground loop Let me know if you want details."

Are saying the extra shielding could create a ground hum? Crap!

I love the equipment, but I'm obviously still learning how it works :)

Also, good call on the pens!

Another question -

With the small number of peope invloved in the recording sessions (4 max, including myself) it would be fairly easy to shut down the phones in the room. What kind of range is typical for this kind of interference? Could phones in other rooms bleed through?

It's not likely that phones in another room will interfere. You may be able to rescue your existing audio track via noise removal filters in some of the newer audio software. Basically, you give it a sample of the noise you want to remove as a 'noise print' then it finds and removes the noise that matches in the rest of your audio file. You are typically presented with options as to the amount of noise removal you want to apply because over doing it will kill some of the desired audio as well.

-gb-

Jay, thanks for clarifying my post about the Beachtek. Admittedly, I was just repeating what I've seen posted here so many times (I don't have a Beachtek myself).

Patrick, I just sent you that MP3 file.

Also, I agree with Greg -- my experience is that you have to be within 6-10' to pick up noise from a phone. I think I used the noise reduction function in Adobe Audition to remove the sound once -- it got most of it out. I still wouldn't count on fixing it with software though, so you'd probably be better doing a retake if you hear it during the shoot.

Folks, with apologies I'll have retract a lot of what I've said, including my swaggering pronouncement about the impossibility of the mixer being the problem. Back in post #3, Sam probably nailed it. In an act of graciousness I'll not soon forget, he PM'd me with a reference.

Basically, audio equipment manufacturers often terminate cable shields at a circuitry ground terminal inside the housing, then run a wire from there back out to the housing. That's fine for audio frequencies, but at rf frequencies that wire is a resistor due to inductive reactance. So the rf is applied to the audio ciruitry. It's called pin 1 error, as Sam tried to tell us. It applies to mics too. The writer of the reference tested 45 cardioid mics and found cell phone (specifically!) interference in most of them. Man!

Here's the reference
http://www.audiosystemsgroup.com/Pin_1_Revisited.pdf

Thank you again Sam, you're a class act.

Glad I could help.

Does anyone know for certain what the output of the mixer is? The spec sheet says electrically balanced. The block diagram appears to be impedance balanced but doesn't really show a resistor. If it is impedance balanced it can work fine into a balanced input and still reject noise.

Sam

Jeremy, thanks for the mp3. This is *very* close to the noises I have.

I do have one program - Soundsoap that I've use for occasional cleanup. Simple and easy to use. I've mostly used it for rumble, never for specific higher frequencies... Worth a try. Otherwise re-recording a couple of lines is easy.

Fred (and Sam) - Thanks for the clarification. Looks like the problem is inherent to the equipment. It'll be interesting to do some testing with different equipment configurations.

In the article he mentions a connector by Neutrik that eliminates this problem, even when there is a Pin 1 error. A google search pulls up dozens of Neutrik connectors. Just curious if anyone has used one of these and if it actually works.

Sam also said,
"Transformer inputs like Beachtech uses helps also."

So would this confirm what others here have said about running from the mixer into the Beachtek into the camera or is this suggesting a different setup altogether?

It's also good to know we're talking about 6' to 10' of interference. That's definitely workable with the equipment at hand for the short term.

The problem appears to be in the mixer so anything after the mixer does not help. If you have to leave phones on in the room either get a phantom power supply to go between the mic and the DXA-2 or get a DXA-6. If the phones can be turned off you can probably get by using the mixer into the DXA-2. That way you are balanced until just before the camera.

Sam

Thanks! Sounds good.

Patrick, Sam and I are looking at different units. Is yours the UB802 or the MX802A (or something else)?

It's the Eurotrack UB802 (cheap job - $100 retail).

Ah, that's what I suspected. The spec sheet and manual for that one, as I said earlier say that the main outs are not balanced. Yet the spec sheet does say that it is a TRS jack, impying impedance balancing. Another call to that support guy might be in order. As of now, to me all four are suspects--the mic, the mixer, the unbalanced run and the cam.

The mixer can be eliminated, as we've learned, by directly grounding the input and output cables to the housing (if they are not already).

I've queried Audio Techica as to the wiring of the AT3031 (I have two of these myself) vis-a-vis the pin 1 error. I'll let you know what they said tomorrow. I'm also going to experiment with my AT3031s and my cell phones.

If the first two are eliminated and the problem persists, the unbalanced run can be addressed several ways.

The UB802 was the first one I looked at. The users manual says electronically balanced. I take that to mean a balanced differential output. The spec sheet says unbalanced. The block diagram looks unbalanced. Who knows?

Sam

Where does it say electrically balanced? Page 7 of the manual says the mains are unbalanced. Clearly Patrick needs to talk to them again if he wants to find out what the story is.

Hey Fred,
We got to be looking at two different things. I was looking at page 9 of the UB802 Users Manual.

Sam