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Old August 24th, 2005, 11:02 AM   #16
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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.
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Old August 24th, 2005, 11:44 AM   #17
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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?
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Old August 24th, 2005, 11:59 AM   #18
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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!
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Old August 24th, 2005, 12:15 PM   #19
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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.
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Old August 24th, 2005, 12:46 PM   #20
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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.
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Old August 24th, 2005, 12:53 PM   #21
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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.
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Old August 24th, 2005, 12:55 PM   #22
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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.
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Old August 24th, 2005, 01:17 PM   #23
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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]
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Old August 24th, 2005, 01:22 PM   #24
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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-
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Old August 24th, 2005, 01:38 PM   #25
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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.
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Old August 24th, 2005, 01:59 PM   #26
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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!
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Old August 24th, 2005, 02:08 PM   #27
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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?
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Old August 24th, 2005, 02:12 PM   #28
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Quote:
Originally Posted by Patrick Swinnea
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-
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Old August 24th, 2005, 02:25 PM   #29
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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.
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Old August 24th, 2005, 02:36 PM   #30
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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.
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