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Old May 29th, 2008, 05:54 PM   #1
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Weird Audio Recording Problem in Certain Buildings

Here's an audio recording problem that I haven't seen mentioned in any forum, but I know its real because I encounter it frequently.

I videotape depositions, usually in law office buildings and skyscrapers, and occasionally I'll come across a building that always has the same problem: The VU meter will show a crazy type of jumping around, as if someone is talking steadily, although no sound can be heard. Sometimes, it creates a static noise the shows up on playback only. On other rare occasions you can hear a very faint whine or buzzy hum over the mixer channels.

While working I don't have much time to experiment, and I can't reproduce the problem at home. Normally I use four Sony ECM-44bs a Shure mixer (267 and 367) and send this signal to the recording decks, which currently are a lite-on DVD recorder and a Mitsubishi DVHS deck. However this phenomenon has occurred with a wide variety of recording decks, and I've observed it for years, so I am confident that the recording equipment is not the problem.

It seems to be associated with certain buildings, as I've observed the problem in the same room with different recording kits. I believe it has something to do with the way the electrical system in the building is grounded, but that's just speculation on my part.

Has anyone else out there encountered this problem? Do you know the cause? Are there any solutions?

Is there a certain kind of power strip one can buy that filters out this weird phantom signal? Any ideas? Thanks.
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Old May 29th, 2008, 06:38 PM   #2
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I'm unsure where you are in the SF area when this occurs, but I have had closeby radar dishes create similar problems for me in the past. But I was within a mile of a radar dome when it occurred (though on the ground, and not up in the air in a building), and it was a single dish so it occurred on each sweep of the dish. If you are in So SF or Redwood City or on the Okaland side near the Alameida Naval Air Station and or Oakland airport there *might* be something akin to that around. Just a thought, though I cannot say with any certainty that is your particular problem necessarily.
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Old May 30th, 2008, 11:09 AM   #3
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Interesting suggestion

Though not what I'm experiencing. This effect is associated with particular offices in certain buildings, and not the one next door, and it occurs in any city.

Bu I am convinced that it is a radio frequency problem, and I'm also 95% sure its coming through the electrical system.
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Old May 30th, 2008, 01:44 PM   #4
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I would power everything on battery. Don't rely on having good AC power on location.
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Old May 30th, 2008, 02:03 PM   #5
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Originally Posted by Ken Reeser View Post
Though not what I'm experiencing. This effect is associated with particular offices in certain buildings, and not the one next door, and it occurs in any city.

Bu I am convinced that it is a radio frequency problem, and I'm also 95% sure its coming through the electrical system.

Ken,

One typical source of random RF energy is the ballasts in fluorescent light fixtures. In an installation of 100 lights, 99 of them might have well operating ballasts, but the 100th might be putting off enough RF to qualify as a radio station.

Next time you encounter the problem, try turning off the lights and see if the problem goes away.

If that's the problem, you're likely running balanced audio lines from the mics to the mixer -- but UNBALANCED lines from your mixer to the recorders.

Unbalanced lines use RCA type (or mini-pin) inputs. Balanced lines use XLR connectors (or sometimes 3-conductor 1/4" phono plugs) but there MUST be three conductors at EVERY connection point. If you just have signal and ground pins somewhere - the connection is NOT balanced and will NOT do a good job or rejecting that kind of RF noise.

You might need to record audio separately on something with balanced inputs if this is the issue.

Good luck.
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Old May 30th, 2008, 04:31 PM   #6
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You're absolutely correct; I am running XLR cables for the mics, but an RCA split into two for the left and right inputs on my recording decks. A balanced line input is not an option.

Is there someway to shield my lines to the record deck? Or otherwise get around that issue? Could I jury-rig a ground from the XLR output ground wire to the recording deck? Like just run a wire between the two?

I will try that with the lights, but wouldn't that occur if the offending ballast was anywhere on the floor of the office was in, and not just the lights in the room? I can't go around turning off all the lights. But its worth investigating, thanks.
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Old May 30th, 2008, 05:03 PM   #7
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Originally Posted by Ken Reeser View Post
You're absolutely correct; I am running XLR cables for the mics, but an RCA split into two for the left and right inputs on my recording decks. A balanced line input is not an option.

Is there someway to shield my lines to the record deck? Or otherwise get around that issue? Could I jury-rig a ground from the XLR output ground wire to the recording deck? Like just run a wire between the two?

I will try that with the lights, but wouldn't that occur if the offending ballast was anywhere on the floor of the office was in, and not just the lights in the room? I can't go around turning off all the lights. But its worth investigating, thanks.
Exactly how are you doing the unbalancing/splitting? What are you connecting to what (pin to pin)? Would a new mixer be in the cards if necessary? (I'm thinking along the lines that my SD mixer has unbalanced consumer line level "tape out" outputs that parallel the balanced main-outs, for example.)

Take a look at the technical notes on the Rane website regarding interconnecting balanced and unbalanced equipment before starting to ground chassis to chassis - sometimes it will make it worse rather than better.
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Old May 31st, 2008, 06:00 AM   #8
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Originally Posted by Ken Reeser View Post
You're absolutely correct; I am running XLR cables for the mics, but an RCA split into two for the left and right inputs on my recording decks. A balanced line input is not an option.

Is there someway to shield my lines to the record deck? Or otherwise get around that issue? Could I jury-rig a ground from the XLR output ground wire to the recording deck? Like just run a wire between the two?

I will try that with the lights, but wouldn't that occur if the offending ballast was anywhere on the floor of the office was in, and not just the lights in the room? I can't go around turning off all the lights. But its worth investigating, thanks.

The typical technique is to get an adaptor at the camera/recorder that does a VERY short unbalanced run. Units like the Beachtek or Studio One audio adaptors mount at a camcorder and allow you to run balanced lines up to a couple of inches from the camera's audio input.

That's typically enough to solve the problem.

There's really no way to run an external wire and stop this because it's not really exclusively a "grounding" issue. Ground loops are just ONE kind of audio problem. Inducted RF like you're likely facing is another.

Balanced audio systems defeat inducted issues by creating two copies of the audio signal - one out of phase with respect to the other, then inverting one and summing them back together - a process that phase cancels common mode noise like inducted hum or RF. Balanced audio is a system that actually CANCELS noise wherever it comes from - not just fixes grounding issues.

As to the distance, I was a employing a little hyperbole when I said it was like a radio station. RF from a typical light ballast or motor or something similar typically isn't strong enough to be a big problem a room away from the source.

Of course, the strength of the RF is something you can't control. So understanding and running properly balanced audio is always a smart thing.

Good luck.
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Old May 31st, 2008, 10:26 AM   #9
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Thanks for These Tips

I will prepare some of these solutions as best I can and see if they work. I'm not recording to the camera, but I will try using a balanced XLR cable instead of an RCA cable for the length to the deck, then use a short adaptor, and see if that helps.

However, the symptoms are present in the mixer (with the crazy VU meter needle), before the output, so I suspect that won't be the cause/cure. Plus, this isn't typical RF noise (and I get plenty of that, too), in that its not audible itself, it just has weird, sometimes audible effects. I am 90% certain the problem is coming through the power lines, through the mixer to the decks. I'd like to cut it off at the source: the wall socket. If running on batteries solves it, then that will prove that thesis.

(I just recalled something: previous recording kits that had this problem used balanced lines throughout, with the exact same symptoms.)

What about "Power Conditioners"? Anyone ever use those? I've read their advertising, but am still unsure what they accomplish. If I could afford it, I'd just buy one like this:

http://www.mapleshaderecords.com/aud.../powercord.php

...and see what happens. But that way could be expensive. Any thoughts on this?
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Old May 31st, 2008, 11:47 AM   #10
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Great ideas guys! All very spot on.

But if it's showing up in the mixer meters, I'm thinking sub-sonics or super-sonics.

To test for sub-sonics, you'd low cut and watch the meters. If they start behaving normally, the building has some sort of resonance from an HVAC unit, an underground subway or maybe even truck and/or buss traffic.

The HF stuff is harder to isolate. Computer-controlled lighting systems sometimes put out a fairly HF whine and then there's motion detectors.

If there are a lot of security NTSC TV monitors around, the horizontal oscillator running at 15,750 kHz in each of them could be getting into your audio. Most folks can't hear it, but a mic can. If you grab some room tone and scrub it slowly in a workstation, it'll come down in pitch to where you can hear it.

For LF, you'd have to grab room tone and double or triple the playback speed to catch the ghost.

Regards,

Ty Ford
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Old May 31st, 2008, 01:12 PM   #11
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Running with Ty's idea about subsonics - is there any relationship between the buildings you experience this is and BART lines or stations?
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Old June 2nd, 2008, 03:15 PM   #12
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I do use the low-cut, but it doesn't affect this problem. As for something like a Bart station, like the radar-dish, the locations are random, not very numerous, but confined to a building. The buildings are in many cities and suburban settings. So the source is not likely coming from outside the building. That plus being around 30 floors up makes anything coming from BART unliklely.

As for HF, that idea sounds interesting! I hadn't thought to check for motion detectors, however I don't know about computer controlled lighting. I have no idea how to check for that. Leaves me with the problem of bocking it still.

I'm going to try to balance my mixer-to-deck cable, with some sort of balancing adapter. But it did just occur to me that, if the wire picking up the interference was the mixer-to-record deck cable, how does it show up on my mixer's VU meter? I will test the idea by disconnecting the cable and seeing if the effect still persists in the mixer's VU meter.

I still strongly suspect the building's power system. Running the mixer on batteries will test that. Maybe.

On a previous suggestion: I just realized that there was one client I frequented often, and the offending effect occurred in each floor of their three floors, every room I set up in, and the same level, over a period of years. So I doubt that it was a faulty fluorescent light ballast.
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