Interference Problem

[Lenp]

I am doing ongoing service on two large board room sound reinforcement systems. They are in large separate office buildings, and not in the same areas of the city.

Both have many wired mikes, mixers, power amps, press feeds and whatever else is needed, but no wireless mics.

In the last year both systems have started to experience sporatic interference. It is a thrashy buzz that in one system comes and goes with no pattern. In the other system, when it occurrs there is about a two second on off cycle that lasts for about 10-15 seconds. In one system the interference is heard in the speakers, in the other it is not heard but is recorded on the digital recording being made with a laptop. I will be getting a copy of a typical rcording for further investigation.

All cables are shielded, professional equipment and proper connectors, all of industry quality. The equipment is grounded and the sound is pristine with no noticable noise on the audio bus when checked with a scope except the expected 'grass'

The rooms are frequently saturated with a 100+ person audiance with their cell phones, colored Berries and what have you as well as the press with the cameras and and their other splendid gear. This when the interference arrives.

An hour plus recording of the system with open mics in an empty room showed no interference.

So the issue is to attempt to identify the source and squash it. Short of turning off the cell phones and other 'toys', which is not an option, does anyone have thoughts on remediating this problem?

Please feel free to Buzzzzzz me if you do!

Len

[MrAl]

Hi Len,

Of course without seeing the system it's hard to say, but i could offer
some troubleshooting tips. I need to know what you can do first
however.

Can you try the system without some connections, that is, disconnect
the mic for one test, disconnect the mixer for another test, and connect
a resistor right at the amp input for another test.

The noise is probably being injected into the system from one of the
external sources, despite good grounding and shielding.

Disconnecting the mic and replacing it with a resistor of equivalent
impedance (keeping the mic cord intact) will tell you if it is the mic
itself, while disconnecting the mic AND cord and replacing it with
a very short cord and resistor will tell you if it is the cord, and
disconnecting the mixer will tell if it's the mixer, etc. If a resistor
connected to the amp (no mic, no cord, no mixer) still produces noise,
the amp is picking up the noise.

The idea is to first find out what is actually picking up the noise.

Audio amps are relatively simple, but yeah, they often pick up noise
and even sometimes the input section acts as a detector and picks
up radio stations.

Sometimes reducing the impedance of the input devices helps enough
if you dont loose too much gain.

[CeaSaR]

Is the sound "dip di dip dip ddddddddddddddd dip dip"? If so, you are
picking up a cell phone trying to contact a cell tower. I hear this all the
time in: computer speakers (no wireless there), car radios, standard
radios, and many other things that involve an audio power amp. Usually,
moving the offending phone away from the area will cure it. As for the
digital recording, it may be the phone of the person running the computer
affecting the incoming recording wire, hence no interference on the main
output amplifier.

For me, this began when our firm first bought cell phones for the crew
chiefs. Whenever you would be in the van and place your cell phone on
the dash, you'd hear that sound from time to time, especially when you
would turn the phone on and it would have to search for the closest cell.
You'd also hear it just before an incoming call. On of our personell
thought their car stereo was going bad and almost took it to the dealer to
get it fixed when I happened to be in the car and heard it. Saved a lot of
time and trouble there. Never happened again as the phone got turned off
before going home!

So, to summarize, keep cell phones away from mixing boards and other
critical high gain amplification. Also keep them away from computer
cables. If this is not an option (it must be, people worked sound for years
before cell phones were invented), then a scheme must be devised and
rigorously followed to lessen the offensive interference.

CeaSaR

PS, you might try bluetooth to move the phone away from the affected
areas while still retaining connectivity.

[jwax]

Does sound like a cellphone. Could you move around the empty room and turn on and off a cellphone yourself? If you could borrow a few different types you may be able to determine the systems most suspectible one, and post a notice.
Get fancy and decode the signal and call the person?
Another reason why most places have a "Turn OFF cellphone" sign.
We're assuming that the interference is not deliberate.

[skibeck]

It does sound like a cellphone near one of the cables thats injecting the noise. I have a website that has a recording of the dreaded GSM buzz. You can hear it at www.stopthebuzzin.com . See if it the same sound. I also sell a product called the "Buzz Shield". The product is simply an anti-static bag that has a logo on it. I am not suggesting you buy my product. Here is what I am suggesting. If you reach a point where you would like to try to shield all the phones in your auditorium I would be willing to send you 100 of these at no charge. Before the meeting you would instruct each phone user to insert their phones inside the bag. The phones will still operate but they will be shielded.

Whats the catch? All I would receive in return is a little publicity with the name of my website and my logo (bee holding a stopsign) on the bags. Anyway I thought it was worth a shot. The anti-static bags really work. You can do a google search on gsm buzz anti static and see where people have successfully tried it. I have had people on my website comment that "what you are selling is simply an anti-static bag", but then they follow up with "I tried it and hey it really works".

Good luck on finding your interference. Let me know if I can help.

Dave

[jwax]

If I understand you Dave, you're suggesting everybody put their cellphones, Blackberrys', etc. in a static bag when in these rooms?
And they'll still work normally, while eliminating the intercom buzzing?
I'd love to see that experiment!
John

[Lenp]

Dave,
Yes the buzzing has some of the same characteristics as your sound bytes.

I suspected cell phones from the beginning but the only way 'bagging' would ever work is to have a hypnotist or a guy with a gun, front and center.

The audience is political wigs and news media covering the meeeting with sometime live cable feeds. To take away the toys would be a mutiny. It's hard to keep them from interrupting the meeting with the ringtones.

All of your clips show the cellphone right near the receiving equipment, but the audience is actually maybe 20 feet away. Maybe the noise is from the board members themselves. (Board menbers are well known to create noise sometimes.)

If I could duplicate the problem, like Al said, there are many methods to isolate and maybe control the noise. The last time I was there we had 3 or 4 people in the room, all had cell phones and we did turn them on and off, with several laying on the boardroom table next to the 8 mics. No noise, not a pop or a click. With a room full of people, the noise crops up several times during a meeting. Maybe one cell phone is the offender. Maybe it's a beat beween a few, Maybe it's the press cameras and the 'duckie' antennas they use for linking systems. Lots of maybe's!

It may be possible to convince all the board members to turn off their cellphones for one session as a test. One problem is, many people do not understand the difference between 'off' and a muted ringer!

And yet, there may be repeaters in the area that swamps the area. We've had problems with wireless mics and signal swamping in downtown buildings in the past.

Dave: Are your bags any different than the typical anti-static bags. If they really do work maybe you should consider a 'mouse pad' like product to lay the phone in or on. It might be more acceptable than a 'plastic bag'.

This sure looks like a Techno-poliltical catch-22

Len

[skibeck]

John,

Yes I am suggesting all ATT and TMobile and Nextel users insert their cellphone into an anti-static bag. Sounds crazy eh? I too would love to see that experiment. Can you say fun with anti-static bags? But I understand Len's point that it would take a considerable effort for compliance.

Len,

Regarding the test you performed with 3 or 4 people in the room with cell phones. Were you able to connect these phones to the cell towers? As you probably know either sending or recieving calls, email, or text messages casues this buzzing. My phone has a sync button that I can depress and create the static. Another option would be to take a pair of computer desktop speakers that are known to have poor shielding to the front of the room. Power them up. Then have your phones generate the static. This will ensure you are creating the interference. Then move around the room.

Are there any wires or devices under the floor where the audience sits? Maybe the noise is jumping onto a wire or something in the floor.

If you suspect your board members have them turn off their phones as you have suggested but also have them place their phones on top of an anti-static bag. So, if they fail to turn off the phone at least you are protected.

Just some random thoughts. Hope it helps.

Dave

[Lenp]

There are two systems:

System #1: Worst offender
All the input cabling is located inside of a custom built 20' wide curved boardroom desk that seats the board. The 8 mic jacks ate on a filler panel that is inside the desk. The mixer and amps at at the end where the recording secretary is seated. Cable runs vary from 6 to 30 feet. The speaker wiring a C.V. system, and the wiring typically isn't shielded. Who knows how it gets there but the speakers are above the audience area, in a bulkhead with wire runs probably 100' or less. Laptop recording is done. No press or AV feeds are connected to the system. The press uses wireles or wired mics.

System #2: Occasional interference
Multiple board tables with fitted mic jacks connected to multipin (Edac) connectors to floor boxes. The 12 mic. inputs run under the floor slab to equipment room, with lengths from 40 to 150 ft. 3 hanging mics for audience response. The speakers are a C.V. system located over the audience gallery and wire is above the suspended ceilings. Multiple audio press feeds and cable network AV feeds are supplied from equipment room.

Since it sure sounds like cellphone noise I'll hunt down that area. The customers understand that it may never be easily resolved so I guess the years of service I've provided offers some crediility There is really no 'hot issue' with this ....as of yet!

BTW: Years ago I was involved installing dial-up dictation systems. One large, 200+ telephone installation at a banking center started to get 'police calls' recorded sporatically. The office building was just up the street from a central police bulding with a rooftop antenna farm. We suspected the worse. RF detection someplace in the miles of twisted pair Beldin cables and CJ terminal blocks. After weeks of isolating areas, by taking them out of service in an attempt to localize the transmissions, we narrowed it to a small area. A cubicle area of 6 desks. We found that One of the employees had (dating myself) a Regency Scanner. Yup, he was keying the dictation trunk and sending the audio over the phone handset. He thought it was a great gag, and even said "Hey, It sure took you guys long enough to figure this out"! . We billed the bank at overtime rate T&M. The company figured our invoice, and their lost production, was more than the trickster was worth. His laughing stopped on his way to the dumpster to get a box to clean out his desk!

Thanks all for the help. I'll repost if there are any updates!

Len

[CeaSaR]

Ever give any thought to the fact that most modern "office" type buildings
are nothing more than metal cages covered in sheetrock and concrete?
Steel support beams, steel joists/trusses, steel floorpans filled with steel
reinforced concrete, metal wall studs, etc. etc. With all of this steel/metal
encapsulating a building, it's amazing that a signal can get out! I would
tend to think that, more than anything, the cell signals would bounce
around inside for a short period of time before finding an escape route.
Perhaps with all those wireless gadgets inside your steel cage there is an
enormous buildup of energy, possibly synchronized, trying to escape, and
that is why only a couple/few of you in the room cannot recreate the
situation. Still, if there is a spot in the audio line that is susceptible to
extraneous noise, you should be able to get some sort of reaction.


CeaSaR