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Old July 18th, 2005, 06:20 PM   #1
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Radio Interference

I just finished a video assignment.
I was going to use the AC outlet at the clients office.
However when I plugged into the AC outlet I got a very powerful radio transmission from Cuba. I was going to have all five people with their own lapel microphone.
I had to end up using the camera microphone and the camera runnining on a battery.
WHY?
Yours truly,
Tom Elliott
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Old July 18th, 2005, 07:29 PM   #2
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Were you using wireless or wired mics?
Was everything OK on battery power?
Pleae give details of the setup.
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Old July 18th, 2005, 08:31 PM   #3
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All mics were wired going through a battery powered mixer. The only thing on ac power was the 4x6 auxiliary monitor.
When the monitor was was disconected, no more radio.
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Old July 19th, 2005, 08:46 AM   #4
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What camera, audio mixer and video monitor were you using? Does your audio mixer, all your wired mics and the connection to the camera use balanced cabling?
Without some further testing it will be difficult to say exactly which pieces in the chain are combining to create the problem. The monitor may just be the final link and not the whole problem.
It could be a bad cable, a problem with the mixer, a problem with the AC wiring, or your camera may be prone to audio problems when using an AC powered video monitor. (There's about a dozen more I could list, but you get the idea.)
In some cases using an isolation transformer like an Ebtech Hum Eliminator between the mixer and the camera can solve the problem, but in this case that's not certain at all.
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Old July 19th, 2005, 11:23 AM   #5
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Ok, here comes the detail.
The office was on the 22n floor facing the Atlantic here in Miami.
The secretaries in the law office complain about the Cuban radio station being heard as they playback the dictation.
====
Camera: Canon GL2
====
Audio mixer:Radio Shack 4 Channel stereo microphone mixer. model 7A5 powered by a nine volt battery or a DC9v100ma transformer that you plug into the normal ac outlet - I only use the battery for I ALWAYs get a 50/60 cycle background hum on the sound track.
====
The mics are off the shelf Radio Shack omni directional battery powered.
====
Video Monitor: Made by Marshall. A small 4x6portable that is ac powered or battery powered. In the past I used it powered off the ac outlets in offices because the battery that came with it does not last long, only about 90 minutes.
====
What is balance cabeling? Would balance cableing mean XLR cables?
Are there balanced/unbalaced XLR cables?
I use/have NO XLR cables or mics.
The monitor was the ONLY item using the power from the buildings ac outlets.
When I disconnected the monitor and used the monitor of the GL2 NO MORE RADIO STATION.
Since I was really upset I had RIPPED the mic cables/mixing board out of the loop (creating a massive mess) and used the camera mic only.
Then I got the over head airconditioning sounds from the vents. At the time of the recording I felt that the ac sound would be disturbing. As I write this and making the dupes for the client the ac sound is not as disturbing as I had first thought.
I did a few tests with the PPRO1.5 sound controls an found I could eliminate alot, not all, of the airconditioning sounds.
Had I not been so hasty in removing the mic set up and been able to use the mics on each person the ac sound would have been totally eliminated since the GL2 was on 100% manual.
That is about it.
Many thanks for the input and help.
Yours,
Tom
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Old July 19th, 2005, 03:56 PM   #6
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Hey Tom,

I put your dilemma to my friend who is a satellite truck engineer and he immediately figured out the problem.

According to him the building is acting as a giant antenna and because Cuba doesn't place RF limits on their radio stations there's tons of it flying around Miami. So when you plugged the monitor in to the wall you plugged into the building ground and therefore picked up the radio station. The RF then traveled along your video cable into the camera. That's why unplugging the monitor solved the problem.

You have to ISOLATE the ground in these circumstances and the cheapest way to accomplish this is by using a ground lifter. These are the little grey 70 cent dohickies (technical term) that convert 3 prong cords into 2 prong cords. He also said the secretaries should ask the lawyers to use these on their dictation machines.

If this doesn't work you may need a video isolation transformer, but these cost a couple hundred bucks. Might be a good investment however if you continue to have problems in Miami.

Post again if this doesn't make sense to you and I will try again.

Good luck!

Stephanie
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Old July 19th, 2005, 05:23 PM   #7
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Quote:
Originally Posted by Stephanie Wilson
Hey Tom,
....
If this doesn't work you may need a video isolation transformer, but these cost a couple hundred bucks. Might be a good investment however if you continue to have problems in Miami.

....
Stephanie
You can make a video isolation transformer that MIGHT work with a pair of 300 ohm to 75 ohm transformers connected back to back. Get one of those little thingys (another technical term) designed to match a 300 twin-lead TV antenna cable to a "F" connector input, looks like a squareish piece of plastic with a couple of screws on one side and a push-on coaxial connector on the other. Here's a picture of one:

- http://www.eenid.us/200-510.html

Get a second that does the reverse, looks like a liitle barrel with an "F" conntector on one end and a 300 ohm twin lead pigtail on the other. Another pitcher:

- http://www.eenid.us/200-500.html.

Connect them back to back with the spade lugs on one to the screws on the other. Instant 75 ohm ground isolation transformer for less than $5.00.
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Old July 19th, 2005, 05:53 PM   #8
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Very interesting, I knew about the 3 to 2 prong conversion adaptor plug, (and the building as an antenna) sometimes it handles the 50/60 cycle hum and sometimes it does not.
the ohm dew-hicky seems like a good idea however the Marshal video monitor I use only has BNC or RCA plugs to attach to the out video of the GL2. just how is the OHM attached to the monitor?? I put the monitor on an articulated arm which is attached to my tripod. I have noticed at some weddings that the video guy has normal 12 volt car battery attached to his rolling tripod to power his camcorder and moble sound equipment. Like I say when all are running off batteries there is to radio and no 5o/60cycle hum.
Next...;-)
Anyway, after searching on the net, it looks like a major portable sound upgrade: shielded xlr cables, mics and mixing board of a special transformer to handle the radio and the 60/50 cyle stuff.
I mean it is solvable for look at all the major concerts that go wireless for the performers mic, and the video coverage.
Thanks again for all the input.
Yours,
Tom
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Old July 20th, 2005, 05:12 AM   #9
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Quote:
Originally Posted by Tom Elliott
Very interesting, ...the ohm dew-hicky seems like a good idea however the Marshal video monitor I use only has BNC or RCA plugs to attach to the out video of the GL2. just how is the OHM attached to the monitor?? ...Tom
As desribed the connectors ar "F" connectors such as a cable TV system but your local Radio Snak has RCA-F and BNC-F adapters for a couple of bucks. Run a short cable from your monitor to the transformer and another from the transformer to the camera video out. Or make up a couple of 75 ohm cables with an "F" connector on one end and an RCA on the other. FYI - ohms are the electrical units of resistance and impedance.
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Old July 20th, 2005, 07:22 AM   #10
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Steve,
Thanks for your patience in this matter of radio interference. I will certainly give it a go for it was sure embarasing in front of the client. A painfull learning experience, only slightly relieved by finding out after packing up that it is a constant problem within the law office.
Right now it seems that the cheapest fastest solution is battery power all the way. And here I thought because of the new technology I was lighting up my location pacakage. 12volt car batteries are not LIGHT.
Hmm, a trickle charge from the ac outlet to the 12volt then to the equipment would that work or is it the fact of being connected in any way to the ac outlet creates the problem?
Any, these forums have always been most supportive.
Thanks.
Yours,
Tom
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Old July 20th, 2005, 07:50 AM   #11
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You're welcome, Tom. I have no idea if that little widget will help or not but for the price it's worth a try. So called "RFI" - radio frequency interference - is a not uncommon problem in almost very electronic application and can stem from a huge number of causes ranging from corrosion to faulty hookups to poor design of the equipment itself to simply being close to a strong transmitter.

I'll never forget the evening many years ago when I had a CB radio in my car. I was sitting in the parking lot of an all-night diner talking when the witress came out to tell me that my conversation was being broadcast loud and clear over their house music/PA system. LOL

There's a good general discussion of the problem and some trouble shooting ideas in the "Radio Amateurs Handbook" from the ARRL. Take a glance at it in your local library.
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Old July 20th, 2005, 07:50 AM   #12
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Quote:
Originally Posted by Tom Elliott
Steve,
Thanks for your patience in this matter of radio interference. I will certainly give it a go for it was sure embarasing in front of the client. A painfull learning experience, only slightly relieved by finding out after packing up that it is a constant problem within the law office.
Right now it seems that the cheapest fastest solution is battery power all the way. And here I thought because of the new technology I was lighting up my location pacakage. 12volt car batteries are not LIGHT.
Hmm, a trickle charge from the ac outlet to the 12volt then to the equipment would that work or is it the fact of being connected in any way to the ac outlet creates the problem?
Any, these forums have always been most supportive.
Thanks.
Yours,
Tom
That trickle charger would likely bring back your headaches with RF interference. You don't have to use car batteries. You can use smaller sealed lead-acid batteries that will run your monitor for a while such as the type used in many steadicam rigs.

Interesting story about that Cuban station. That's true about the other countries not applying limits to their output power like we do. In fact, the old ZZ Top song entitled "Heard it on the X" is all about high powered AM transmitters just over the border from Texas (with studios in Texas). The "X" in the song title refers to the fact that Mexican radio and tv station call letters begin with X as opposed to W and K assigned in the U.S. Anyway, the power levels on these stations can be so high that on humid nights, you can see corona discharge around the towers and birds that happen too close to those towers get fried. So, imagine how strong the RF field can be there in Miami. When you plugged in your power supply, it was converting the AC outlet to 12vdc which means it has diodes to rectify the AC current into DC current. Those diodes will also rectify an AM radio signal and as Stephanie's sat truck friend indicated, the whole building electrical system was your antenna. Just add audio amplifier and you have an AM radio receiver. It's really that easy to receive AM radio especially when the rf field is as strong as it is in that building (probably all of Miami for that matter.)

regards,

-gb-
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Old July 20th, 2005, 08:33 AM   #13
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Never, ever use a 3-prong to 2-prong electrical adapter to defeat the safety ground. That's not what it's made for and this can be deadly. In this case it most likely wouldn't do any good anyway. It's not the ground connection that acts as an antenna (for FM anyway) for RFI but the hot and neutral lines.
Using an un-grounding adapter can solve a ground-loop hum, but you should still only use an isolation transformer to do this without breaking the safety ground.
In addition, cheap microphone elements like those in your mics and the dictation machines can act as radio receivers themselves, especially when used with unbalanced cables and an unprotected mixer. Adding the AC powered monitor that is also probably lacking good RFI protection makes it a worst case scenario for interference in your harsh environment.
Video isolation transformers are available for about $80.
http://www.markertek.com/SearchProdu...ff=2&sort=prod
The AT899 lavalier mic is a good bargain with excellent sound, small size and a variety of clips, as well as phantom or battery power. $200 each.
Good battery powered mixers are much more expensive, but there are cheap ones available such as the Behringer MXB-1002 for $100.
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Old July 20th, 2005, 09:12 AM   #14
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Quote:
Originally Posted by Jay Massengill
Never, ever use a 3-prong to 2-prong electrical adapter to defeat the safety ground. That's not what it's made for and this can be deadly. In this case it most likely wouldn't do any good anyway. It's not the ground connection that acts as an antenna (for FM anyway) for RFI but the hot and neutral lines.
Using an un-grounding adapter can solve a ground-loop hum, but you should still only use an isolation transformer to do this without breaking the safety ground.
I was going to mention in my previous post that it wasn't the building ground, but the hot and neutral lines acting as the antenna. Using a two prong adapter wouldn't have helped much for the rf interference. He wasn't hearing FM radio, that requires a lot more 'demodulation' components than does simple AM radio. Anytime you pick up interference that you can actually understand, as in the example above, it is AM.

regards,

-gb-
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Old July 20th, 2005, 01:06 PM   #15
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I beg to differ with Jay on two points:

1) 3 to 2 prong adapters are used ALL THE TIME in the broadcast industry to avoid problems between shore and satellite truck power. I've run every kind of video and/or audio equipment at one time or another on a 2 prong adapter and haven't fried a thing.

2) A hum bucker wouldn't have solved the problem.

Stephanie
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