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Radio Freq. or Electromagnetic Interference, any solutions?

Discussion in 'Recording' started by RockNRoll, Jun 26, 2007.

  1. RockNRoll

    RockNRoll Active Member

    Hello all,

    After trying to troubleshoot the noise issue we've had in our studio for a long time, it seems that it could very well be RFI or EMI issues.

    In that case, if the source of these interferences are potentially occuring outside of the studio, are there any possible measures or solutions that can be utlilized in order to minimize the degrading affects it has on audio signals within the studio?

    Basic noise pattern we encounter consists of a high-pitched chirping sound that occurs intermittently. We hear it in our monitors and headphones. We even hear it in basic consumer PC speakers plugged into a laptop with our studio gear turned off. Guitar pick-ups also are affected by these emissions and the degree of audible noise varies as you physically move the guitar around like an antenna.

    Short of having to move our studio, we're willing to try any possible solution to try and resolve or at least alleviate these specific issues. Appreciate any tips and advice!

    Thanks!
     
  2. Boswell

    Boswell Moderator Distinguished Member

    Can you post a link to a sample of the interference? Some types of interference have characteristic acoustic signatures, and it may be possible for us to narrow down the source by hearing what the effect is.

    It's worth checking that there are no mobile (cell) phones switched on within a 100m radius of the studio.
     
  3. RockNRoll

    RockNRoll Active Member

    Hello Boswell,

    Sure, I will try and see if I can record a sample later today and post up a link soon. Thanks!
     
  4. Thomas W. Bethel

    Thomas W. Bethel Well-Known Member

    How close do you live to a radio or TV tower? do you live next to or around an airport especially one with ground search radar? Is there a cell phone tower in your vicinity? Can you link the sound with some appliances or HVAC coming on or going off? Do you have a oil fired furnace?, Does anyone do welding near you? Do you have any neon signs in your house? Do you have an electrostatic air cleaner or sump pump in the place were you are hearing the sound? Can you turn off parts of the house and see if the sound goes away? Do you have anyone that uses diathermy equipment near your studio?

    Any of these can cause problems and maybe what you are experiencing.
     
  5. RockNRoll

    RockNRoll Active Member

    Woh...thanks Thomas, those are some interesting things to look out for...I'll try and survey the area again with those tips in mind. Meanwhile, appreciate any further tips or suggestions from anyone that has experienced such issues. Thanks again. I will also try and record an audio sample of the noise we are encountering. Thanks again!
     
  6. Space

    Space Well-Known Member

    Are you using any type of power conditioners in this room/on the gear , like say Furman stuff?

    Lets assume you have a code required smoke detector that has a battery and that battery is low, it could produce a chirp...I'm just saying:)

    Fact is it could be coming from INside the room. If you place enough computer gear with enough outboard gear and add a TV somewhere close to the monitor, no wait, that is how my room is and it is free of interference.

    Nevermind...
     
  7. taxman

    taxman Active Member

    You should try to find an amateur radio club in your area (Ham Radio). If need be, you can contact the Amater Radio Relay League in Newington CT. A local club should have a techniucal guy who is good at finding RF interference. Someone with appropriate test gear may be able to find the source. Once they find the source, you can determine appropriate treatment.
     
  8. RockNRoll

    RockNRoll Active Member

    Hello all,

    I managed to record some of the noise sample from our studio. This is coming from a basic type of PC speaker. From the noise sound sample, you will be able to hear some kinda humming noise in the background, but the noise to watch out for is the high-pitched noise frequency that kinda chirps away...That particular high-pitched noise has been a common culprit in different areas and gears in the studio and those PC speakers have been able to pick up on those noises which is why I decided to record it right off the speakers as a sample........Just wondering if anyone has heard or come across such noises and if there are any suggested possible solutions if it pops up in any part of our system. Thanks for any tips or feedback.

    Here's the link:
    http://www.4shared.com/file/19175086/67d01782/Noise_Interference.html
     
  9. sheet

    sheet Well-Known Member

    Holy crap. That is a nasty mess isn't it?
     
  10. Boswell

    Boswell Moderator Distinguished Member

    Do you have your laptop switched on all the time your studio gear is operational? The prominent frequencies in your sample (ignoring the low frequency buzz) are at 8340 Hz and 12510 Hz (2nd and 3rd harmonics of 4170 Hz), with subsidiary peaks at 8006 Hz and 12204 Hz. These could possibly be beat frequencies from switching power supplies or screen update rates.

    Either way, the character of the interference is unlike rectified RF from an external transmitter, and I think you should go round power cycling everything in the building to eliminate it from being the cause.
     
  11. RockNRoll

    RockNRoll Active Member

    Thanks Boswell for your well detailed response! When you refer to switching power supplies, are you referring to using different power supplies to different gears throughout the studio? and for screen update rates, are you referring to LCD monitors in the studio? Do laptops possibly introduce unwanted noise around studio gear? Thanks for your help!
     
  12. Space

    Space Well-Known Member

    First thing I would question is the computer itself...but thats just me:) Then I would inspect cables and their connections. It reads/sounds like a poor connection. And then I would dump the computer speakers because they can amplify an already existing noise issue.

    Ya know if ya place a mic in front of a speaker it can feedback, same as if you place a guitar pickup in front of a speaker.

    Lot of things could be producing this noise.

    Be interesting to find out what really is.
     
  13. Boswell

    Boswell Moderator Distinguished Member

    Laptops contain many switching power regulators as well as the one in the mains adaptor. The screen drivers are the worst in terms of interference as they have that lovely flat aerial to radiate with. In addition, most laptop mains adaptors have 2-pin mains connectors, meaning no ground connection.

    You have to be really methodical in tracking this sort of problem down. You are half way there in saying that you hear the high-pitched sounds on the laptop speakers without any of the studio gear switched on, but what about the other way round? Do you hear it in the studio monitors with the laptop switched off and the mains adaptor unplugged from the wall?
     
  14. MrEase

    MrEase Active Member

    OK I've seen many of these replies and listened to your "cheep"! I'm not sure I agree with some of the responses and from listening I can see no way of telling whether the noise is caused by radiated RF (I would agree it's not a local radio station) or by RF on your power lines.

    What is needed is some form of logical diagnostic test sequence. You mentioned in your original post that even guitar pick ups get this noise - but the question as to the source is still not answered.

    First I would ask if you have a little battery powered practise amp - say a Pignose or Fender Mini Twin.for example. If not see if you can borrow one. If you try this set up with just a guitar, do you get the interference? If so the problem must come from radiated RF. If not, try plugging the practise amp to the wall with a wall wart. I think you will now be getting my drift... Basically the first step is to determine how the interference is getting into your systems i.e. mains or air borne.

    Next, although I have heard your noise I would try to find out when it happens i.e. does it only happen during the working day and stop in the evenings when every normal person has gone home? If so the source is probably some local industrial unit. Do you have any welding companies close by? Arc welding can produce some pretty horrible wideband RF problems both air and mains borne. Are there any microwave dishes around locally - and if so are you in their line of sight? If the problem only occurs in the evenings, do you have a radio ham in the building?

    I could go on but I hope I have given you some basic diagnostic ideas. Let us know what you find out and I'll try to help further.

    Edit:
    P.S. Where are you based? If you post the google earth co-ordinates we could have a look at your vicinity.
     
  15. RockNRoll

    RockNRoll Active Member

    Hiya MrEase,

    Thanks for your response. Yes, that's a good idea, I will try to zero in on the fact whether it's a mains issue or airborne issue. I will try out your suggestions and see if I can get a hold of a battery powered practice amp. I will come back to post any findings. Thanks again all for your patience and advice.
     
  16. MrEase

    MrEase Active Member

    So did you get anywhere with the testing?
     
  17. taxman

    taxman Active Member

    I reiterate my recommendation to try an Amateur Radio Club. Those guys find RF for fun. Its called fox hunting. They use wide band receivers and highly directional antennas. Coupled with a spectrum analyzer, they could find your source in a jiffy. They could also neutralize the interference. The American Amateur Radio Relay League, ARRL.org, publishes much material on the mitigation of RF interference.

    MW
     
  18. MrEase

    MrEase Active Member

    That's fine for finding the source of the interference but it does not mean that you can easily shut the source down (even if the source is an illegal radiation)!

    It is more logical to find HOW the interference is causing the studio problem first. Once that is known there are steps that can be taken to minimise or eliminate the problem within the studio itself. It is certainly not worth taking a scatter gun approach though as this could cost a small fortune with no solution. Of course if the studio was local to me I could take a spectrum analyser round but I certainly wouldn't bother doing that on a first call.
     
  19. rmburrow

    rmburrow Active Member

    RockNRoll: Are you within the United States? Do you have access to a GPS? If so, use the GPS to get the geographic coordinates (D,M,S N Lat., D,M,S W/ Lon) outside your studio location. Go to Home | FCC.gov and search for AMQUERY, FMQUERY, and TVQUERY. The program will ask for the site coordinates and a search distance. Put in 10 km (6 miles). These programs will drop out the call sign (and tech info in detail mode) for facilities within the search distance. This will tell you what stations are near by. If the separation between your location and the station(s) is less than 3 km (2 miles), you may have a possible interference source. RF interference ("RFI") can be a "tough dog" to find and eliminate, but one time through will make you cognizant of the symptoms in the future.

    Typically, you can hear a strong AM signal through your equipment (via direct audio rectification from cables, etc. acting as an antenna). If you hear the station's call letters, you can go back to AMQUERY on the FCC web site and get the technical info. FM interference may make a "swishing" noise, and television interference may have a regular timing pulse and "hash" from the data stream.

    Last but not least of a possible interference source is a MIDI box. I found this one the hard way on a live recording of an organ equipped with one of these. A spectrum analyzer test revealed peaks around 350 kHz and 1 MHz. The interference was there with the organ turned on, and disappeared when the organ was turned off. This interference got into the house PA system as a subtle hiss, and into my (tubed) condenser mics. Had to change mics to get a clean recording. These MIDI boxes are supposed to pass FCC Part 15 radiated and conducted interference standards for sale in the US, but a lot of digital stuff gets out there untested.

    Eliminating or minimizing RF Interference:

    1) Have you tried an isolation transformer on the AC supply to your studio gear? For standard U.S. 120 volt service, the neutral (white) wire of a receptacle is grounded on a bus at the electrical service panel. The ground prong (green wire) is also grounded at the electrical panel. When the equipment is plugged in, a ground loop is set up between the neutral and the safety grounds at the piece of grounded equipment. If the sum of lengths from the electrical panel to the piece of equipment approach 1/4 wavelength at a nearby transmitter's frequency, you may have interference. The isolation transformer breaks the ground loop through the neutral and preserves the safety ground.

    2. Are your mic or peripheral cables 1/4 wavelength long at the frequency of any nearby transmitter identified from FCC or other reliable data? If so, try different length cables. Unbalanced or high impedance cables are more susceptible to RFI.

    3) You may have to test each item of your equipment individually to determine which unit(s) are susceptible to RF interference.

    4) Ferrite beads work at FM or TV frequencies as well as RF chokes (Ohmite Z-50 or Z-144 depending on frequency). Bypassing also helps with RF interference but the components have to be chosen to minimize the interfering signal and not have adverse effects on desired audio/video signals.

    5) Shielding of speaker leads may help, as well as bypassing the speaker leads to ground at the amplifier. Unshielded speaker leads act as an antenna. Use of a single point ground for your equipment is essential. If you find RFI on a piece of equipment, contact the manufacturer and inquire if "fixes" are available to "harden" the equipment against RF interference. If the interference is in speaker cables, try shielded cables, with the shield ground connected to the ground of the amplifier. DO NOT use the shield as an audio carrying conductor! If the interference is still there, connect good quality MYLAR or ceramic 0.01 uF 400 volt capacitors from each speaker lead to the amplifier ground (bypassing the RF to ground). The 0.01 uF capacitor has a low impedance at radio frequencies, and high impedance at audio frequencies (across the 8 ohm speaker load); the purpose of the bypass capacitors is to keep the RF out of the amplifier by shorting it to ground at the speaker terminals. Use good quality capacitors since you don't want to short out your amplifier(s)!

    5A: Flat, parallel wires like zip cord or most speaker wire acts like a transmission line at RF frequencies. (Remember the old flat 300 ohm "twin lead" antenna wire for FM or TV?). If you can find twisted pair or twisted pair shielded cable, try that on your speakers instead of the flat stuff and listen for a reduction. Check the Belden catalog...

    Hopefully, those suggestions will work most of the time to get rid of RFI.

    Good luck...
     
  20. DonnyThompson

    DonnyThompson Distinguished Member

    There have been so many intuitive responses here and it seems that everything has been covered, but I will ask one question that may have been overlooked....


    Are you in a building/rental space that also occupies other businesses... like a beauty salon? small machine shop? injection molding?

    Regardless of whether you have your own electric bill, many buildings do not isolate the electricity from one business to another; and conditioning that power isn't something the electric company cares one bit about...

    just shooting in the dark here...
     

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