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Author Topic: Inductive loop leaking into ClearCom Freespeak headsets  (Read 1991 times)

Pete Erskine

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Inductive loop leaking into ClearCom Freespeak headsets
« on: March 01, 2018, 02:41:27 pm »

In this theatre they use an inductive loop hard of hearing system.  Any FS pack with a dynamic mic picks up the program when a channel is keyed, allbeit 20 dB down with light weight headsets. 

Electret mics have no problem and dynamic mics in the Sony Cameras are ok.  Just the FS mic-in is picking it up.

any help is welcome....
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Keith Broughton

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Re: Inductive loop leaking into ClearCom Freespeak headsets
« Reply #1 on: March 01, 2018, 03:27:34 pm »

In this theatre they use an inductive loop hard of hearing system.  Any FS pack with a dynamic mic picks up the program when a channel is keyed, allbeit 20 dB down with light weight headsets. 

Electret mics have no problem and dynamic mics in the Sony Cameras are ok.  Just the FS mic-in is picking it up.

any help is welcome....
Can you determine if it's the actual capsule or the wiring in the boom that is the antenna?
I can't recall if the wiring in the boom is shielded or not.
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Pete Erskine

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Re: Inductive loop leaking into ClearCom Freespeak headsets
« Reply #2 on: March 01, 2018, 03:30:56 pm »

Can you determine if it's the actual capsule or the wiring in the boom that is the antenna?
I can't recall if the wiring in the boom is shielded or not.

I think its the unbalanced connection to the FS BP. I wrapped a headset in shielding and still picked it up.
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Jason Glass

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Re: Inductive loop leaking into ClearCom Freespeak headsets
« Reply #3 on: March 01, 2018, 05:36:42 pm »

I think its the unbalanced connection to the FS BP. I wrapped a headset in shielding and still picked it up.
Hi Pete,

Do you have any large snap-on ferrite beads that you could experiment with? One might also try soldering RF blocking capacitors into the headset connectors; admittedly a total PITA.

Sent from my Nexus 5 using Tapatalk

Pete Erskine

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Re: Inductive loop leaking into ClearCom Freespeak headsets
« Reply #4 on: March 01, 2018, 06:22:38 pm »

Hi Pete,

Do you have any large snap-on ferrite beads that you could experiment with? One might also try soldering RF blocking capacitors into the headset connectors; admittedly a total PITA.

Sent from my Nexus 5 using Tapatalk

no....  Ill look for some on the weekend...
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Ron Hebbard

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Re: Inductive loop leaking into ClearCom Freespeak headsets
« Reply #5 on: March 01, 2018, 06:39:07 pm »

In this theatre they use an inductive loop hard of hearing system.  Any FS pack with a dynamic mic picks up the program when a channel is keyed, allbeit 20 dB down with light weight headsets. 

Electret mics have no problem and dynamic mics in the Sony Cameras are ok.  Just the FS mic-in is picking it up.

any help is welcome....
One ugly solution that worked well for me with a Beyer DT-108 in a high EMI environment was to glue a second identical mic capsule on the rear of the existing mic and wire the two mics in series aiding.  With the second mic physically reversed, its induced voltage cancelled the induced voltage in the original mic.  With the wearer's voice primarily being picked up by the original mic, they were heard on the intercom loud and clear.  Looked ugly as sin and was a little weighty on the boom's tilt lock but worked superbly.  This was worn by an assistant LX while shuffling mid-performance patches in a large repertory theatre's dimmer room where the board operator was hoping to hear "Completes" on the various patch changes as they occurred.  The LX patch was a single conductor 'telco' style patch in the mid 1970's.
There you go and worth every penny I charged you. 
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Ron Hebbard.
« Last Edit: March 01, 2018, 06:56:16 pm by Mac Kerr »
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brian maddox

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Re: Inductive loop leaking into ClearCom Freespeak headsets
« Reply #6 on: March 01, 2018, 09:19:01 pm »

In this theatre they use an inductive loop hard of hearing system.  Any FS pack with a dynamic mic picks up the program when a channel is keyed, allbeit 20 dB down with light weight headsets. 

Electret mics have no problem and dynamic mics in the Sony Cameras are ok.  Just the FS mic-in is picking it up.

any help is welcome....

Okay, this is a fun one.  I guess that wired systems [including the cameras] are sufficiently shielded/grounded that they don't have the same issue, but since the FS is obviously not grounded the interference has no where to go?  We know that mic is unbalanced, so...

Please let us know if you come across a solution as it only a matter of time before i encounter the same thing...
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Henry Cohen

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Re: Inductive loop leaking into ClearCom Freespeak headsets
« Reply #7 on: March 01, 2018, 09:55:12 pm »

. . . but since the FS is obviously not grounded the interference has no where to go?

A common misconception. Is an airplane grounded? Yet there's a mass of high power radios and electronics that work well together. Three issues that contribute to RFI/EMI noise: 1) Lack of a sufficiently low impedance return path to the point of origin (usually the 0VDC rail) for the current created by the EM field that doesn't go through electronics or board traces; 2) Improper, or no shielding that suppresses RFI, that, again, has a sufficiently low impedance direct return path to 0VDC; and 3) Signal lines that are not balanced and thus can't mitigate EMI.   


Quote
We know that mic is unbalanced, so...

Which makes it more susceptible to EMI/RFI as there's no opposite magnetic field to negate the current in the shield.

I think Ron's solution of the opposite polarity mic element is brilliant (though I would have thought they would need to be wired in parallel).
« Last Edit: March 01, 2018, 09:58:25 pm by Henry Cohen »
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brian maddox

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Re: Inductive loop leaking into ClearCom Freespeak headsets
« Reply #8 on: March 01, 2018, 10:26:49 pm »

A common misconception. Is an airplane grounded? Yet there's a mass of high power radios and electronics that work well together. Three issues that contribute to RFI/EMI noise: 1) Lack of a sufficiently low impedance return path to the point of origin (usually the 0VDC rail) for the current created by the EM field that doesn't go through electronics or board traces; 2) Improper, or no shielding that suppresses RFI, that, again, has a sufficiently low impedance direct return path to 0VDC; and 3) Signal lines that are not balanced and thus can't mitigate EMI.   


Which makes it more susceptible to EMI/RFI as there's no opposite magnetic field to negate the current in the shield.

I think Ron's solution of the opposite polarity mic element is brilliant (though I would have thought they would need to be wired in parallel).

I knew if i made a sufficiently vague and boneheaded question based on my admittedly murky knowledge on this subject, someone who actually Knows Things would explain it in a way that i can remember.  So thanks, Henry.  I actually DID understand your answer.

Will i remember it in 2 days/months/years?  That's another matter entirely....  :)
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Keith Broughton

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Re: Inductive loop leaking into ClearCom Freespeak headsets
« Reply #9 on: March 02, 2018, 06:59:07 am »


It's my understanding that a induction loop system is a magnetic field  and not RF, so I'm not sure ferrite beads would be effective.
Rons suggestion is a good one.
This is how "hum bucker" pickups work on guitars.
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Re: Inductive loop leaking into ClearCom Freespeak headsets
« Reply #9 on: March 02, 2018, 06:59:07 am »


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