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  1. Member
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    Hello,

    When recording with my Tascam DR-40 audio recorder, I notice it picks up GSM cellphone signals. Sometimes, the signals are so strong as to ruin the recording altogether.

    Since that recorder supports "Stereo XLR/TRS mic/line input with phantom power (supports +4 dB line level input)", I was thinking of getting an XLR microphone.

    I have a couple of newbie questions:
    1. Are XLR microphones imune from cellphone interference?
    2. Are entry level mics good enough for amateur recording? Should I get a self-powered mic or one that will be powered by the recorder through phantom power?

    Thank you.
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  2. aBigMeanie aedipuss's Avatar
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    afaik it's not even possible for a portable recorder to intercept and record cellphone signals. first the signals are digital and the recorder has no decoder. second gsm frequencies are in the 380MHZ to 1900MHZ range well out of the recorders range of 20-20k htz
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    "a lot of people are better dead" - prisoner KSC2-303
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  3. Member
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    What are those, then ?

    I get the same issue with my Sony camcorder, although not as bad.

    https://vocaroo.com/i/s0jx9tDASq24
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  4. Mod Neophyte Super Moderator redwudz's Avatar
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    Have you tested this with your own cellphone to see if a cell phone is truly the origin of the interference?
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  5. aBigMeanie aedipuss's Avatar
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    it's in the audible spectrum. maybe a fax machine neary?

    Image
    [Attachment 47735 - Click to enlarge]
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  6. Yes, that noise is cell phone signals being picked up by your recording equipment. I get it too -- especially when my smartphone is next to a powered PC speaker. I hear the noise then a second or two later the phone rings.
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  7. Member
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    An XLR connection, if properly shielded and wired end-to-end, will cancel out RFI like this. The key is what's called balanced audio.
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  8. Take a large ferrite ring (those designed for common mode) Click image for larger version

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ID:	47740 and pass trough it mic wire few times (wound simple coil), you can also try to use few ferrite beads like this
    Click image for larger version

Name:	1280px-A_collection_of_Snap-On_-_Clamp-on_ferrite_beads.jpg
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ID:	47739, ferrite beads or ferrite ring core should be located close to input on your recording devices as possible.

    Originally Posted by JVRaines View Post
    An XLR connection, if properly shielded and wired end-to-end, will cancel out RFI like this. The key is what's called balanced audio.
    yes but this signal may leak in to devices on all wires also on shielding and as high frequency signal it may be not efficiently attenuated due poor CMRR of most audio inputs (they are signed to deal with sub-MHz mostly power network and LF signals) also parasitic capacitance may be responsible for this... balanced passive (transformer) may behave better but nowadays there is plenty of balanced active (transformer-less) circuits...
    Last edited by pandy; 5th Jan 2019 at 13:45.
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  9. Member
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    Originally Posted by jagabo View Post
    Yes, that noise is cell phone signals being picked up by your recording equipment. I get it too -- especially when my smartphone is next to a powered PC speaker. I hear the noise then a second or two later the phone rings.
    I forgot all about this. I used to get hear a vibrating hum on my speakers in the late 90's from all my GSM phones. So it's not limited to newer phones. I haven't owned a non-GSM phone after the mid-90's so don't know if all phones did that.
    Last edited by lingyi; 5th Jan 2019 at 14:40.
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  10. aBigMeanie aedipuss's Avatar
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    Originally Posted by lingyi View Post
    Originally Posted by jagabo View Post
    Yes, that noise is cell phone signals being picked up by your recording equipment. I get it too -- especially when my smartphone is next to a powered PC speaker. I hear the noise then a second or two later the phone rings.
    I forgot all about this. I used to get hear a vibrating hum on my speakers in the late 90's from all my GSM phones. So it's not limited to newer phones. I haven't owned a non-GSM phone after the mid-90's so don't know if all phones did that.


    is the captured noise caused by the phone vibrating on a table with the mic and shaking the mic at those frequencies? otherwise i'm not getting how an audible tone is getting recorded without being heard.
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  11. Originally Posted by aedipuss View Post
    Originally Posted by lingyi View Post
    Originally Posted by jagabo View Post
    Yes, that noise is cell phone signals being picked up by your recording equipment. I get it too -- especially when my smartphone is next to a powered PC speaker. I hear the noise then a second or two later the phone rings.
    I forgot all about this. I used to get hear a vibrating hum on my speakers in the late 90's from all my GSM phones. So it's not limited to newer phones. I haven't owned a non-GSM phone after the mid-90's so don't know if all phones did that.


    is the captured noise caused by the phone vibrating on a table with the mic and shaking the mic at those frequencies? otherwise i'm not getting how an audible tone is getting recorded without being heard.
    No. I used to hear it just before the neighbor's phone rang too (at least 20 feet away). It's EMI noise induced into the amp/recording device. I don't think it's literally the MHz/GHz cell signal itself but some other EMI from the phone.
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    I agree, I don't think it's the phone signal itself since like jagabo's experiences, it always happened seconds before the phone rang / was connected to the network*, so it's something in the phone itself that's causing the interference. It wasn't a steady hum, more like a pulsed on-off rhythmic tone (always the same) that I even got over my car speakers.

    *As I recall from my VoiceStream days, cell phones aren't continually connected to towers. Rather they poll during set intervals for the nearest tower even if they're stationary. The old Nokia phones had a secret setting that could extend the polling time (half DTMF?) increasing battery life at the expense of having a longer connect time if moving between towers. I think some Bluetooth devices have this capability also.
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    Ahhh...here's an article about it which seems to be limited to GSM phones: https://www.rfvenue.com/blog/2015/05/06/how-to-prevent-cell-phones-from-interfering-wi...udio-equipment

    And a sample of the sound I heard: https://www.youtube.com/watch?v=f9nOqTN10Lk

    @jagabo - Does this bring back annoying memories? ;-p
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  14. Originally Posted by lingyi View Post
    And a sample of the sound I heard: https://www.youtube.com/watch?v=f9nOqTN10Lk
    Yes, that's the sound. What I heard was a lot shorter though. That sample had 14 bursts before the continuous buzz. I seem to recall about 3 bursts then the buzz. And the buzz stopped withing a second or so.

    Originally Posted by lingyi View Post
    @jagabo - Does this bring back annoying memories? ;-p
    It wasn't very loud so it wasn't very annoying. Of course, it would have been if it appeared in audio recordings.
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    The interference I heard would vary by where I was. I really think it has to do with the polling of the towers. Just as sometimes it's takes longer than normal to hear the other phone ringing or sometimes you never hear it ring and the other person picks up.
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    Thanks much for the infos.

    Before buying an XLR microphone, I'll experiment by just asking everyone within a couple of meters from the microphone to set their phones to plane mode, and see if the issue goes away.
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  17. Originally Posted by yetanotherlogin View Post
    Thanks much for the infos.

    Before buying an XLR microphone, I'll experiment by just asking everyone within a couple of meters from the microphone to set their phones to plane mode, and see if the issue goes away.
    Using those snap-on ferrite beads will not hurt anything and you will be impressed how "magically" such small thing may improve situation - you can search somewhere in your old cables - perhaps there is somewhere such ferrite.

    And explanation is quite simple - strong EM field may induce real current that will trig semiconductor joins to act abnormally (junction work as detection diode and follow power envelope so bursts of energy are easily recognizable) - i saw myself CRT's displays turned OFF by 4W UHF radios, some people reported permanent damage of H deflection switch due of this (and those switches for example BU208 are quite robust as they work in under high voltage and high power so they are not so easy to broke).
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  18. Member Cornucopia's Avatar
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    @aedipuss, if EMI/RFI leaks into an analog line, even if the main carrier is highly ultrasonic, there will be what amounts to ring modulation artifacts when that interference signal blends with the "wanted" signal. This includes both sum frequencies and difference frequencies, and it is quite possible that those difference frequencies are in the audible range and can be noticeable, especially with particularly strong phone ring signals.
    Add to that the possibility during digital capture of there being enough ultrasonic signal in the resulting analog blend that it "overpowers" the anti-aliasing filters and so alias/foldover signals (birdies) are created in the resulting digital code due to the equivalent of subsampling.

    This is unfortunately a common occurrence in location recording, and is something even pros have to watch out for. It even has a name - the "galloping horses" sound.

    Scott
    Last edited by Cornucopia; 6th Jan 2019 at 12:55.
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