Detecting Mobile Phone Transmissions With a Sound Card

Anyone who had a cheap set of computer speakers in the early 2000s has heard it – the rhythmic dit-da-dit-dit of a GSM phone pinging a cell tower once an hour or so. [153armstrong] has a write up on how to capture this on your computer. 

It’s incredibly simple to do – simply plug in a set of headphone to the sound card’s microphone jack, leave a mobile phone nearby, hit record, and wait. The headphone wire acts as an antenna, and when the phone transmits, it induces a current in the wire, which is picked up by the soundcard.

[153armstrong] notes that their setup only seems to pick up signals from 2G phones, likely using GSM. It doesn’t seem to pick up anything from 3G or 4G phones. We’d wager this is due to the difference in the way different cellular technologies transmit – let us know what you think in the comments.

This system is useful as a way to detect a transmitting phone at close range, however due to the limited bandwidth of a computer soundcard, it is in no way capable of actually decoding the transmissions. As far as other experiments go, why not use your soundcard to detect lightning?

23 thoughts on “Detecting Mobile Phone Transmissions With a Sound Card

  1. If I remember well, GSM is TDM (Time-division multiplexing) and 3G and 4G are CDMA (Code-division multiple access). So GSM mobile phones send only during very short and intensive bursts, which is what we hear in speakers.

      1. Yeah, No way you’ll get any useful information out of it though. The sound card only picks up the switching on/off of the receiver and GSM modulation involves a lot more than that.

        A simple RTL-SDR should suffice though.

  2. In highschool I worked in a small team which built an phone detector in a simmilar way.
    We recorded signals of about 217Hz (the modulation frequency we thought, not sure if thats right).
    Additionally we disturbed transmission on 900MHz to make the phones reconnect when we wanted them to.
    Probably not legal at all but it was a fun project to work on :)

    1. Probably not, as I can still hear 3G phones placed too close to my desk speakers. Instead of the dit-dit-trr sound they make a much faster and quieter crackling sound.

    1. … Welcome to 2017, where this is done everywhere :)

      So, there’s multiple projects to decode GMSK of GSM, there’s a whole set of SDR-based 2G, 3G and 4G basestations, there’s of course analyzers that come with these and also a lot of these that have been separately developed.

      I mean, every modern handset is to some extent an SDR, and every modern basestation is an SDR, to a much larger extent, even.

    1. in the 70s we had a neighbor with a 50′ tall radio antenna, and about the time Roller Derby would come on was usually when he liked to get on the radio and transmit, got to where youd sit leaning with fists clenched trying to watch the tv then BUZZZ blah blah blah and half the screen was snowy. but the problem sorted itself out…

  3. .–. .-. . – – -.– … ..- .-. . -.-. . .-.. .-.. .–. …. — -. . … -.. — -. ·—-· – ..- … . — — .-. … . -.-. — -.. . – — -.-. — — — ..- -. .. -.-. .- – . .– .. – …. – …. . – — .– . .-. ·-·-·- -.. .. – -.. .. – -.. .. – -.. .- -.. .- -.. .- -.. .. – -.. .. – -.. .. –

  4. A friend’s nextel had each burst “hitting” the enter button on paying for gas at a convenience store. The clerk barked “stand back” so they could void the extra entries!

  5. A friend has a Galaxy Grand Prime with Verizon. It blasts interference into his speaker system connected to his computer.

    What’s weirder is the audio recording software he uses causes a regular pulsing tick in the speakers, only when it’s recording. It only affects the speakers, not the digital recording so he puts up with it. The newer versions of the software don’t cause this but he won’t use them due to a feature he likes and uses in the earlier version having been removed.

    The company knows about the tick in the older version but refuses to release a patch, or to put the removed feature back into a new release.

  6. I used to pick up someone’s phone conversation whenever I plugged my Yamaha keyboard into my cheap guitar amp. Presumably they used some kind of wireless landline handset, or maybe it was ham? Is it based on the same principles described above?

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