Listening to P25 “Fed” Radio with a $20 RTL-SDR Dongle

A guy takes the feed from an RTL-SDR $20 software defined radio and
pipes the output to an open source digital decoder package. All of a sudden all the gaping vulnerabilities we saw in Matt Blaze’s “Why (Special Agent) Johnny Still Can’t Encrypt”* are accessible to anyone.

I will note that, as far as I know, listening to unencrypted public safety radio in all its flavors is still legal.

And, key point here, because it works on commodity hardware & open source software, one can do this as just one person. (Which we all ultimately are.) Being free from dependency on large companies and the like may seem less important for day to day things, but in security it’s critical.


“This is another illustration of the amazing utility of the RTL-SDR and SDR# software. Here proct0r links the output from SDR# to a program known as DSD. DSD is an open source command line program originally coded in C by anonymous for Linux for the purpose of decoding digital protocols via a PC soundcard. The program accepts audio taken from the discriminator output from a scanner or amateur radio receiver fed to the soundcard input. DSD is then run (using the appropriate arguments, depending on the mode and parameters involved) with the resulting demodulated speech directed to the soundcard output jack. Additionally, various protocol decodes (P25 among them) provide a running display of certain packet data in the terminal window.”



  1. I’ve been waiting years for this, I refuse to spend $500 on a digital police scanner.

  2. Reblogged this on Reality Check and commented:
    More information to “listen in”.

  3. Reblogged this on Signal Corps.

  4. Does this software work with Windows?

  5. […] Patriot groups interested in radio communications recently came across another use for the RTL-SDR: decoding APCO P25 signals. […]

  6. No kidding? This is what this article is all about? It was/is complete gobbly-gook to me. I have no experience with RTL-SDR nor have any idea what it means.
    I always wanted a police scanner, never priced one, so never realized a scanner could cost $500. But to make my computer into a police scanner using the above software is foreign to me. I would probably wind up with a $1000 computer repair bill.

  7. Joe Circuitboard · · Reply

    Then you certainly would not want to spend $3000 on a WinRadio G315 with all the decoders that works full spectrum as either a pc card insert or a usb plug in.

  8. Hello. Does the software run with Windoze? Is this easy to set up for someone not too tech? Thanks!

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