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> RTL-SDR is a very cheap ~$25 USB dongle that can be used as a computer based radio scanner for receiving live radio signals in your area (no internet required

> RTL-SDR is a very cheap ~$25 USB dongle that can be used as a computer based radio scanner for receiving live radio signals in your area (no internet required). Depending on the particular model it could receive frequencies from 500 kHz up to 1.75 GHz. Most software for the RTL-SDR is also community developed, and provided free of charge. https://www.rtl-sdr.com/about-rtl-sdr/ https://www.rtl-sdr.com/about-rtl-sdr/

themodelplumber 7y ago
I believe you can still get in trouble with these in the US; last I checked, for example, New York has a law forbidding the use of police-radio-monitoring equipment to monitor police frequencies when placed in a vehicle, at least. I wouldn't be surprised to hear there are other cities with stricter laws, even NYC itself.
- jyriand 7y ago
  Sorry for the stupid question, but why isn’t police using some kind of signal encyption, if they don’t want others listening in?
  - mschuster91 7y ago
    Because replacing analog radio with digital is expensive as everyone using it must be issued with new hardware (cops, fire trucks, EMS, public transport, military, secret services) and all relay/transmitter stations must be replaced too. Sometimes you actually have to build new stations especially underground/in big buildings because what in analog could be a bad, but understandable signal is undecipherable gibberish for a digital radio. It took Germany until something like ~3 years ago to finally do the switch.
    - adrianN 7y ago
      I've heard multiple times that you sometimes get understandable speech out of an analog radio where a digital signal wouldn't work. That doesn't make sense to me. If there is enough signal left for a human to understand speech, surely there is enough signal left for a computer to understand something if you have an appropriately chosen amount of ECC applied in your protocol.
      - j16sdiz 7y ago
        Analog radio usually use lots more bandwidth, maybe that's why.
        philpem 7y ago
        Not always. 25kHz "wide NFM" is still common where licences have been grandfathered in, but 12.5kHz "narrow NFM" is becoming more common. 12.5k NFM has the same signal bandwidth as a timesliced (dual timeslot -- two virtual channels per physical RF channel) DMR or APCO P25 signal.
        jcims 7y ago
        Instantaneous use is the same but channel management does probably make P25 more efficient for a bunch of departments to independently operate in limited spectrum.
      - philpem 7y ago
        The problem is twofold: "graceful degradation" and the effect of digital compression. Digital schemes like TETRA and DMR compress the audio and add error correction. The audio bitrate for DMR is somewhere around 2400 Baud. This means a lot has to be thrown away. As a result, some things confuse the codec (usually AMBE). I have Scottish and Irish friends with strong accents; AMBE makes their voices both indistinguishable and unintelligible. The second problem is what's sometimes called "the cliff-edge effect" or "graceful degradation". An FM signal degrades progressively as the signal gets weaker. If there's a temporary 'fading', you'll get a bit more hiss for a second or two. An experienced dispatcher or radio operator will often be able to "hear through" the hiss. Digital signals don't degrade gracefully. The audio quality is perfect up to a point, but after that point things fail quickly. You get corrupted, missing or glitchy audio, loss of control packets (more lost audio), loss-of-sync (no audio until the next SYNC burst, ~1-2 secs)... Incidentally, analog speech scramblers have similar issues. Rolling Spectrum Inversion scramblers send an FSK data-burst every time the seed (ephemeral key) changes. If your radio misses one of these bursts, the rx/tx audio will be screwed up for a second or two.
    - jcims 7y ago
      I think it's more about the technical difficulties associated with effective key management. All law enforcement and emergency services in my area use P25 trunked radio for comms. Encryption is available on their radios. They don't use it.
      - mschuster91 7y ago
        > I think it's more about the technical difficulties associated with effective key management. TETRA, the German digital police/ems/... network, uses SIM cards for that part. Pretty easy, if you ask me.
  - Grollicus 7y ago
    additionally to the Expense, in some situations (for example in a burning house) where the radio reception gets bad an analog radio degrades better: the signal gets noisy but you can propably still understand what's said, and you can ask for repeats and piece together something out of a few retransmissions. With digital radio on the other hand, a bad signal results in lost chunks. There the transmission is clear until it gets choppy and that's a lot harder to make anything out of.
    - heavenlyblue 7y ago
      BTW, does anyone know why aren't there codecs that would increase the speaking latency but spread out the information over a wider range of the signal, so that the lost chunks could simply be reconstructed from last and the next chunks received?
      - Grollicus 7y ago
        That's just another ECC and things like raptor codes to exist, which could achieve analog-ish characteristics in digital transmissions. I suspect this kind of encoding works badly with encryption, as decryption needs a perfect preimage otherwise the decrypted message will simply be garbage. Also, even a slightly increased delay makes for very awkward conversations.
- Jnr 7y ago
  Out of the box those devices act as digital TV and radio receivers. In fact I have been using them for just that for many years. Weird that someone can get in trouble for having a TV/radio receiver.
  - iaarn 7y ago
    That's like saying, out of the box a knife acts as something I use to cut bread, weird that someone can get in trouble for carrying one to stab people with it.
    - eeZah7Ux 7y ago
      No. Criminal law does not work by comparison. An SDR is not a knife.
    - mcv 7y ago
      Did the researcher actually stab someone with his RTL-SDR? I thought the arrest claimed just the ownership of the bread knife was enough reason for suspicion.
    - wongarsu 7y ago
      I don't know about the US, but German law has classes of knives you can only legally carry for purposes compatible with society. I am allowed to carry a leatherman for electrical work or to peel an apple, carrying that same knife for self defense or to play with it in front of the train station is illegal. You don't have to do anything, just carrying it in a suspicious context can be enough to have it confiscated.
      - madengr 7y ago
        I believe the USA federal definition of a weapon is a blade over 3”. Though they still won’t let you carry a less than 3” blade on a plane.
        micmil 7y ago
        The federal definition is essentially irrelevant unless you're traveling across state lines or live in DC. Such things are handled by the states in all other cases.
- baybal2 7y ago
  Something along the same lines works in Canada - by the law, every radio station must be registered with CRTC. A rule that is rarely enforced, but which was used few times in nineties to expel foreign embassy staff for just owning cellphones.
  - gruez 7y ago
    Can't they already expel any diplomat for any reason?

themodelplumber 7y ago

I believe you can still get in trouble with these in the US; last I checked, for example, New York has a law forbidding the use of police-radio-monitoring equipment to monitor police frequencies when placed in a vehicle, at least. I wouldn't be surprised to hear there are other cities with stricter laws, even NYC itself.

jyriand 7y ago
Sorry for the stupid question, but why isn’t police using some kind of signal encyption, if they don’t want others listening in?
- mschuster91 7y ago
  Because replacing analog radio with digital is expensive as everyone using it must be issued with new hardware (cops, fire trucks, EMS, public transport, military, secret services) and all relay/transmitter stations must be replaced too. Sometimes you actually have to build new stations especially underground/in big buildings because what in analog could be a bad, but understandable signal is undecipherable gibberish for a digital radio. It took Germany until something like ~3 years ago to finally do the switch.
  - adrianN 7y ago
    I've heard multiple times that you sometimes get understandable speech out of an analog radio where a digital signal wouldn't work. That doesn't make sense to me. If there is enough signal left for a human to understand speech, surely there is enough signal left for a computer to understand something if you have an appropriately chosen amount of ECC applied in your protocol.
    - j16sdiz 7y ago
      Analog radio usually use lots more bandwidth, maybe that's why.
      - philpem 7y ago
        Not always. 25kHz "wide NFM" is still common where licences have been grandfathered in, but 12.5kHz "narrow NFM" is becoming more common. 12.5k NFM has the same signal bandwidth as a timesliced (dual timeslot -- two virtual channels per physical RF channel) DMR or APCO P25 signal.
        jcims 7y ago
        Instantaneous use is the same but channel management does probably make P25 more efficient for a bunch of departments to independently operate in limited spectrum.
    - philpem 7y ago
      The problem is twofold: "graceful degradation" and the effect of digital compression. Digital schemes like TETRA and DMR compress the audio and add error correction. The audio bitrate for DMR is somewhere around 2400 Baud. This means a lot has to be thrown away. As a result, some things confuse the codec (usually AMBE). I have Scottish and Irish friends with strong accents; AMBE makes their voices both indistinguishable and unintelligible. The second problem is what's sometimes called "the cliff-edge effect" or "graceful degradation". An FM signal degrades progressively as the signal gets weaker. If there's a temporary 'fading', you'll get a bit more hiss for a second or two. An experienced dispatcher or radio operator will often be able to "hear through" the hiss. Digital signals don't degrade gracefully. The audio quality is perfect up to a point, but after that point things fail quickly. You get corrupted, missing or glitchy audio, loss of control packets (more lost audio), loss-of-sync (no audio until the next SYNC burst, ~1-2 secs)... Incidentally, analog speech scramblers have similar issues. Rolling Spectrum Inversion scramblers send an FSK data-burst every time the seed (ephemeral key) changes. If your radio misses one of these bursts, the rx/tx audio will be screwed up for a second or two.
  - jcims 7y ago
    I think it's more about the technical difficulties associated with effective key management. All law enforcement and emergency services in my area use P25 trunked radio for comms. Encryption is available on their radios. They don't use it.
    - mschuster91 7y ago
      > I think it's more about the technical difficulties associated with effective key management. TETRA, the German digital police/ems/... network, uses SIM cards for that part. Pretty easy, if you ask me.
- Grollicus 7y ago
  additionally to the Expense, in some situations (for example in a burning house) where the radio reception gets bad an analog radio degrades better: the signal gets noisy but you can propably still understand what's said, and you can ask for repeats and piece together something out of a few retransmissions. With digital radio on the other hand, a bad signal results in lost chunks. There the transmission is clear until it gets choppy and that's a lot harder to make anything out of.
  - heavenlyblue 7y ago
    BTW, does anyone know why aren't there codecs that would increase the speaking latency but spread out the information over a wider range of the signal, so that the lost chunks could simply be reconstructed from last and the next chunks received?
    - Grollicus 7y ago
      That's just another ECC and things like raptor codes to exist, which could achieve analog-ish characteristics in digital transmissions. I suspect this kind of encoding works badly with encryption, as decryption needs a perfect preimage otherwise the decrypted message will simply be garbage. Also, even a slightly increased delay makes for very awkward conversations.
Jnr 7y ago
Out of the box those devices act as digital TV and radio receivers. In fact I have been using them for just that for many years. Weird that someone can get in trouble for having a TV/radio receiver.
- iaarn 7y ago
  That's like saying, out of the box a knife acts as something I use to cut bread, weird that someone can get in trouble for carrying one to stab people with it.
  - eeZah7Ux 7y ago
    No. Criminal law does not work by comparison. An SDR is not a knife.
  - mcv 7y ago
    Did the researcher actually stab someone with his RTL-SDR? I thought the arrest claimed just the ownership of the bread knife was enough reason for suspicion.
  - wongarsu 7y ago
    I don't know about the US, but German law has classes of knives you can only legally carry for purposes compatible with society. I am allowed to carry a leatherman for electrical work or to peel an apple, carrying that same knife for self defense or to play with it in front of the train station is illegal. You don't have to do anything, just carrying it in a suspicious context can be enough to have it confiscated.
    - madengr 7y ago
      I believe the USA federal definition of a weapon is a blade over 3”. Though they still won’t let you carry a less than 3” blade on a plane.
      - micmil 7y ago
        The federal definition is essentially irrelevant unless you're traveling across state lines or live in DC. Such things are handled by the states in all other cases.
baybal2 7y ago
Something along the same lines works in Canada - by the law, every radio station must be registered with CRTC. A rule that is rarely enforced, but which was used few times in nineties to expel foreign embassy staff for just owning cellphones.
- gruez 7y ago
  Can't they already expel any diplomat for any reason?