5 ms·

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.
    - anilakar 7y ago
      I still haven't fathomed why ETSI chose AMBE+2 for DMR. It's a codec made by an American company for compressing American English speech and it works reasonably well for that purpose. However, most speakers in my native language sound as if they were drunk or having a stroke. Even Motorola has recognized that problem and added options in their programming software that makes the DSP boost higher frequencies before feeding the signal to the vocoder. As to the graceful vs cliff-edge degradation of signals I've yet to see real comparisons where both analog and digital radios use the same frequency, radiated power and bandwidth (with co-channel interference!). With digital systems, the degradation will surely be much more noticeable, but the real question is whether the critical signal level is above or below that of the analog system.
      - mschuster91 7y ago
        > I still haven't fathomed why ETSI chose AMBE+2 for DMR. It's a codec made by an American company for compressing American English speech and it works reasonably well for that purpose. However, most speakers in my native language sound as if they were drunk or having a stroke. A lack of ethnic diversity in vendors and standardization committees, which has also led to issues such as the "racist soap dispenser" (https://gizmodo.com/why-cant-this-soap-dispenser-identify-dark-skin-1797931773 https://gizmodo.com/why-cant-this-soap-dispenser-identify-da...). When most or all testing is exclusively done on White US-english male persons, issues such as a lack of sensitivity/quality for anything diverging from that norm only crop up when it is way too late to fix.
  - la_barba 7y ago
    There is lots of discussion to be had here, including that analog signal degradation is linear while a digital signal simply 'vanishes' if the SNR gets below a threshold. I suppose you could alternatively keep the existing analog infrastructure and apply an ECC to the analog signal too.
- 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.

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.
  - anilakar 7y ago
    I still haven't fathomed why ETSI chose AMBE+2 for DMR. It's a codec made by an American company for compressing American English speech and it works reasonably well for that purpose. However, most speakers in my native language sound as if they were drunk or having a stroke. Even Motorola has recognized that problem and added options in their programming software that makes the DSP boost higher frequencies before feeding the signal to the vocoder. As to the graceful vs cliff-edge degradation of signals I've yet to see real comparisons where both analog and digital radios use the same frequency, radiated power and bandwidth (with co-channel interference!). With digital systems, the degradation will surely be much more noticeable, but the real question is whether the critical signal level is above or below that of the analog system.
    - mschuster91 7y ago
      > I still haven't fathomed why ETSI chose AMBE+2 for DMR. It's a codec made by an American company for compressing American English speech and it works reasonably well for that purpose. However, most speakers in my native language sound as if they were drunk or having a stroke. A lack of ethnic diversity in vendors and standardization committees, which has also led to issues such as the "racist soap dispenser" (https://gizmodo.com/why-cant-this-soap-dispenser-identify-dark-skin-1797931773 https://gizmodo.com/why-cant-this-soap-dispenser-identify-da...). When most or all testing is exclusively done on White US-english male persons, issues such as a lack of sensitivity/quality for anything diverging from that norm only crop up when it is way too late to fix.
- la_barba 7y ago
  There is lots of discussion to be had here, including that analog signal degradation is linear while a digital signal simply 'vanishes' if the SNR gets below a threshold. I suppose you could alternatively keep the existing analog infrastructure and apply an ECC to the analog signal too.
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.