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Comment by jyriand | Hacker News Reader

Sorry for the stupid question, but why isn’t police using some kind of signal encyption, if they don’t want others listening in?

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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.

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.

Analog radio usually use lots more bandwidth, maybe that's why.

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.

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.

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.

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.

> 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-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.

It's more than purely testing[0], it's a lack of self-awareness. I remember seeing marketing / engagement material targeting worldwide customers and using American football as the illustration. Wrong kind of football if you're aiming beyond NA.

[0] Not to detract from the skin colour issues we've seen countless examples of, like the example you mentioned or the Google Photo fail https://news.sky.com/story/google-photo-app-labels-black-cou...

I expect it's partly due to the market ecosystem when DMR was created. The standard is codec-agnostic, but in practice DMRA (and thus all the manufacturers) settled on AMBE. If APCO P25 predated DMR, that'd at least partly explain the codec choice.

I'd absolutely love to see a Codec2 based DMR radio, even if it were a firmware mod to an existing radio.

As for the degradation - same. The only data I've seen is from radio manufacturers and the DMRA. Practically it's going to depend on the specific radio, the antenna and so on. You could measure sensitivity as dBm vs error rate for digital or dBm vs SINAD for analog and plot the two together.

Once you cross the line of what the ECC/FEC can handle, things tend to break down very quickly. Most DMR radios will cut out entirely if they see a lot of Post-FEC uncorrectable bit errors.

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.

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.

> 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.

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.

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?

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.

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