[dancsi]

This reminds me of how the student meal subsidies are implemented in Slovenia, and in my opinion it was quite unwieldy. You call a phone number and place your phone's earpiece on another device with a microphone. Then, some personal data is transmitted using (ultra?)sound. I remember it being quite unreliable, but that might be down to using the telephone network as the data carrier.

[tut-urut-utut]

I remember back in the time, during the eighties, one of the radio stations broadcasted ZX-Spectrum games over the air. You would record that noise to a tape, and later you could load to your ZX and play. It worked remarkably good.

[noobermin]

Generally datasette formats were meant to work with the extremely lossy media of tape. Also, I'm not sure but telephone audio tends to be much more compressed compared to radio. That on top of the fact you are going from earpiece to mic with an air gap and the background noise, it's probably much much worse.

[ale42]

Regular telephone audio is usually filtered 0.3-3.4 kHz (this on analog phone lines). Digital phone lines use PCM with A-law (or µ-law in the US and some other places) logarithmic sample encoding, with 8 kHz sampling rate, getting more or less the same result of an analog phone line, possibly with less distortion. FM radio has much higher bandwidth, usually up to around 15 kHz and with better basses too.

[moffkalast]

> (ultra?)sound

Well it's not ultra I'd say, that screeching is quite audible. Very similar in sound to an old phone modem.

I think the system it uses is the same as for Moneta, which can be used in much the same way but gets billed to the sim account instead. I'm sure other countries also use the same principle for some services.

[Aachen]

> using (ultra?)sound

Phone lines have (or at least had) narrow frequency ranges. I'm not an expert but I'd assume this is just normal sound like any old modem.