The story goes like this:
Person 1 is at the car with his mobile phone. Person 2, who is at a sufficiently large distance (approximately at least 200 m) calls person 1. Both persons put their mobile phone on the loudspeaker. Person 1 holds his mobile phone up to the receiver of the remote control for the central locking. Person 2 holds the remote to his phone and presses. The car will open with person 1 now. In other words: the signal from the remote control is ‘tunneled’ via the GSM connection. This seems very unlikely to me, and no one can prove it to me, but the burden of proof seems to be on me…
Answer
Dear Brecht,
In the early years of fixed telephony, a lot of research was done on which frequencies should ideally be sent in order to be perfectly intelligible on the other side. Where music uses frequencies up to 20 kHz (20000 Hz), it is sufficient for speech to limit the signals to 4000 Hz. This has the added advantage that the unused frequencies can be used for other things (other telephone calls, data, video) with special techniques. However, these are never made available to the calling user: after all, the telephone operator can charge money for them! Most central car locking systems use frequencies around 433.92 MHz. These signals are therefore never transmitted through the speech channel, let alone can be picked up by the microphone of your mobile phone or transmitted by the loudspeaker. In short, I don’t think this story is true…
Answered by
prof. dr. ir. Jo Verhaevert
Wireless communication
http://www.ugent.be
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