Ultrasounds Beacons

The transmitting device generates and radiates a sound whose frequency spectrum has energetic content only at frequencies that are not audible by the average human ear (f > 19kHz). These frequencies are chosen in order to still be detectable by the microphone and circuitry of any smartphone, tablet, phablet or personal computer (f < 22kHz). The sound signals are defined in such a way that they can be uniquely paired with integer positive numbers expressed with N bits on a binary base.

• Coding: given the information content to be transmitted (N-bit binary numbers), the coded signals are generated by the combination of N waves, one for each of the N information bits. The single wave will have different parameters values depending of the value of the n^th bit. The resulting signal is synthesised with a given desired time length, normalised in amplitude, converted into digital domain, compressed if needed and finally written into memory.
• Transmission: The processor cyclically reads the digital signal from the memory and de-compresses it, if needed. The signal is then converted into analogue domain, sent to a programmable gain amplifier and radiated by a high efficiency loudspeaker. The amplifier gain and the radiated sound pressure level are set differently for different desired transmission distances.
• Receiving and decoding: The microphone embedded into the decoding device is reached by a sound pressure level that it’s turned into an electrical signal varying with time. This signal is converted into digital domain and then processed as follows: a time/frequency analysis is performed on the digital signal and for each bit of information the algorithm measures the parameters values associated to that bit, thus checking if it’s a 1 or a 0. The algorithm also compare the information energy with a minimum energy level, taken as equivalent noise level. This allows to discard the decoding results on which the uncertainty is too high and it also allows the definition of the global sensitivity level for that decoding.