Normal laptop speakers and microphones can transmit inaudible ultrasound signals between devices.
The author implemented a return-to-zero frequency shift keying (FSK) encoder and decoder using WebAudio APIs.
A demo web application enables sending and receiving text messages via ultrasound at about 10 bits per second.
The current implementation is fragile, with no error correction and limited speed, but the source code is available for enhancement.
Ultrasound-based data transmission is already used by conferencing software for device proximity detection.
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