Ultrasound Modem
Send short text messages between two nearby devices over near-ultrasonic sound, using on/off keying of a 17 kHz tone.
Transmit≈ 22s
Turn the volume up. Most adults can barely hear 17 kHz, but pets and kids might.
Receive
14 kHzSignal 0%20 kHz
not listening
Received
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How it works
- Each character is sent as a 12-bit preamble (
111000111000) followed by its 8 bits, using on/off keying of a 17 kHz sine wave. - Every bit lasts 500 ms, with a one-second gap between characters. It's slow on purpose so cheap speakers and mics can keep up.
- The receiver watches the FFT bins around 17 kHz, samples once per bit period and looks for the preamble.
- It uses the same protocol as the original page, so old and new versions can talk to each other.