acoustic data transfer

on one device pick send, on another pick receive, then place the speakers within a few feet of the second device’s microphone. keep the volume up. text is encoded as binary fsk: 1100 hz = 0 · 2100 hz = 1 ·700 hz = sync.

15 / 24 chars · 120 bits + 8 checksum

frame duration ≈ 13.8 s

• this is a real modem. the same fsk modulation that connected the world to the internet in 1985 is running entirely in javascript above your speaker.
• no network is involved. the only path between the two devices is air pressure.
• tone-based pairing is everywhere. retail check-in apps, mobile games, and conference badges have used near-ultrasonic data-over-sound for years — usually without telling you.