SoundLink v2 · OFDM

Send a file from one phone to another using only sound.

Pick a file or type text. Put the receiving phone within arm's reach, its Receive tab listening.
Also: save as audio file

The whole transmission as a WAV (2 passes) — send it later; the receiver plays it near a listening phone or drops it into the Receive tab.

All local — nothing leaves your device. Custom OFDM modem: Auto probes the link and bit-loads 199 carriers (1.8–8 kHz) with 16‑QAM/QPSK/BPSK per carrier, coherent detection with pilots, Reed–Solomon FEC with erasures; fixed DQPSK/DBPSK modes as fallbacks. Real-world rate ≈ 400–1500 B/s depending on the phones and the room.

Questions

Can you really send a file with sound?

Yes. The file is modulated into audio and played through the speaker; the other phone's microphone hears it and demodulates it back. It is a modem, exactly like a dial-up one — the medium is the air in the room instead of a phone line.

What does it sound like, and will people hear it?

It is audible, and it sounds like noise or a warbling tone. This is not an ultrasonic tool that works secretly. Anyone in the room will hear it and anyone in the room could record it, so treat it like reading something aloud.

How much data can it carry?

Small files — a photo, a contact card, a short note, a link. Audio over a room is the narrowest of the three modes by a wide margin. Use it when there is genuinely nothing else.

When is sound the right choice over QR?

When the phones cannot see each other but can hear each other — in a pocket or a bag, across a table without lining up a camera, or where a camera is not allowed. Sound also does not need either person to hold anything steady.

What ruins a sound transfer?

Background noise, distance, and volume. Put the phones close together, turn the sender's volume well up, and stop talking over it. A quiet room a few centimetres apart works far better than a noisy one at arm's length.