Rattlegram lets you transmit short text messages over COFDMTV encoded audio signals. COFDMTV is based on COFDM (coded orthogonal frequency-division multiplexing) technology:

160 ms long OFDM symbols
6.25 Hz per sub-carrier
1/8 guard interval
differentially encoded PSK (phase-shift keying) modulation
systematic polar codes for forward error correction
Currently Modes 14, 15 and 16 are supported: Using either SPC(2048, 1392), SPC(2048, 1056) or SPC(2048, 712) with CRC32C aided SCL decoding and di