<p><em>Rattlegram</em> lets you transmit short text messages over COFDMTV encoded audio signals. COFDMTV is based on COFDM (coded orthogonal frequency-division multiplexing) technology:</p><ul><li>160 ms long OFDM symbols</li><li>6.25 Hz per sub-carrier</li><li>1/8 guard interval</li><li>differentially encoded PSK (phase-shift keying) modulation</li><li>systematic polar codes for forward error correction</li></ul><p>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 differential QPSK modulation for the payload. Only a bandwidth of 1600 Hz is needed and the transmission of either 85, 128 or 170 bytes take about one second with the leading noise and fancy header disabled.</p>