300 baud modem sound (Bell 103)
The oldest modem here: two pairs of tones and nothing else. The data is a real ASCII login banner you can decode yourself.
Heads up: modems are loud. Playback starts at −12 dB and runs through a limiter.
AT console (Hayes commands)
Event list (the spectrogram as text)
Two tones each way
The Bell 103 data set (AT&T, 1962) is frequency-shift keying at 300 bit/s. The calling (originate) modem sends 1270 Hz for a one and 1070 Hz for a zero. The answering modem sends 2225 Hz for a one and 2025 Hz for a zero. There is no answer-tone negotiation and no training: the answerer sends its mark tone, the caller replies with its own, and data starts.
Our answerer sends DIALUPSOUNDS BBS - 300 BAUD / login: as 8-N-1 characters, and the caller types guest. To decode it, set the speaker to M2 (always on), download the WAV and run minimodem --rx -f file.wav --mark 2225 --space 2025 300. Our test suite does exactly that.
Bell 103 is a Bell System specification rather than an ITU-T recommendation; its European cousin is V.21, the same FSK idea on different frequencies, still used inside every V.8 handshake for the menus.