Dial-up modem sound
A dial-up connection, synthesized from the ITU-T recommendations: dial tone, the number, ringback, the answer tone, and the full negotiation to carrier. The spectrogram shows who is talking and what they are saying.
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)
What you are hearing
A dial-up modem could only send sound down a phone line, so before any data moved the two modems held a conversation in sound. The famous noise is that conversation. Each part has a job, and each job is written down in an ITU-T recommendation. We generate every segment from those texts, so the frequencies, timings, and bit patterns are the real ones rather than a sound designer's impression.
Press Dial and you get the whole call. Your modem picks up and waits for dial tone, dials the number in DTMF (dual-tone multi-frequency) digits, and you hear the far phone ring. The server modem answers with a 2100 Hz answer tone. Then comes a quick exchange of menus at 300 bit/s ("I can do V.90, V.34, V.32bis…", "let's use V.90"). After that, both modems probe the line with a 21-tone chord, train their equalizers on a burst of hiss, and on a 56k call the server plays a ladder of PCM (pulse-code modulation) codes, the digital impairment learning, that sounds like slow bongs. Then the speaker clicks off, because by default a Hayes modem mutes itself at carrier detect.
The phases at a glance
| Phase | Who | Sounds like | Spec |
|---|---|---|---|
| Dial tone, DTMF, ringback | network / caller | steady chord, beeps, ring | E.180, Q.23 |
| Answer tone (ANSam) | answerer | a wavering high whistle with clicks | V.8 §7.2, V.25 |
| CM / JM menus | both | two warbling FSK tones | V.8, V.21 |
| INFO, tones A/B, L1/L2 probing | both | short bursts, two pure tones, the chord | V.34 §10.1.2 |
| S, PP, TRN, J, MP | both | the hiss | V.34 §10.1.3 |
| Sd, TRN1d, DIL (56k only) | server | buzz, then bongs | V.90 §8.4 |
Pick a different modem and the call changes: V.34 has no DIL, V.32 bis has its two-tone warble instead of the chord, and a 300 baud Bell 103 call is just two tones carrying a real login banner you could decode yourself. Try Busy, No carrier, or Fax answers for the calls that went wrong.
Want the long version? The dial-up handshake, explained walks through every phase with players. The FAQ covers the short questions, and what is approximate is listed in our open questions.