DialUpSounds is a synthesizer for one very specific sound: two modems negotiating over a phone line. You choose the modems, the country and the state of the line; it builds the whole call from the ITU-T recommendations and draws it as an annotated spectrogram you can play, slow down, scrub and download.
We built it because the existing dial-up sounds online are either recordings of one particular call or sound-designed impressions of "modem noise". Both are fine for a listen. Neither lets you ask what a part of the sound is, or hear what a V.34 call sounds like next to a V.90 one, or how the same call changes on a noisy line. The recommendations that define these signals are public. So we read them and wrote the signals down as code.
How we check it
Accuracy is the whole point, so the test that decides whether a build ships was written separately from the code it checks, from the specification text alone. It runs our renderer and decodes the audio with its own decoders: the V.8 menus bit by bit, the V.34 INFO sequences with their CRC, DTMF digits, the 21 line-probing tones, G.711 tables, answer-tone modulation and cadences. The 300 baud segment must decode with minimodem, an independent open-source modem program. Where the recommendations leave a choice open, or where we simplified (most notably the data that follows CONNECT), we say so in the captions and in our open questions.
Who to thank
Oona Räisänen's "The sound of the dialup, pictured" (2012) taught a generation what the parts of this sound are. The authors of spandsp and other open modem implementations made it possible to cross-check our reading of the specs. The ITU-T publishes its recommendations free of charge.
The small print
Everything runs in your browser. There is no server-side audio, no account and no upload. The site uses Google Analytics 4 to count visits (see privacy). The source code is MIT-licensed on GitHub. Found a mistake in a clause? Tell us; corrections are the most useful thing you can send.