56k modem sound (V.90)
The full 56k handshake: V.8 menus, line probing with the roles reversed, the training hiss, and the digital impairment learning that only V.90 and V.92 calls have.
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 makes a 56k call different
V.90 (ITU-T, September 1998) works because one end is not really analogue. The server modem sits on a digital line, so it sends G.711 codewords straight into the telephone network at 8000 per second. Only the last stretch of copper to your house is analogue. Your modem's job is to work out which voltage each codeword turns into by the time it reaches you. That learning is what you hear after the hiss.
Roles swap in phase 2
In a V.34 call the caller probes on 1200 Hz and the answerer on 2400 Hz. V.90 §8.2.3.1 reverses that: the home (analogue) modem sends INFO and tone A on 2400 Hz plus a quiet 1800 Hz guard tone, and the server sends on 1200 Hz. Watch the green and amber bands trade places on the spectrogram.
The DIL
The home modem asks for a test pattern with Ja, the DIL descriptor (Table 12/V.90): how many segments, which codewords, a sign pattern and a training pattern. The server plays it back as DIL (§8.4.1). Because the server controls the exact codeword, the home modem can see what robbed-bit signalling, digital pads and μ-law/A-law conversion did to each one. Our descriptor asks for 32 segments stepping up through the codes with a 500 Hz sign pattern: that is the slow, rising bong. Real modems chose their own descriptors, so the exact pattern varied between brands.
Why 56k was never 56k
The downstream rate is chosen in steps of 8000/6 bit/s, from 28 000 to 56 000 (V.90 §5.1), and the result depends on the line. Our line presets map to 52 000 (clean), 50 666 (typical), 46 666 (noisy) and 44 000 (long distance). Upstream stays V.34-style, at up to 33 600 bit/s.
Approximate: the PCM data encoder after CONNECT (no modulus encoder or spectral shaping) and the constellation choice. See the explainer for every phase.