If I plot the spectrum of a short modem signal, what should I expect > to see?
> It just looks like a lump in the frequency domain, but I'm not sure I > did it right.
Old, slow (< 9600 bps) modems used relatively simple FSK and PSK modulation. The classic Bell 212A (long the standard 1200 bps dialup modem), for example, used 4-ary differential PSK.
New modems use trellis-coded modulation with multi-dimensional signal constellations. Analyzing them is not trivial (graduate-level signal processing). Decoding them isn't either.
Exactly what are you trying to accomplish?
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