TELECOM Digest OnLine - Sorted: Re: Spectrum of a Modem Signal

laura halliday (marsgal42@hotmail.com)
10 May 2006 15:07:54 -0700

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mike7...@gmail.com wrote:

> 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?

Laura Halliday VE7LDH "Que les nuages soient notre
Grid: CN89mg pied a terre..."
ICBM: 49 16.05 N 122 56.92 W - Hospital/Shafte

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