Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!pasteur!ucbvax!ucsd!rutgers!mcnc!unccvax!dya
From: dya@unccvax.UUCP (York David Anthony)
Newsgroups: sci.electronics
Subject: Re: 9600 baud modems
Message-ID: <1044@unccvax.UUCP>
Date: 19 Jul 88 14:08:11 GMT
References: <1127@nunki.usc.edu> <478@ns.UUCP>
Organization: Univ. of NC at Charlotte, Charlotte, NC
Lines: 43

In article <478@ns.UUCP>, logajan@ns.UUCP (John Logajan x3118) writes:
> In article <1127@nunki.usc.edu>, grking@nunki.usc.edu (Greg King) writes:
> > 9600 baud modems can operate over normal telephone lines.
> > the bandwidth of normal telephones is about 3 kHz.
> > how do these modems work?
> 
> With a bandwidth of 0-3000 hertz, you can simultaneously send tones at any
> arbitrarily spaced frequency intervals, say every 10 hertz.  The presence or
> absence of a tone can have some information meaning.  Thus you could, in
> the 10hz case, send some 300 bits of information simultaneously.  The more
> tones you send, they lower in amplitude they have to be, because the aggragate
> volume could exceed that allowed.  But by proper phase choice, the rate of
> volume increase to number of tones added, is much less than a linear relation!

	This is how **some** 9600 baud modems work. Real 9600 baud
modems work by quadrature amplitude modulation of a 1700 Hz (or so)
carrier wave. Groups of 4 bits are encoded through the usual scrambler
device to yield four scrambled bits (ensuring not having runs of
ones and zeroes of significant length, so that the carrier clock
may be recovered).  Each four bits represents a unique amplitude and
phase.  The recovered clock then is multipled against the recovered
QAM-encoded audio, yielding a magnitude and a phase.  A trivial circuit
decides which of the quadbits the recovered magnitude and phase
corresponds.

	Now, much digital signal processing is required to keep
the QAM signal error-free; but the technology in real, Honest-to-God
9600 bits/sec modems (such as the UDS 9600 A/B, or the Rockwell
R96MD/R96FAX) is not frequency division multiplexing. It is QAM.

	Now, with trellis encoding of the original quadbit patterns,
(forward error correction) you can obtain a better bit error rate
for a given signalling rate, or pack in even more symbols per baud.
This is the principle behind the UDS 14.4 Trellis (14.4 kbps)
and the Rockwell 14.4 chipset to be introduced. They are also QAM
modems.

	Note, that a baud is the number of symbols per second
being transmitted. A 9600 bit/sec modem operates at 2400 baud.
Each symbol represents a unique magnitude and phase for each
group of 4 bits.

York David Anthony
dataSPan, Inc.