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Path: utzoo!watmath!watdcsu!herbie
From: herbie@watdcsu.UUCP (Herb Chong [DCS])
Newsgroups: net.audio
Subject: Re: CD Reflections - 44.1k?
Message-ID: <854@watdcsu.UUCP>
Date: Mon, 21-Jan-85 10:18:13 EST
Article-I.D.: watdcsu.854
Posted: Mon Jan 21 10:18:13 1985
Date-Received: Tue, 22-Jan-85 04:58:14 EST
References: <15100001@hpfcmp.UUCP> <3411@mit-eddie.UUCP> <1420@hplabs.UUCP> <755@clyde.UUCP> <258@petrus.UUCP> <272@mtxinu.UUCP>
Reply-To: herbie@watdcsu.UUCP (Herb Chong [DCS])
Organization: U of Waterloo, Ontario
Lines: 53
Summary: 

In article <272@mtxinu.UUCP> ed@mtxinu.UUCP (Ed Gould) writes:
>When I studied signal theory briefly about 12 years ago, there was
>a theorem stating that it was *impossible* to push more information
>through a signal than the bandwidth of the signal, e.g., one can't
>send more than k bits per second through a k Hz bandlimnited channel.

This is almost true; your memory isn't quite right.  There is also the
different signal levels to consider.  Suppose I were to agree decide
that the S/N of the channel was sufficiently high to use a 4-level
encoding of the data being transmitted (2 bits in one pulse).  The
pulses are distinguished by which one of the 4 levels is received.
Thus, I'm transmitting 2*n bits/s in a channel bandlimited to n bits/s.
This is equivalent to saying I'm transmitting 2*n baud.  1 baud != 1 bit/s
in general.
 

>Telephone voice-grade channels are 2700 Hz limited, filtering to allow
>signals only from 300 Hz to 3000 Hz.  So how do 4800 and 9600 bps
>modems work over dialup circuits?  (The telco carriers, by the way,
>are strict about bandlimiting their signals, since they frequency-
>multiplex them onto higher-bandwidth channels.)

They don't transmit at 9600 bps, they transmit at 9600 baud.  See above.

>The answer seems to be that the theory that generated that theorem
>wasn't completely correct.  Maybe the Nyquist theorem shouldn't be
>regarded as gospel, either.

I'm afraid Nyquist knew what he was talking about.  Theoretically, in a
completely noise-free channel, it is possible to transmit all the information
known to Man in a single pulse of a specified amplitude within any non-zero
bandwidth.  When there is noise, there is a limit to the information transfer
rate.
 
>-- 
>Ed Gould		    mt Xinu, 739 Allston Way, Berkeley, CA  94710  USA
>{ucbvax,decvax}!mtxinu!ed   +1 415 644 0146


Your's for enlightenment,

Herb Chong, BASc
Computer Consultant 

I'm user-friendly -- I don't byte, I nybble....

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