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Path: utzoo!linus!philabs!cmcl2!csd1!condict
From: condict@csd1.UUCP (Michael Condict)
Newsgroups: net.followup
Subject: Re: Modem Users Beware: BELL $$$
Message-ID: <129@csd1.UUCP>
Date: Thu, 20-Oct-83 10:04:25 EDT
Article-I.D.: csd1.129
Posted: Thu Oct 20 10:04:25 1983
Date-Received: Fri, 21-Oct-83 23:55:10 EDT
References: <2515@utcsrgv.UUCP>
Organization: New York University
Lines: 25

By my reckoning, a 4800 baud modem link requires a bandwidth of at least
9600 Hz (assuming one avoids any inefficiencies associated with translating
between digital and audio), while the phone co. currently only guarantees its
voice lines a frequency response up to approx. 5K Hz, which is certainly less
than that.  Thus, where do you get this figure that a 4800 baud modem only
needs 8% of a digitized voice line?  And why does it matter whether the line
is digitized or not?  You can't beat information theoretic restrictions.
To get a frequency response up to 10K Hz you need a band-width of at least
10K Hz, regardless of how you code the information (unless of course you
take advantage of some knowledge of the signal, such as the redundancy of
human speech, to reduce the quantity of information that needs to be trans-
mitted -- but that wouldn't help you in this case, because the redundancy in
human speech compared to modem transmissions means that even less bandwidth
is required for the voice line as compared to the modem).

If I have displayed a grievous lack of knowledge of communications here, please
just tell me by mail, to avoid embarassing me and boring others.  If not,
please tell me what's wrong with my analysis (which, by the way, indicates that
2400 baud modems are on the theoretical limit of error-free transmission with
a 5K Hz bandwidth and should therefore not be expected to be very reliable;
and 4800 baud modems should require two independent phone lines, which I have
heard that they do).

Michael Condict		...!cmcl2!csd1!condict
N.Y.U.