Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!unmvax!ncar!ames!amdcad!sun!pitstop!sundc!seismo!uunet!portal!cup.portal.com!ISW
From: ISW@cup.portal.com (Isaac S Wingfield)
Newsgroups: sci.electronics
Subject: Re: Dial tone generation
Message-ID: <14094@cup.portal.com>
Date: 30 Jan 89 01:59:35 GMT
References: <349@serene.UUCP>
Distribution: usa
Organization: The Portal System (TM)
Lines: 16

"Precision dial tone", I think, refers to the fact that,
pre-DTMF, the dial tone generator was just that: a rotating
machine!

With the advent of DTMF, it became necessary to control the
harmonic content of the dial tone so it didn't gimp up the
DTMF decoder; hence "precision". If you think about the DTMF
frequencies, and the precision dial tone, none of the possible
harmonics from any of those could be mistaken for an instance
of any other tone.

BTW, on the precision dial tone: the frequencies are 350 and 440 Hz,
with amplitudes chosen so that the lower produces 100% double
sideband modulation of the upper. The signals are actually just
summed, but if you look with a 'scope, that's what you'll see.
Isaac                 isw@cup.portal.com