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From: lewis@tramp.Colorado.EDU (LEWIS WILLIAM M JR)
Newsgroups: sci.electronics
Subject: Re: Touch-tone freq.
Keywords: What are they?
Message-ID: <1991Mar4.220938.668@csn.org>
Date: 4 Mar 91 22:09:38 GMT
References: <1991Mar4.193718.3020@ncsu.edu>
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The Touch Tone (R) dial transmits two frequencies whenever a button is
pushed.  The low-band frequencies are selected by the row on the dial and
the high-bands by the columns.  Thus

	697 Hz ----- 1 ----- 2 ----- 3 ----- Spare
	             |       |       |       |
	770 Hz ----- 4 ----- 5 ----- 6 ----- Spare
LOW	             |       |       |       |
	852 Hz ----- 7 ----- 8 ----- 9 ----- Spare
	             |       |       |       |
	941 Hz ----- * ----- 5 ----- # ----- Spare
	             |       |       |       |
                    1209    1336    1477    1633 Hz

                                HIGH

The spares are used mainly on military sets for turning privacy/secrecy
gadgets on and off and other, similar non-sense.

The frequencies were selected after much, much work in the hope that
strange noise hits, the customer whistling etc., etc. would not register
as a valid digit.  A full description can be found in the "Bell System
Technical Journal" between 1960 and 1965.