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From: kitty!larry@uunet.uu.net (Larry Lippman)
Newsgroups: comp.dcom.telecom
Subject: Precise Dial Tone & A Tone of the Past: No-Such-Number
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Date: 24 Sep 89 16:58:21 GMT
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In article  gabe@sirius.ctr.columbia.
edu (Gabe Wiener) writes:
> Does anyone know when the common telephone dialtone (i.e. 350Hz and 440Hz
> played together) was adopted?

	The standardization of dial tone to a dual frequency of 350 Hz
and 440 Hz with a transmission level of -13 dbm was first proposed by
AT&T and developed into CCITT Document AP III-84.  I don't know the
exact date of this CCITT standard, and don't have a copy handy, but I
believe it was around 1963.  This standard applies only to North
America, however.  Many European and other countries use a single
frequency, most commonly 425 Hz.

	In is important to understand that this dial tone is produced
by LINEAR mixing of two sinusoidal tone sources of 350 Hz and 440 Hz,
and is NOT one frequency modulated by the other, which was the method
employed with many previous dial tone supplies.  The reason for the
linear mixing is to reduce harmonics which may fall into the DTMF
frequency domain and interfere with the interpretation of the first
DTMF digit.  The previous dial tone supplies were extremely rich in
harmonics.

	The previous North American dial tone "standard" was 600 Hz
modulated by 120 Hz; I use the word "standard" loosely here since in
practice there was quite a variation because there was no technical
_reason_ why the tone had to be precise.  In older electromechanical
CO's dial tone could be produced by a variety of apparatus, including
motor-driven tone alternators, motor-driven pole-changing interrupters,
electromagnet-driven pole-changing interrupters (i.e.,vibrators),
ferroresonant AC-line powered devices, and in later years solid-state
devices of varying stability.

	Interestingly enough, with one exception, I have never seen
nor even heard of a call progress tone generator which used vacuum
tubes.  Technology in this area went directly from the
electromechanical to the solid-state.  The one exception was the
"no-such-number" tone generator, which used vacuum tubes and made its
debut around 1940; it has been affectionately called the "crybaby
tone".  This call progress tone (for lack of a better term) began
rapidly disappearing in the later 1950's with the rapid implementation
of intercept recorders which replaced it.  The no-such-number tone had
pretty much disappeared in the Bell System by 1965.

The last "holdout" I am aware of which used this tone was Rochester
Telephone, in Rochester, NY, and quite to my surprise I heard this
tone when I misdialed a toll call to the Rochester area about four
years ago.  I was so taken aback - not having heard this tone for
probably 20 years - that I was almost motivated to fetch a tape
recorder and record it for posterity. :-)

<> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
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