Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: How can I restrict outgoing phone calls?
Summary: Touch-tone service availability
Message-ID: <3237@kitty.UUCP>
Date: 17 Jun 89 04:27:35 GMT
References: <43afec9f.16321@apollo.COM> <25535@agate.BERKELEY.EDU>
Organization: Recognition Research Corp., Clarence, NY
Lines: 56

In article <25535@agate.BERKELEY.EDU>, hughes@math.Berkeley.EDU (Eric Hughes) writes:
> >I don't know how many know this (for all I know, everyone does :^)
> >but a very simple way to make any TouchTone (tm?) phone ...  not able
> >to dial out, simply reverse the red and green wires (easiest to
> >access them in the wall jack). You will hear a dialtone when you lift
> >the handset, but when you press the buttons, you don't loose the
> >dial- tone.

	DTMF dial telephones manufactured within the past 15 or so years
generally work with either line polarity.

> This is true of older Western Electric dial pads, which were engineered
> not to produce tones were the line polarity was reversed.  I believe the
> name for it was something like Tone Gaurd.  From what I understand, before
> the phone company allowed subscriber line equipment, this polarity 
> reversal was how they enforced touch tone availability.  (Don't believe
> me, though.)

	Sorry, I don't believe you. :-)

	The original DTMF dial design was an _extremely_ clever design
which employed only one germanium transistor with a dual resonant tank
circuit to simultaneously oscillate at two frequencies.  This was a
"minimum component" design, and Bell Labs engineers obviously deemed it
unnecessary to provide a full-wave bridge rectifier for use on either line
polarity.  The low-cost, minimum component design goal was the sole reason
why the DTMF dial was polarity-sensitive.

	In later years, Western Electric offered an applique consisting
of a full-wave rectifier and a couple of resistors on a small circuit
board to make the 35-type DTMF dials polarity-insensitive.  This applique
was called a "polarity guard", and had a P-number designation (which I have
long forgotten).

	Enforcement of DTMF service availability was performed solely in
the central office, and was accomplished as follows:

1.	In the comparatively few SxS CO's which were converted to DTMF
	service (through shared register-senders inserted between linefinder
	and first selector), only DTMF customers were assigned to linefinder
	groups so equipped.

2.	In #1 and #5 Xbar CO's the DTMF class of service was effected by
	assigning DTMF customers to only certain line link frames (or in
	some cases only certain verticals within a LLF) which were associated
	by the marker with only certain originating registers equipped with
	DTMF receivers.

3.	In ESS CO's, from day one, DTMF availability was always a parameter
	specification for every subscriber line, and the denial of service
	was therefore accomplished in software.

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