🚀 go-pugleaf

In article <3310002@wdl1.UUCP>, bobw@wdl1.UUCP (Robert Lee Wilson Jr.) writes:
> A rotary dial phone (and some electronic push-button phones in a mode
> which imitates the rotary dialer) just open the line intermittently, once
> if you dial 1, twice for 2, ... ten times for 0. There are specifications
> for how long the line is open and the space between pulses: Mechanical
> dials have a governor mechanism which sets the rate at which the dial
> returns, and the switch contacts open and close the line during the return
> rotation.

	The "open" specification is usually expressed in "percent break",
and is nominally 60% break for most electromechanical switches; i.e., a
rotary dial has a normally-CLOSED pulsing contact which opens for 60%
of the intradigital pulsing interval.  The "speed" specification for
normal subscriber telephone applications is nominal 10 pulses/sec.
This means that the intradigital pulsing interval is 100 ms, with an
open of 60 ms.  Therefore, dialing a "1" is equivalent to one line open
of 60 ms; dialing a "2" is a 60 ms open, a 40 ms closure, and another
60 ms open; etc.
	For some PBX and Dial Service Assistance (operator) applications,
there are "high speed" dials which run at 20 pulses/sec.  These are now
pretty much obsolete, however.

> A touch-tone phone sends a pair of tones for each digit. I don't recall
> the exact frequencies, but there are two bands, each with several
> frequencies to choose. Each row on the keypad determines a tone from one
> group and each column determines a tone from the other, so that each key
> has a unique pair of tones. The spec here is (I assume from observation,
> i.e. I haven't read it) just a minimum duration, perhaps with some
> ...

	Touch-tone (which used to be a trademark of AT&T) is more properly
called dual-tone multifrequency (DTMF) dialing.  Tones are divided into
a "low group" (697, 770, 852 and 941 Hz) and a "high group" (1209, 1336,
1477 and 1633 Hz).  A digit consists of one frequency from the low group
and one frequency from the high group (dual-tone :-) ).  There are
obviously 16 digit-combinations, but most telephones have only 12 such
combinations (0-9, * and #); some VERY old telephones did not even have
the * or # keys.  The other four digits are used in special applications,
most notably AUTOVON (they are usually used to assign a "priority" to
calls).  Normal central office dial registers do not respond to these
other four digits.
	Most DTMF dial registers in central offices and PBX's will respond
to DTMF signaling at the rate of 10 digits/sec, with each tone being
present for at least 50 ms, and with an interdigital interval of at least
35 ms.
	The actual low and high group DTMF frequencies may seem to be a
bizarre choice.  In fact, they were carefully chosen by Bell Labs to
minimize harmonic interference, and to be _least_ likely to be spoofed
by speech.

> Decoding the rotary at the switching office was traditionally done by a
> special stepping switch, and the 'phone companies had enormous amounts
> invested in that kind of technology. When touch tone capability was first
> added to any given exchange, new switching gear had to be added. It made
> sense, perhaps, to charge extra for the users of that gear. I don't see
> any reason now for a premium, and I don't recall that I pay any such to
> PacTel. Presumably every line (after some electronic conditioning) just
> goes to some specialized IC which is stamped out in enormous volume, can
> handle both kinds of inputs, and costs almost nothing compared to the
> labor costs of installing connections, etc.
> 
> I would be interested if anyone does know of a reason for a differential
> cost at this stage...

	I agree!  Additional charges for DTMF service should be abolished! 

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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