Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!ames!decwrl!sgi!wdl1!bobw
From: bobw@wdl1.UUCP (Robert Lee Wilson Jr.)
Newsgroups: sci.electronics
Subject: Re: phone dialing
Message-ID: <3310002@wdl1.UUCP>
Date: 16 Jun 88 18:24:12 GMT
References: <361@tiger.oxy.edu>
Lines: 41

The difference in the transmitted signal is considerable:

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.

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
requirement on maximum skew in start/stop times between the tones of a
pair: After all the system doesn't seem to care how long you hold the
button beyond some minimum, and the tones _are_ generated for as long as
you hold the button. Original touch tone phones had discrete transistors
and LC tone generator circuits switched by a matrix of contacts
corresponding directly to the rows and columns. The simultaneity of the
two contacts for a key couldn't have been too great: That's why I assumed
above there was no terribly strict spec for skew.

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...

Bob Wilson
(Disclaimer: As usual, my company wouldn't back up my opinions if they
even knew what they were!)