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!)