BARTH@AI.AI.MIT.EDU (Richard Barth) (10/21/87)
Re: From: haynes@ucscc.UCSC.EDU.ucsc.edu (99700000)
Subject: Re: TTY/TDDs and Baudot
> (2) What method is used for communication? Is it a form of
> Frequency-Shift Keying, and if so, what are the frequencies
> used?
Not quite frequency shift. Actually it's on-off keyed, with the tone
on for the space frequency. That is the critical frequency, and I
don't remember what it is. The other frequency (mark) is used only
to have a signal as strong as the space tone so that noise and echoes
don't trigger the space tone detector. The mark tone goes off after
a short delay so that the station at the other end can send without
any send-receive switching. So the frequency of the mark tone is not
at all critical.
Uh, well, sort of. The original patent by Bob Weitbrecht used on-off
keying, as stated. One tone was used only to actuate the echo
suppressors on the phone line; the other tone (and the absence
thereof) was the only one recognized by the receiving modem. Until his
death a few years ago, Weitbrecht and his company continued to build
his equipment that way. He was about the only one that did, however.
More modern design, as used by everybody else in the business and as
described in the draft EIA standard for TDDs, calls for FSK.
The Space tone is 1400 Hz; standard Mark tone is 1800, although
equipment built according to the Weitbrecht design don't consider the
1800 number as critical. As long as it's far enough away from 1400 not
to interfere with the detector there.