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From: brian@ucsd.edu (Brian Kantor)
Newsgroups: comp.dcom.telecom
Subject: Re: Interesting Police Technology
Message-ID: <8340@accuvax.nwu.edu>
Date: 27 May 90 14:44:53 GMT
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X-Telecom-Digest: Volume 10, Issue 391, Message 4 of 12


The older data terminals (such as the "MODAT") didn't send ASCII, and
it wasn't 10-bit async frames.  What was actually being sent was the
raster image of the characters to be printed, a scanline at a time, on
the assumption that if mobile chop (brief periods of squelch closure
due to nulls in coverage in a moving car) or ignition noise pulses
were to eat some of the data stream, you'd only lose a few dots and
the human eye/mind could easily fill in the missing data, since the
image of a character contains a LOT of redundant information (just
consider how easily you can read a bad photocopy).

Those were the days when microprocessors like the 8080 had only been
on the market a year or so, and they cost $125 each, so there weren't
many outside the lab.

Nowadays more sophisticated error-correction systems are used,
although I'm not up on the exact details.  I'll try to find out -
although I'm not in the two-way business anymore, lots of friends
still are.

In any case, I've done some snooping, and nothing I have here will
decode the radio teleprinter stuff I can hear on my spy radio, which
means that it's either encrypted and/or it's not ASCII, Baudot, AX.25,
SITOR, nor some other common codes.


	- Brian