"Harold Hallikainen" writes:
> My first FSK demod was vacuum tube based with 88mH telephone loading
> coils. I then moved to the NE565. I ended up on the XR2211, which is like
> an NE567, but has an FM output and an FSK comparator. Works nice!

	In early 1980, I built a 300-baud demodulator using a NE565
and it worked reasonably well. I fed the comparator output to a
741 op amp and was able to get a little better performance out
of it by hanging a shaper made from 2 555's and some 7400's such
that the pulse had to be at least half a pulse-width long before
the output actually went low or high. This made it less
susceptible to noise on the input.

	One of the local FM radio stations in Oklahoma City at
the time fed 300-baud data over its 67-KHZ SCA signal and I was
curious as to what it was.

	It turned out to be a commodity price ticker service for
wheat and pork bellies, etc. Really exciting. It was kind of
interesting but I could not have done anything with the
information I was reading as you were really supposed to be
paying for that, I am sure.

	Every so often, messages would come across instructing
users of the service to press certain key sequences on their
terminals to perform simple programming tasks. The data appeared
to be formatted for screen display at times and then, at other
times, it seemed to be the fill for specific record type
applications. It might have even been TCP/IP or something
similar as there would be sequence numbers and some binary
garbage and then a bit of text.

	After awhile, I lost interest in the project and all the
components gradually got repurposed in to other projects.

	A few local hams in Oklahoma City were using 300-baud
FSK in much the way that RTTY had been used for years and I
would also connect the demodulator to a receiver and receive
those transmissions. This was before packet so there was no
formatting of the data or anything special. I did make the
demodulator able to receive either the answer tones which were,
I think, 2205 for space and 2125 for mark or 1070 and 1270 for
the Originate pair.

	It worked pretty well as long as the audio was not
noisy.

	This dredges up lots of old memories.

Martin McCormick WB5AGZ  Stillwater, OK 
Systems Engineer
OSU Information Technology Department Telecommunications Services Group
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