Another place to look at early technology is the Page Communications
patent (US Patent 2999925, filed in 1959, issued in 1961) that is
titled "Variable Decision Threshold Computer." It is the earliest
place I have seen Automatic Threshold Correction (ATC) discussed. You
can find it at the www.uspto.gov web site by using the patent number
search.
If you cannot implement ATC, the next best thing is to make sure that
whatever circuit which you use allows you to select an "FM" mode.
"FM" detection is just a case of first bandpass filtering the
composite Mark and Space audio tones and then hard limiting the signal
before sending it to a discriminator.
Without either one of the above, you will misprint when there is any
significant selective fading present. Although not included in the
tittle, the same '925 Page patent above also mentions a method of
achieving diversity decoding of RTTY signals. But you will need more
than one antenna (or an antenna like a turnstile that provide two
different polarization concurrently) and two independent receivers.
Bear in mind that for all practical purposes, unless you are
connecting terminal unit (TU) to a mechanical teletypewriter, you will
still need a computer to use a TU.
Someone/something has to generate the Baudot code, including the
decoding and encoding of the LTRS/FIGS shift. To make other decoders
today happy, you will want to also generate the diddles and the USOS
encoding/decoding.
What that means is that you will probably also have to write a couple
of lines of code in software to use any homebrew hardware that you
build.
Finally, back in the good old days, manufacturers always included
diagrams (even the '925 patent has circuit diagrams). Both the KAM
circuit diagram and the ST-8000 circuit diagram show where to tap to
get a cross ellipse tuning indicator with an oscilloscope, for example
-- remember that a stand alone decoder is not that useful unless your
hearing has perfect pitch.
If you are lucky, someone nearby has a copy of the HAL ST-8000
Technical Manual (first volume has detailed description and second
volume contains the circuit diagrams). You don't want to replicate an
ST-8000 (it will take you man-months, if not man-years of tedious
work) but it will give you an idea of what is inside the Rolls-Royce
of RTTY hardware decoders. The ST-8000 also has a squelch circuit that
allows you to use two of the units for diversity reception.
73
Chen, W7AY
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