[RTTY] Subject: Early RTTY generation/decoding

[Kok Chen]

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