[GreenKeys] Fwd: ST-8000 vs ST-8000A - short form

[Dino Papas]

Bill Henry was kind enough to share the history of the ST-8000 and  
ST-8000A and I thought some of you would be interested in hearing  
about the fascinating development cycle for the two TU's.  Makes me  
appreciate mine all the more!

Thanks to Bill for taking the time to share.

73 to all.....

Dino KL0S
---------------------------------------------
Begin forwarded message:	

From:  ghenry at halcomm.com
Subject:  Re: [GreenKeys] ST-8000 vs ST-8000A - short form
Date: June 5, 2009 3:12:37 PM EDT
To: 	  kl0s at cox.net

ST-8000:  I designed this modem in 1984-1985.  It has all the bells  
and whistles I had ever wanted in a RTTY demodulator.   The design  
made heavy use of the then-new switched capacitor filters offered by  
Reticon.  The filter center frequencies are set by the switch clock  
frequency. So, there are five frequency synthesizers on the digital  
board - input band-pass filters, Mark filter, space filter, and low  
pass filters.  Each synthesizer is set by the front panel controls.   
And, there is also another synthesizer for the TX audio tones.  Drew  
White, K9CW, designed the synthesizers and the digital control  
circuitry.  I had a lot of fun designing the CRT.  It used a very  
neat rectangular tube made by AEG Telefunken - only cost $35 in  
1983.  Last time I bought some (1990?) they were $500 and had a 100  
pc minimum order - not available at all, now.  I spent a lot of time  
tweaking the circuits of the ST-8000, playing cassette taped I had  
made of various "nasty signals" over the years.  I did side-by-side  
print outs with the same signal passing through several modems -  
ST-6, MPC-1000, ST-6000, 1280, and the under development ST-8000.  I  
tweaked until the ST-8000 DID work better than the others.  I also  
devised my own HF simulator and tested all modems against it - again  
tweaking to be "the best".  Obviously this is a cut and try approach  
and no doubt there are some conditions I may have missed.  But the  
design has now stood the test of time.  This design also used most  
every part in our inventory and each modem took almost 20 hours to  
build.   It was NOT a cost-efficient design.

The ST-8000A:  This design evolved to meet hard specifications set by  
the government, specifications that were identical to those in the  
1280A/M manual.  I started with the ST-8000 design but then the  
ST-8000A design developed a life of its own.  At the time, the  
military had a lot of sheltered enclosure RATT set-ups that used the  
1280A/M and it was therefore an inflexible spec. that whatever modem  
was bid had to be "form, fit, function', and connector"  
interchangeable with the 1280A/M.   That turned out to be a BIG deal,  
but we did it.  The resulting ST-8000A has the basic form of the  
ST-8000 but with many "minor" changes to meet the form, fit,  
function, and connector requirement.  And then there were the  
reliability and host nation requirements.  The reliability has to be  
25,000 hours MTBF or better.  There is no way to include a CRT tube  
and its HV supply and meet that requirement.  So the scope had to be  
taken out.  Low cost commercial IC's used in the ST-8000 had to be  
replaced with MIL-883B rated components - for example, trading a  
$0.25 dual op-amp for a $17.00 MIL job.  The resulting modem has a  
computed MTBF of 50,000 hours (MIL-HDBK-217, Individual Parts Stress  
Analysis).  Also, the modem had to work in all NATO countries and be  
compatible with their safety and power standards.  Therefore, it is  
designed to work with 115/230VAC, 47 to 440 Hz power sources.  And  
safety standards in other countries are usually much tighter than we  
have with UL approval.  This impacted the design of the transformer  
and all AC components and even the vent hole size in the covers.   
And, the connectors had to be the same round MIL-jobs used on the  
1280A/M.  Further, the most important connectors (data and audio I/O)  
are NOT standard - the pin inserts are rotated with respect to  
"normal" connectors of that size and pin count.  All this added up to  
a lot of extra cost - and extra labor.  It took 28 hours to make each  
modem and almost that long to do Q/A, testing, and boxing of each  
one.  The resulting modem will withstand a harsh environoment.  But,  
some specifications such as selectivity and sensitivity are slightly  
less than those of the ST-8000 - but still in excess of the 1280A/M  
spec's required by the government.  You can stack any mix of  
ST-8000A's or 1280A/M's in a rack and hook them up and all will work  
the same, even with respect to the Remote Control and diversity  
features.  That was a primary requirement.

Diversity:  The ST-8000A will even work diversity with a 1280A/M (or  
with another '8000A, of course).  But, the diversity systems in the  
ST-8000 and ST-8000A are very different.  The ST-8000 uses "Selection  
Diversity", the best of two RTTY signals is chosen based on S/N.  The  
data output to the printer is switched to the modem that has the best  
signal.  The ST-8000A and 1280A/M use "Combination Diversity". In  
this case the Mark outputs and Space outputs of each modem are  
paralleled and the resulting vector-sum Mark and Space audio signals  
are detected and used to drive the data output.  In theory,  
Combination Diversity will produce copy even through short fades of  
Mark or Space signals (selective fading).  But, if you have any  
interference in either Mark or Space channels of either modem, the  
interference may dominate the detector and what might have been an  
otherwise good copy may be trashed by the interference.  Selection  
diversity shines when completely different antennas and different  
receiving frequencies can be used.  Combination works best on just  
one frequency and with similar antennas.  The Dovetron MPC-1000 also  
used combination diversity.  Hank and I lost a lot of sleep arguing  
this issue in the wee hours at the RTTY Journal Hospitality Suite at  
Dayton for several years in a row.  There probably is "no best  
solution" - but I'd never admitted that to him!

That's more than the "short form" - but my fingers wouldn't stop.

Bill, K9GWT



At 06:48 PM 6/4/2009 -0400, Dino Papas wrote:

> Bill -- I wonder if you'd be able to share a little with the RTTY
> groups of the layman's version of what the ST-8000A was designed and
> built for vs. the ST-8000 (my favorite as well, and still looking for
> a TT/L-2!).  I think the guys may be interested.  Obviously it's a
> winning piece of gear since it's still on the market!
>
> 73 -- Dino KL0S
>