Re: [SLUG] 45 baud Baudot output is need, the old TTY standard

From: Chuck Hast (wchast@gmail.com)
Date: Mon Feb 23 2009 - 21:21:25 EST


On Mon, Feb 23, 2009 at 8:52 PM, Donald E Haselwood
<dhaselwood@verizon.net> wrote:
> Interesting problem. It's been a long time. I gave away my
> model 15 Teletype and typing reperferator around 1975.
>
> It isn't clear from your post what you are trying to do. If
> the "box" is a ham radio, then the usual method of driving
> the radio is program that generates the RTTY signals via
> the sound card, i.e. the standard is a two-tone audio
> shift. The modulated audio then goes to the mic input to
> the radio and the radio transmits using SSB. There is no
> need for a UART running at 45 baud.
>
> This might be useful--
> http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=17558
>
> Before sound cards and processors that were fast enough and
> had the MMI instructions, the method was to connect the
> computer to a box called the TNC via the serial port. The
> UART usually ran at 1200 baud. The TNC then translated the
> data speed and code and modulation/demodulation to the
> radio. Again there was no need for the PC UART to run at
> 45 baud.
>
> If the PC is going to directly drive a radio with 45 baud
> baseband (i.e. only the carrier is frequency shifted; no
> audio modulation), there is more involved than just the
> baud rate. The program that translates the typing into
> level 5 codes has to handle thing such as LTR/FIG
> (letters/figure) shift insertion, as well as collapsing the
> 8-bit input codes to the limited code set.. IIRC some of
> the 5 level machines had an option to unshift at the end of
> the line so a bogus figure shift didn't continue beyond the
> current line (this was used when plain text was the
> predominat data). I don't remember what it was for the
> amateur standard. Push-to-talk also has to be implemented.
>
> I suspect that the programs that are out there will be
> MSDOS, since the days of driving a teletype machine
> directly with the 5 level 45 baud days were well before
> Linux (and Windows). The model 33 Teletype (w the paper
> tape reader/punch) was 8 level ASCII codes and were the
> standard input/output for the PDP8 and PDP11 machines and
> the old, slow 5 level machines were only used for ham
> radio, (which used audio modulation and hence the eventual
> migration to using the sound card, for which there are mnay
> programs available).
>

Don,
Before that they were called TU (Terminal Units) and they went from the
simple with some filters to detect the Mark and Space tones with a zero
crossing detector and some sort of keyer to handle the 90 to 150vdc
20 or 60ma loop current, to the really advanced ones in the later years of
RTTY, that had filters that could be set for shift, tones, and low pass filters
for speed. These expensive ones could be hooked up to operate in diversity
mode so if you took a fade on one receiver you would probably not take the
same fade on the 2nd one depending on the diversity mode uses. I had a
Dovetron which was sort of the Mercedez Benz of TU's at one point, it could
copy about anything as long as it could dig the tones out of the air, and of
course it was a legit code and not encrypted...

Brings back fond memories of those noisy but reliable machines. I had both
the M15 and the M28 machines, oh and a M35, all of them ASR's along with
some KSR's and a skin tight M28 RO taken out of submarine service...
Did you ever see a M15 do 100 wpm, it was scary to say the least, they
did not last very long either...

I had a M28 that had a "3 on the floor" gear shift, it did 66, 75 and 100wpm.
All you had to do was open the dome and reach back by the motor and
move the "stick" to the speed you wanted. If you wanted 60wpm you had
to swap the 66 gear for the 60 gear, and that was it.
Oh and there were military machines that had a governed DC motor you
could speed up or slow down, they had a set of tuning forks and on the back
of the motor was a drum with black line markings you just used the turning
fork, after striking it you looked through a set of blades that opened and
closed, and you changed the speed until the black marks stopped moving
against the white background.. Ohh the things we did to get the different
speed rates. Only problem with that governed motor was it generated a lot
of noise and could kill a sensitive HF receiver.

-- 
Chuck Hast  -- KP4DJT --
To paraphrase my flight instructor;
"the only dumb question is the one you DID NOT ask resulting in my going
out and having to identify your bits and pieces in the midst of torn
and twisted metal."
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