[GreenKeys] Decoding RTTY for a Teletype with a Computer / Want to Buy

Harold Hallikainen harold at mai.hallikainen.org
Sun Nov 1 14:04:53 EST 2020 

I see the Teletype as an electromechanical UART (Universal Asynchronous
Receiver/Transmitter). With 60 wmp Baudot, the loop current is typically
60 mA when idling (it's possible to use series wired holding magnets at 20
mA). This idle state is the "mark" condition. When there is no current, it
is the "space" condition. These terms come from Morse's original recording
telegraph that would put dits and dahs on paper tape by marking the tape
when there was current, and leaving a space when there was not. On a UART,
the marking condition is normally a binary 1 with space being binary 0.

For 60 wpm Baudot, the loop is interrupted when the transmission of a
character starts. This "start bit" lasts 22 ms. The start bit is then
followed by 5 data bits, with the least significant bit being transmitted
first. After the last data bit, a stop bit is sent. The stop bit is always
a mark. It is transmitted for 31 ms, but the receiver stops after 22 ms to
allow for speed variation between the transmitter and receiver. On UART
chips, you get this by saying you want 1.5 stop bits. UART chips normally
allow you to send 1, 1.5, or 2 stop bits.

The loop current runs a single electromagnet (in the model 15, I think
earlier models like the model 12 had separate electromagnets for each bit
with a distributor to route the current to the appropriate magnet). When
the loop current is interrupted for the stop bit, a clutch is released
allowing the shaft of the decode portion of the machine to rotate. Each
bit is separated out mechanically and sent to code bars on the type basket
that each move to one of two positions. Slots in the code bars then allow
one pull bar to fall into the slot allowing the character to be typed.
It's a mechanical marvel. There's a nice slow motion video at
https://vimeo.com/353151381 .

The electromagnet has a resistance of a few ohms, so it would take very
little voltage to cause 60 mA to flow through it. However, since the
electromagnet has a large inductance, the current builds very slowly when
the circuit is closed (the time constant T=L/R where L is big and R is
small). So, it's standard to use a high voltage power supply with a large
resistor to limit the current. Since T=L/R, the now high resistance allows
the current and the magnetic field to build quickly.

This high voltage makes a computer interface interesting. Typically a high
voltage transistor is used to key the loop. The transistor has to handle
the loop voltage plus the "inductive kickback" when the loop current is
interrupted. The faster the current through the electromagnet falls (and
the magnetic field falls), the higher the voltage across the
electromagnet. A common trick in relay drivers is to put a diode across
the coil clamping the voltage to the supply voltage. But, this slows the
current fall, slowing the release of the relay. Typically a clamp circuit
or snubber is added to limit the high voltage. Limiting that high voltage
spike too much, though, limits how quickly the selector magnet releases,
so it's a compromise. There is a GK member who has USB loop keyers
available.

You can often find Teletypes for free. I keep a list of stuff that shows
up on Green Keys at
https://docs.google.com/document/d/1I6TgY5PEQ35c6IVqWWrV5lmEvdRVZxkj8FaYxUj87qM/edit?usp=sharing
. I don't see anything in Florida right now, but I don't manage to list
everything. At the top of that page are notes on searching the GK list for
equipment. At the bottom of that document is more info (from a GK member,
not me) on getting and transporting equipment.

On "Terminal Units" (actually modems), it can, of course, be done in
software. But hardware-based TUs are available for low prices. I recently
bought a Flesher TU-170 Terminal Unit for $30, shipping included. It will
demodulate 170 Hz shift AFSK and generate the corresponding AFSK. It has a
loop supply and keyer in it, so it's just a matter of connecting the
Teletype (keyboard and magnet in series) to a couple terminals. I see one
on eBay right now for $50 plus shipping. I also see one on QRZ at
https://forums.qrz.com/index.php?search/107506137/&q=Flesher+TU-170 .

Good luck!

Harold
https://w6iwi.org/
https://w6iwi.org/rtty



>     Chris, do you have any basic texts on how teleprinters work?
> I think the members here can suggest a couple which I am sure are
> available free on the web.
>     A Teletype is really a sort of mechanical computer. It takes
> a form of binary code as direct current pulses at its input and
> translates them into print characters by a purely mechanical
> means. The source of the binary signals is DC but the DC can be
> supplied by a number of devices. The most usual method for
> transmitting Teletype signals via radio is by frequency shift
> keying, one frequency for mark a second for space, but Teletype
> can also be sent by CW. FSK has some important advantages over CW
> but the results at the machine are the same. The device between
> the receiver and the Teletype converts the audio signals (they
> can also be RF signals) into the DC pulses needed by the
> machine.  A computer can decode the Teletype signal from an audio
> source and print it on the screen or send it to a computer
> printer. I think some can generate a digital signal which can be
> used by an external device to create the DC signals to drive the
> Teletype machine. It is still the Teletype that "decodes" the
> incoming signals into characters.  The whole arrangement can be
> pretty simple. A computer can make a very effective "detector"
> for incoming signals because it can discriminate between wanted
> signals and noise or interference very effectively. I think the
> loop current supply (the name of the device that drives the
> magnets in the Teletype machine) can be made fairly cheaply.
> There are also several "detectors" for converting from the audio
> frequency shift to DC pulses, which are then used for the
> machine. These also are not too expensive. There are converters
> to go from the Beaudot binary output of a Teletype to any other
> code. The range of characters on earlier machines is limited but
> later ones were used as computer terminals. If you have an
> earlier type of machine, like a Model 15, I don't know what other
> applications there are for it other than sending and receiving
> teleprinter signals. I am interested in what you are considering.
>      I don't know if this is helpful, my own understanding is
> limited, but I have been fascinated by Teletype machines all my
> life and have studied about them a lot.
>
> On 11/1/2020 7:43 AM, Chris DeMuro wrote:
>> Hey everyone!
>>
>> I have a question, specifically about decoding RTTY on
>> teletypes. When I get a Teletype, I intend to use it for
>> several things, of which RTTY is just one, but I'll get to that
>> later.
>>
>> As I understand it, and please bear with me as my understanding
>> is limited, the RTTY audio signal is fed into some sort of
>> decoder, and then the serial text output, either as ASCII or
>> Baudot is fed into the Teletype to be printed.
>>
>> I don't have much money for this Teletype endeavor (again, I'll
>> get to that) so I am trying to save a buck wherever possible. I
>> was wondering if it is possible to use a computer to replace
>> the role of the decoder, and send the text out of a serial port
>> to the Teletype. I was trying to figure out a way to do this
>> using FLDIGI, but I couldn't find any way of doing it.
>>
>> Now before you say "But that defeats the purpose of using a
>> Teletype! Why even bother using a mechanical machine? Just make
>> your RTTY QSOs on a computer if you feel that way!" The reason
>> I am okay with using a computer as a RTTY decoder to feed text
>> to a Teletype is the same reason I enjoy typing out news
>> articles and such on a manual typewriter. I enjoy using the
>> machine. I am not overly concerned with how authentic the
>> experience is. I intend to use a modern radio and other such
>> modern amenities. I think it would be cool to get an authentic
>> station set up, but that is not my first priority.
>>
>> As a final note, I mainly run Linux, but if anyone has a method
>> to do this with Windows software, I can make that work, so
>> don't hesitate to let me know about that just because I run Linux.
>>
>> Now that I have rambled on long enough about that, and
>> hopefully expressed my question well enough that someone can
>> understand it, let me begin the next part of this message.
>>
>> I am looking to purchase a Teletype. I will take any model,
>> though models easier to service would be preferred. I would
>> like it to be working or only in need of minor repair, as I do
>> not have much experience with this. An ASCII model would be
>> ideal, but I will definitely take a Baudot machine. I am in
>> Central Florida, and I have a budget of about $300. Apologies
>> if that is too low. Again, I am new to all this. I have some
>> family going up to Georgia later this month, so I can probably
>> arrange to get something from that area as well.
>>
>> Please let me know if you have a machine you're looking to
>> sell. I have been fascinated by these machines for a while now,
>> and I can absolutely guarantee it would go to a good home, and
>> be put into good, long-term service.
>>
>> Anyway, thanks to anyone who can get through this long email. I
>> am too long-winded for my own good. :)
>>
>> Thanks so much to everyone who has assisted me thus far on this
>> mailing list, and I look forward to hearing from you!
>>
>> Thank you,
>>
>> Chris DeMuro
>> KN4OBK
>>
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>>>>> https://teletype.net/gksearch
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>>>>> http://comcents.com/tty/greenkeyssearch.html
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>
> --
> Richard Knoppow
> 1oldlens1 at ix.netcom.com
> WB6KBL
>
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>>>> Jordan Spencer Cunningham's GreenKeys Search Tool:
>>>> https://teletype.net/gksearch
>>>> 2002-to-present greenkeys archive:
>>>> http://mailman.qth.net/pipermail/greenkeys/
>>>> 1998-to-2001 greenkeys archive:
>>>> http://mailman.qth.net/archive/greenkeys/greenkeys.html
>>>> Randy Guttery's 2001-to-2009 GreenKeys Search Tool:
>>>> http://comcents.com/tty/greenkeyssearch.html
>
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