Mike,
RTTY uses 5-bit Baudot code.
As a result, it is only able to encode a maximum of 32 symbols. The alphabets
already make 26 already, the numerical digits make up another 10, then you have
comma, period, dash, slash, carriage return, line feeds, space, and a couple
others (including NULL). There is no way for them to all fit into 32 pigeon
holes.
Because of that, the Baudot code is partitioned into two groups. The first
groups contains alphabets, the second group contains digits and punctuations.
Things like Space, NULL, Carriage Return and Linefeed are present in both
groups.
The way the transmitter signals the receiver which group to use is to send two
extra special characters, called LTRS and FIGS.
On a Teletype keyboard, you will find two keycaps, one called LTRS and one call
Figs. The operator hits one of these keys before he sends a sequence of
letters or a sequence of digits.
Think of these like the way you think of the Shift Lock key on a modern
keyboard.
When you hit LTRS, you are telling the other end to interpret the Baudot code
as coming from the LTRS set.
If you look at your QWERTY keyboard, the top row of alphabets are QWERTYUIOP
Now look diagonally above it and you will see 1234567890.
Guess what, the arrangement of keys are also the arrangement of codewords for
the Baudot code. So, 1 and Q shares a codeword, 2 and W shares a codeword, 3
and E shares a codeword, etc.
To send "123", the transmitter sends FIGS first, then the codes for 1 then 2
and then 3.
If you get an error hit when the FIGS is being received, you will still be in
LTRS "shift", and instead of printing 1 2 3, your decoder will print the LTRS
equivalent for them, viz. Q W E.
By the way, if you look at the dash (minus), it falls to the right of the
letter P, so you go to the next line on the keyboard -- i.e., minus has the
same codeword as an A.
This is why you might see things like AQWE APYR printed on your screen. (The
two are equivalent to -123 and -064.)
This is also why you see TOO all the time. If you check your keyboard it is
just 599. The FIGS simply got changed to something else by noise. Very often
it is due to USOS turning the state of the decoder to LTRS shift and the FIGS
that was sent is not correctly received.
The other direction also happens. If you lost a LTRS character through noise,
you could be printing 5603 instead of TYPE.
With software modems and computer keyboards, you no longer have to explicitly
hit a FIGS or LTRS key anymore. The computer keeps track of what you are
typing and it inserts the LTRS and FIGS automatically for you, behind your back.
Most RTTY ops in the past simply look at the keyboard (in a real teletype
keyboard, 1 and Q, etc shares the same keycap) and never miss a beat. With
modern keyboards, you just go up and diagonally to the left to decode what they
print. Again, few RTTY contesters will miss a beat.
And, as repeated here by multiple people (I think 4 people in the last day
alone!), many software will show the FIGS/LTRS conversion for you. As they
have said, no big deal.
If you want to learn more about this piece of the RTTY mechanism, go here:
http://en.wikipedia.org/wiki/Baudot_code
There is even a picture of a real Teletype keyboard (and it is green :-), where
you can find the LTRS and FIGS keys, and can see that Q and 1 shares a key,
etc. Scroll down the page a little (int the section called Details), and you
will find the encoding table.
Now that you have (literally) the Decoder Ring, you are one more step closer to
RTTY Nirvana :-)
I have mentioned the above in the RTTY section of Chapter 16 of the ARRL
Handbook (from 2010 onwards), but in fewer words. You can read other things
like Diddles described in that chapter too, if you are curious about other RTTY
"stuff."
73
Chen, W7AY
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