The Baudot code is a 5 bit code and those of you who are familiar with
Binary Notation will know that the maximum number of values we can have
with
5 bits is 32. That means that each unit of transmission, one keystroke if
you like, can contain any one of 32 possible values. If you look up a table
of Baudot codes you will see there are 32 values listed, one code for each
letter of the alphabet plus a few other codes for other things such as a
space and a Carriage Return. But, what if we want to send a number such as
"9" or a question mark? These are not mentioned in that table because all
32
codes are already used.
The solution is rather similar to the Typewriter or Computer Keyboard where
we have the Shift key to get various additional codes from the keyboard.
Most keys will produce a different result if we hold down the Shift key as
we type. Well, one of those original 32 codes is a special code known as
FIGS (for Figures Shift). The convention is that when we want to send a
number or some other special character such as a punctuation mark, we can
do
that by firstly transmitting a FIGS code.
Then instead of using that original table of 32 codes, we have a second
table of codes to use, and that second table includes all ten numeric
digits
and various punctuation marks. Provided both sides of the conversation
observe the convention, the sender can send a FIGS and start using the
second table; the receiver will see the FIGS code and it too will interpret
all data that follows from the second table.
With just 5 bits of data we then have almost 64 different codes we can send
and receive. (I say almost because there is some duplication in the two
tables, including a space and a Carriage Return but that is not important
here). Even that many codes is not enough to handle all 26 letters of the
alphabet in both UPPER and lower case, so RTTY systems always operate in
upper case only.
If we wanted to type a big number (say "13579") we don't have to send FIGS
before every digit. We send that code only once and the receiver then will
take EVERYTHING we type from now as if it belongs in the second table. When
we want to revert to the normal alphabetic table of codes we can send
another special code, this one called LTRS (for Letters Shift). Then
everything goes back to normal, using the original alphabetic table of
codes.
Normally we don't have to concern ourselves with these FIGS and LTRS codes.
Our computing equipment will take care of those things for us. We just type
away and rely on the system to generate and send those codes when
necessary.
It is quite possible to lose bits here and there when receiving a RTTY
signal, whether it be because of fading, interference, frequency drift, or
whatever. One of the big problems with lost data is the possible loss of a
FIGS or LTRS code! Say we had sent "13579" and then typed "HAPPY BIRTHDAY".
Our equipment would have sent a LTRS code before the first "H" but what if
the receiver did not copy the LTRS code we sent? Can you imagine what
happens? As far as the receiver is concerned we are still sending numbers
or
other codes from the numeric table! So our "HAPPY BIRTHDAY" is going to
come
out looking something like "#-006 ?845#$-6". And EVERYTHING we type from
then on is going to look just as strange until we happen to send another
LTRS code later. It is for this reason that many systems include an option
to "Un-shift on space". If you have a multi mode TNC capable of handling
RTTY, you will probably have this option in your TNC. If that option is ON
then your receiving system will imply a LTRS code every time it receives a
space. So if you seem to be copying lots of funny numbers from a strong,
well tuned signal, try setting that option ON.
this information is not mine, as I not to express itself to me in English
have copied the text of the page NB6Z Web, I hope does not get upset.
You can see the interesting one I articulate in:
http://home.teleport.com/~nb6z/rtty.htm
Saludos de Roberto EA2RY
http://www.ea2ry.com
ea2ry@ea2ry.com
Correo revisado con Norton Internet Security 2004
e-mail reviewed with Norton Internet Security 2004
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