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Description:
RTTY or Radio Teletype is
a direct machine to machine
communications mode using the Baudot (or Murray) code.
They experimented with FSK, or Frequency Shift Keying and found it performed
much better. With FSK, the transmitter is shifted up in frequency
every time a Mark is to be sent, reverting
to the lower frequency for a Space. The amount of the
shift is usually 170 Hz for Amateur Radio use although many commercial
Teletype signals use other shifts, notably 425 Hz and 850 Hz.
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.
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.
Today, RTTY is still a popular mode especially on
the HF bands, and the advent of the "Glass Terminal",
firstly the Dumb Terminal and now the Personal Computer, has
brought this mode to even more operators the world over.
Many specialised RTTY systems were developed for
the Amateur enthusiasts but have been
superseded now by the Personal Computer with one of
the Multi Mode TNCs or sound card DSP programs which handle
RTTY and many other modes besides.
The latest Computerised RTTY equipment generally allows
us to use the mode better, quieter, more efficiently,
using less power and occupying less space than the old
TTY machines, but the limitations of the mode remain.
References:
- http://www.teleport.com/~nb6z/rtty.htm |
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