USER'S GUIDE
DATA TRANSMISSION CODES
PK232UG Rev. E 9/87
G-2
190
Some newcomers to digital radio are unsure which digital code is best to use on a given band
under given conditions. This brief tutorial outlines some differences between Baudot and ASCII
and explains some of the advantages and disadvantages of both codes.
BAUDOT CODE
History
The Baudot/Murray code dates from about 1874, when Emile Baudot, a Lieutenant in the French
Telegraph Service, developed the Baudot distributor for telegraphy. This code uses a 'start' bit and
a 'stop' bit to mark the beginning and end of each character, and five information or data bits to
identify each character.
With only five information bits, the total number of possible combinations equals 25 or 32. But our
English language has 26 letters (counting upper case only!), 10 numerical digits and 9 common
punctuation characters. That adds up to 45 alphanumeric characters. How can we transmit at least
45 different characters if the available code only permits 32 code combinations?
The Baudot/Murray codes resolve this seeming paradox by the same stunt used on your old elec-
tromechanical typewriter - you shift to FIGS or UPPER CASE. Now you have the numerical digits,
the punctuations and other useful things like parentheses, cent sign, dollar sign, number sign, 'at'
sign, asterisk, percent sign, ampersand, underscore, etc. You can handle 64 possibilities. That's
fine – if you never need to handle anything but text message traffic or connect to computers!
Code Variations
Over the years, different versions of the same code have evolved, creating some confusion in both
domestic and international traffic. Western Union, the Bell System, the Weather Bureau, the
F.A.A., the Armed Forces and the Associated Press all developed their own versions of the same
code. Some users couldn't get together on the same keyboard configurations.
When the International Telex Network was formed before World War II telex users had yet anoth-
er version of the Baudot/Murray code - the International Telegraph Alphabet Number 2.
Limitations
Although it is the most widely used data transmission code in the world (the International Telex
Network is still the largest of all record communications networks) the Baudot/Murray code has
two serious drawbacks:
1. The code makes no provision for parity or any other built-in method of detecting transmission
or reception errors. This means the receiving system has no way of telling if an error has oc-
curred;
2. The code is sequential --a specific control character defines the identity of the following series
of characters for a period of time until a new control character is recognized. FIGURES SHIFT
is an example.
The two control characters which identify the shift, or 'case' are 'LETTERS' and 'FIGURES'. Those
who operated in the days of electromechanical teleprinters and Teletype™ machines know what
happened when the data was kicked into upper case by a noise hit, and how frustrating it was to
have lines of unintelligible stuff.
