RTTY / ASCII
38
RTTY and ASCII Modes
KAM only
RTTY Theory
RTTY is a 5 level (5 bit) BAUDOT code, which limits the number of possible characters to 32. This
obviously does not provide enough combinations for all 26 letters of the alphabet and the numer-
als and punctuation. Therefore, in order to allow more combinations, two special characters have
been reserved as a FIGURES shift and LETTERS shift. When you first enter the RTTY Mode, the
system is in the LETTERS shift, thus allowing normal alphabetic characters to be typed. When you
type any numbers or punctuation, the KAM automatically inserts a FIGURES shift prior to sending
the character. One FIGURES shift is sent prior to the beginning of a string of numbers, but if a
space is typed, a new figures shift will be required. You may manually insert a FIGURES shift by
typing a Ctrl-N or a LETTERS shift by typing a Ctrl-L. If this is done, the KAM will not insert the
FIGURES shift for you, for that sequence of numbers.
The standard mechanical teletype machine always "un-shifted" when it received a space character,
thus any numerals or punctuation immediately following a space will require a FIGURES shift to in-
sure, that the receiving machine is in the FIGURES mode. The KAM simulates this un-shifting
based on the setting of the USOS command.
When operating in RTTY, standard procedure is to transmit approximately one line of RYs prior to
actually typing your message. This allows the receiving station time to properly tune his station
and adjust his receiving equipment. This was particularly true with the older mechanical units, but
has carried over into today's computer terminal RTTY operation. A typical sequence, then, for call-
ing CQ might look like this:
RYRYRYRYRYRYRYRYRYRYRYRYRYRYRYRYRYRYRYRRYRYRYRYRYRYRYRYRYRYRYRYRYRYRYRY
CQ CQ CQ DE WK5M WK5M WK5M
CQ CQ CQ DE WK5M WK5M WK5M
CQ CQ CQ DE WK5M WK5M WK5M K K K
The RY has been used for the synchronizing and testing purpose because in the 5 level Baudot
code, this represents a complete reversal of the bit pattern, therefore the receiving mechanical
machine had to completely reverse all levers all levers and gears within the time frame of one
character. It was felt that if the machine could do a complete reversal like this, it could copy any
other combination without problem.
RTTY is transmitted using FSK (Frequency Shift Keying) in which two possible conditions for each
bit of data are sent as two distinct frequencies. These are called MARK and SPACE, and the differ-
ence between these two frequencies is called the SHIFT. Most amateur RTTY is transmitted with a
170 Hz shift, but other common shifts used are 425 Hz and 850 Hz. With FSK, the frequencies are
generated by changing the actual carrier frequency of the transmitter. Another method of transmit-
ting RTTY is to generate the tones within a piece of equipment and then modulate the carrier with
the tones. This is called AFSK (Audio Frequency Shift Keying). The KAM supports both methods of
transmitting RTTY and ASCII signals as well as all three of the standard shifts. In addition, the
KAM can use non-standard shifts by selecting MODEM SHIFT and setting your own choice of MARK
and SPACE tones.
ASCII is an 8 bit code, allowing 256 possible combinations and thus overcomes the shortcomings
of RTTY concerning numbers and punctuation. Because all possible numbers, letters and punctua-
tion can be defined within this set of of combinations, no LETTERS or FIGURES shift is required.
Transmission of ASCII is accomplished using FSK or AFSK as is done in Baudot RTTY and many of
the operational practices have been carried forward from RTTY to ASCII.
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