Basic Rtty Theory - AEA COMPUTER PATCH CP-1 Instruction Manual

Radioteletype (abbreviated RTTY) is a form of telegraphic com m un­
ication em ploying typew riter-like machines for generating a coded set
of electrical impulses when a key is pressed on the keyboard and
converting a received set of such impulses into the corresponding
character.
W ith the application of home computers to the amateur's use, we are
now able to use a com puter to do all decoding and generation of the
coded set of electrical
practically no maintenance, whereas old teletype gear required
frequent oiling and maintenance.
More im portant than the computer, is the terminal unit (TU) which
handles the conversion of the tw o-toned signal into a string of DC high
and low (1 or 0) level signals. This is called a dem odulator because it
demodulates the tones into something the com puter can understand.
A TU should also have some method of generating a two-tone signal
from a high/low going signal. This is called a modulator. When a
dem odulator and m odulator are com bined into the same package, it is
referred to as a modem.
The CP-1 is a modem. The dem odulator in the CP-1 employs a
sophisticated dual filtering system and an automatic threshold circuit.
The two tones (received from the air) are separately demodulated and
then fed into a threshold circuit. The threshold circu it then
autom atically decides which is the stronger of the two filter signals and
allows that one to pass. However, when one of the signals fade out or is
jammed or removed altogether, the threshold circu it automatically
switches over to the clear tone.
This sort of a system is especially nice under poor signal conditions
When a condition called selective fading is present. Selective fading is
when one of the RTTY tones fades away and then reappears. When this
happens, the threshold circu it autom atically adjusts to the other tone.
Another nice feature of the threshold circu it is if interference
eliminates one of the RTTY tones or if one of the tones is jammed, the
threshold circuit will again switch over to the tone remaining in the
clear.
It is obvious that a dual filtering system which uses both filters and a
threshold circuit, w ill perform much better than a single filte r system or
a phase-locked loop system that uses only one filte r for decoding.
U.S amateurs are allowed to use 60 wpm, 67 wpm, 75 wpm, and 100
wpm Baudot, and 110 baud and 300 baud ASCII on the HF bands.
ASCII is a code made up of 7 bits and 1 parity bit. However, the parity
bit is left out. Baudot is made up of a 5 bit code and includes a start bit
and stop bit. The start bit sets the sending and receiving mechanisim in

BASIC RTTY THEORY

impulses.
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Further, 'computers require