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1. RADIOTELEX COMMUNICATION
1.1 General
Telex subscribers can attest to radiotelex as a reliable and efficient
method for sending and receiving teleprinter connections. Telex
subscribers, especially those who often use HF-band radio circuits,
will also attest that the telex connection is subject to interference
from a variety of sources, including atmospherics, fading and noise
disturbance. This interference plays havoc with radio signals, re-
sulting in the receiving of information different from the intended
information. Thus a means must be provided to prevent mutilation
of radio signals by interference on HF-band radio.
Radiotelex communication today owes its reliability and efficiency
to error detection and correction. The ITU-R defined both a con-
stant-ratio code for automatic error detection and requirements for
the error correction in Recommendation M.625-3.
1.2 Code Description
The DP-6 employs a 7-element synchronous code providing 2
7
=
128 combinations. Among these 128 combinations, there are 35
constant-ratio combinations having a ratio of 3 (Y) mark bits to 4
(B) space bits. Thus ratio is used to test the validity of each re-
ceived character.
Of the 35 combinations, 32 are used for the required alphanumeric
teleprinter signals. The remaining three 7-element codes are used
exclusively for operational purposes. These are:
Idle Signal (ARQ Mode), Phasing Signal 1 (FEC Mode)
Idle Signal
RQ Signal (ARQ mode), Phasing Signal 2 (FEC Mode)
Transmission rate is 100 bauds. If the 4B/3Y ratio is disturbed due
to interference, the output of the receiver is blocked to restrict the
mutilated character from passing on to the teleprinter.
Frequency Shift
The frequency shift is 85 Hz with a center frequency of 1700 Hz,
as specified in ITU-R Recommendation M.625-3.
Space Tone Frequency 1700+85 = 1785 Hz
Mark Tone Frequency 1700-85 = 1615 Hz