Furuno DP-6 manual Radiotelex Communication, General, Code Description, Frequency Shift

1.RADIOTELEX COMMUNICATION

1.1General

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 27 = 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

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