TWINPLEX CCIR 476

A FDM (4 frequency) 2 channel simplex synchronous ARQ system using the CCIR 476 alphabet. 2 channels of 3 characters are sent, made up of 7 bits per character. A complete cycle has a duration of 450 msec and consists of 210 msec transmit, 240 msec receive.

WORD-, BIT-, CHARACTER or NOT-INTERLEAVED decode is selectable along with 6 different ways of decoding the 2 'channels' called F7B-1 to F7B-6 respectively. The sending station can also individually program the actual audio shifts used.

The 4 types of Interleave are arranged as follows

(in the following chart, letters refer to the characters being sent, e.g. normal SITOR would be ABC DEF GHI JKL, and numbers refer to the bits of the letter, so if the letter A was sent by normal SITOR it would be A1..A7 i.e. all 7 bits are sent one after the other) Normal case and Bold Italics differentiate between the two channels.

WORD- ABC ABC DEF DEF GHI GHI JKL

BIT- A1 A1 A2 A2 A3 A3 A4 A4 A5 A5 A6 A6 A7 A7

CHAR- A A B B C C D D E E F F

NOT- ABC DEF GHI JKL MNO PQR

The four frequencies F1 to F4 that are transmitted can be sent in 6 basically different ways and this leads to the 6 different modes F7B1 to F7B6,