Code, Telegraphic

Code, Telegraphic

 

a conventional coding system accepted in telegraphy, in which each letter or sign is represented by its own combination of pulses of electric current. An elementary unit (a code element) is the shortest possible unit; all other units are composed of such elements. The number of elementary units used to denote each symbol in a telegraphic code may be different for different signs (nonuniform code), or it may be the same for all signs (uniform code). The number of values an elementary unit may assume in the transmission process is called the base of the code. According to this criterion codes may be classified as binary, ternary, and so on. Uniform codes are divided into five-element codes, six-element codes, and so on, depending on the number of elementary units used to denote a symbol.

In the nonuniform Morse code (see Figure 1), symbols are represented by current units of different duration—short elementary units (dots) and units of triple duration (dashes). Morse code is rarely used in commercial telegraphy because it is not economical in operation and is not suitable for printed reception.

The most widely used code is the five-element uniform code no. 2 (see Figure 2), which was recommended in 1932 by the International Consulting Committee on Telephony and Telegraphy. Code no. 2 as used in telegraph apparatus in the USA is shown on the right side of Figure 2. In this code three characters do not coincide with the two-register international code no. 2 (in

the figure these three characters are enclosed by a heavy rectangular line). In the USSR correspondence is received and transmitted by telegraph apparatus in both the Russian and Latin alphabets. Therefore a three-register code no. 2 (Figure 3) with Russian and Latin alphabets has been adopted as the basic code in the USSR. This code differs from the two-register code mainly in the addition of the Russian alphabet and a register that shifts the apparatus to operation in the Russian alphabet. In the code no. 2 the individual combinations of current units are distinguished only by the sequence of the plus and minus (or interval) current units. For uniform codes, one of which is code no. 2, the number of possible combinations N is determined according to the formula N = mn, where n is the number of code elements and m is the base of the code.

Redundant codes are used if interference is strong in channels assigned to radio telegraphy and data transmission. Such codes make it possible not only to detect but also to correct an error in the code combination received by the telegraph apparatus.

REFERENCE

Tomashevskii, B. A., S. D. Chantsov, and G. U. Osipenko. Kurs telegrafii. Moscow, 1963.

V. V. NOVIKOV

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