AUTOMATIC PRINTING EQUIPMENT 405 



The apparatus associated with loaded cables will in practically all 

 cases be installed in the same offices as the equipment in use on non- 

 loaded cables and, in order to avoid the necessity for duplicating the 

 operating and maintenance staffs, it should be of such a nature as 

 to permit of its being operated and maintained by men familiar with 

 the operation of apparatus in use on land lines and ordinary cables. 



Codes 



The signalling speed attainable on any telegraph circuit, the effect of 

 interference upon the received signals, and the design of the operating 

 equipment depend to a certain extent upon the telegraph signal code 

 used. A great variety of codes have been devised from time to time with 

 a view to effecting greater economy of line time or greater freedom from 

 the effects of interference, but only a few of them have been generally 

 adopted in commercial practice. These may be divided into two 

 general groups: the two-element codes which are composed of various 

 combinations of positive and negative current impulses, of which the 

 continental Morse and the Baudot codes are well-known examples, 

 and the three-element codes in which a zero or no-current interval is 

 employed to separate individual pulses of a group or as a third element 

 in the combinations. The cable code and three-unit code are examples 

 of the three-element type. 



The codes in each of these two groups may be subdivided into two 

 classes, those known as uniform codes in which all characters are 

 composed of the same number of equal time units and those known as 

 non-uniform codes in which the impulses forming the characters vary 

 in length, number or both. The non-uniform codes are well adapted 

 for use where the received signals are translated manually, but are 

 not so well suited to automatic translation as the uniform codes on 

 account of the mechanical and electrical complications introduced by 

 the necessity for distinguishing between signal combinations of 

 varying length. 



Of the uniform codes which have been used in automatic printing 

 telegraph systems, the Baudot or two-element five-unit type of code 

 possesses advantages over the three-element three-unit type of code 

 which make it much better suited for automatic operation of sub- 

 marine cables. The three-element three-unit code employs, as does 

 also the cable code, a zero interval of unit length in forming the signal 

 combinations representing each character or letter and the shape of 

 the received signals must be sufficiently refined to make this zero 

 interval easily distinguishable (see A, Fig. 2) in order to pre^■ent 

 contusion in translation. Even with the best shaping obtained to date 



