254 BELL SYSTEM TECHNICAL JOURNAL 



Key West and Havana and has also been used on some non-loaded 

 cables and quite extensively on land lines. The multiplex distributor 

 method is used widely on land lines and has also been used to some 

 extent on non-loaded cables. Of the two the multiplex distributor 

 method makes more effective use of the line when the frequency- 

 range is limited to about 100 cycles per second or less and the carrier 

 current method is more effective when a considerably wider frequency- 

 range is available. Since the frequency-range provided by the New 

 York-Horta cable extended to about 60 cycles per second, the multiplex 

 distributor method was the more effective means for providing multiple 

 channels on this cable and was accordingly adopted. 



With the multiplex distributor method of separating channels 

 several different systems of operation employing different signal codes 

 are possible and several different codes have been practically applied. 

 Among these are the cable-code, the three-unit three-element code and 

 the five-unit two-element or Baudot-type code. To determine which 

 of the several possible systems can give the greater speed of operation 

 is an extremely complex problem since it requires consideration not 

 only of the number of characters or letters and their frequency of 

 occurrence in messages but also of the line characteristics and the 

 nature of interference. From the practical point of view, however, 

 the multiplex system, which employes a code of the Baudot type, 

 has the great advantage of availability of perfected transmitting and 

 printing apparatus and, in view of this advantage, there seems little 

 doubt of this being the best system for the immediate practical 

 realization of the possibilities of a loaded transoceanic cable. In 

 this system the line-time is divided into as many parts as there are 

 channels of communication and each of these parts is divided into 

 five units. The line is thus used in effect to transmit five successive 

 signal units of either positive or negative polarity from one transmitter 

 to its corresponding receiver, thereby sending one letter or character 

 over one channel. It is next used to send similarly another letter on 

 another channel and so on until a letter has been sent over each channel , 

 whereupon a second letter is started over the first channel. 



Although multiplex distributors for land lines had long been avail- 

 able, the standard apparatus was not suitable for realization of the 

 full advantage of the permalloy-loaded cable. This was appreciated 

 from the first, and long before the manufacture of a loaded cable was 

 started the development of a system for operating it was undertaken. 

 In several important respects the apparatus developed for the cable 

 is different from that used on land lines. 



Two-way operation is provided by automatic reversal of the direction 



