THE TYPE PI CARRIER SYSTEM 353 



be obtained by using angular modulation (frequency or phase) with a 

 large modulation coefficient. 



Cost was again a major factor in the choice between double sideband 

 and single sideband amplitude modulation. Past experience with other 

 carrier systems has indicated that filters are a major part of the cost of a 

 system and, when frequency space is available, double sideband filters 

 are, in general, less expensive than those for single sideband. In addition, 

 the cost of a single sideband system would be increased because of the 

 problem of obtaining the necessary carrier supply at the terminal. 



The frequency plan developed for the PI carrier system is shown in 

 Fig. 1. The unusually wide carrier spacing of 12 kc was adopted in order 

 to minimize filter costs. Since the remote terminals are generally distribu- 

 ted along the line, it was not practical to use double modulation to ac- 

 complish filtering in the most efficient frequency range. Instead, filtering 

 was done at line frequencies. Every effort was made to achieve channel 

 filter designs with maximum efficiency of element utilization. Advantage 

 was taken of the more leisurely rising characteristics of the double side- 

 band filters permitted by the wide frequency spacing. 



The stackable frequency arrangement Avas provided for non-repeatered 

 operation, because when the lowest two carrier frequencies are used to 

 provide a channel, it can be used over substantially longer distances than 

 channels using higher frequencies. The grouped arrangements were pro- 

 vided for repeatered systems to reduce the cost and number of the re- 

 peater filters and amplifers needed to separate the two directions of 

 transmission. The staggered grouped arrangement can be used with the 

 normal grouped arrangement on a pole line having poor crosstalk coup- 

 ling in order to increase the effective coupling loss between carrier 

 channels on different pairs. The grouped and stackable arrangements 

 cannot be used on the same pole line, because certain frequencies would 

 be used for both directions of transmission. This would produce large 

 differences between transmitted and received carrier power at terminals 

 and repeaters which would lead to intolerable crosstalk. 



A number of terminal arrangements were studied in order to implement 

 the above frequency plan. The arrangement for a remote terminal shown 

 in Fig. 2 was chosen as the simplest terminal meeting all of the system 

 requirements. It is very similar to the channel terminal arrangement 

 used in the Type Nl carrier system, another double sideband amplitude 

 modulation sj^stem used for long distance trunks of the Bell System. The 

 several shaded portions in the figure show the breakdown of the terminal 

 functions into individual sub-units, which are the basis for the equipment 

 arrangements discussed in Section 5 of this paper. A number of the other 

 important features that make up the terminal arrangement are discussed 

 in the following sections. 



