352 



THE BELL SYSTEM TECHNICAL JOUKXAL, MARCH 1957 



The blocking out of the frequency range was controlled by a number 

 of factors. Cost considerations required that the carrier frequencies be 

 kept above the voice frequency range. If carrier extended into the voice 

 range, the voice frequency circuit would be lost on a carrier pair. One 

 of the carrier channels applied to that pair would have to be used to re- 

 place it. Thus, the addition of four carrier channels to a pair would yield 

 a net gain of only three channels. This in turn would increase the net 

 cost per gained channel. Filter costs determined how close to the voice 

 frequency band the carrier frequency range could be placed and in con- 

 junction with the number of channels required how closely the channels 

 could be placed to each other. 



Crosstalk considerations restricted the carrier frequency range to below 

 about 100 kc in order to reduce the cost of line treatment and rearrange- 

 ment of pairs on existing rural lines. By using this frequency range it 

 appeared possible to apply more than one carrier system to crossarms on 

 an open-wire route. The rapid rise in attenuation with frequency of steel 

 wire used on rural lines dictated that the range of frequencies be kept low. 

 As a result of these two sets of considerations, development work on 

 the PI carrier system was concentrated in the 8- to 100-kc range. 



Amplitude modulation of the carrier frequencies was chosen over 

 other forms of modulation because of the simpler terminal circuitry 

 and equipment and because of the saving in bandwidth. Use of ampli- 

 tude modulation and the use of compandors, discussed in a later section, 

 were felt to compensate for possible transmission advantages that could 



STACKABLE 



NORMAL 

 GROUPED 



STAGGERED 

 GROUPED 



J I L 



APART 



FREQUENCY 



I 



_l I l_ 



J I L 



P=PILOT FREQUENCY FOR 

 REPEATER REGULATION 



J I 



6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 102 

 FREQUENCY IN KILOCYCLES PER SECOND 



ARROWS SHOW USUAL 

 DIRECTION OF TRANSMISSION 



f TOWARD CENTRAL OFFICE 



I FROM CENTRAL OFFICE 



Pig. 1 — Type PI Carrier Frequeucy Plan. 



