THE TYPE PI CARRIER SYSTEM 357 



The +4 dbm output level coupled with noise and crosstalk considera- 

 tions indicated that 30-db bare carrier line loss between terminals would 

 be possible. A survey of existing and planned Bell System rural telephone 

 lines indicated that substantial amounts of entrance cable and open wire 

 would be encountered in potential carrier layouts. Calculations of carrier 

 frequency loss of those facilities showed that 30-db loss would not be 

 sufficient to care for all of the necessar}^ rural applications, which con- 

 firmed the need for carrier repeaters. 



3.5 Compandors 



Compandors were incorporated in the PI system because their several 

 advantages more than offset their added cost. The crosstalk and noise 

 advantage provided by their use reduced the need for expensive line 

 treatment to reduce crosstalk. In addition, the compandor noise advan- 

 tage permitted lower received carrier levels to be used, thus increasing 

 the permissible carrier line loss. Compandors also eased the requirements 

 on terminal and repeater filters, thus reducing filter cost. 



The compandor in the PI sj^stem is a simplified version of the syllabic 

 compandor used in Type Nl and carrier systems, but its performance 

 is comparable to those units. The new problem of matching the com- 

 pressor and expander characteristics in PI terminals operating in differ- 

 ent ambient temperatures has been simplified by the use of silicon-alumi- 

 num junction diodes in the compandor variolossers and control circuits. 



3.6 Channel Regulation 



Channel regulation was necessary to provide satisfactory transmission 

 performance and keep maintenance adjustments to a minimum. The 

 regulation was designed to compensate for daily and seasonal carrier 

 circuit net loss variations caused by changes in line attenuation with 

 temperature. It would be desirable to have the terminal regulation range 

 equal to 30 db, the maximum line loss that can be spanned between the 

 terminals, to ease engineering layout considerations. However, cost con- 

 siderations led to a 15-db range, with span pads used where required by 

 system layout to adjust the received carrier power to the center of the 

 range of the regulator. 



The regulation in the receiving amplifier is of the backward-acting 

 type. A reference control signal, derived from the receiving amplifier 

 output, is used to vary the loss of the balanced diode variolosser at the 

 amplifier input. The regulator "stiffness" of 1.6-db change in channel 

 voice frequency output for 15-db variation in carrier input is obtained 



