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THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1957 



circuit from its input. This results in carrier being transmitted to the 

 central office terminal, where it causes relays in the terminal to recreate 

 the customer's line short across the line connecting the ccntnil office 

 carrier terminal and the central office switching eciuipment. 



Dialing at the customer's instrument alternately opens and shorts 

 the voice frequency extension at each dial pulse. Opening of the line 

 operates a relay in the remote terminal which short-circuits the transmit- 

 ting amplifier input and causes the relays in the central office to follow 

 the pulsing of the received carrier, recreate the dial pulses there, and 

 operate the central office smtching equipment. 



3.8 Repeater 



Repeaters are used in the PI system whenever the line transmission 

 loss exceeds the maximum that the terminals can accommodate. The 

 repeaters use transistors for gain instead of electron tubes, but otherwise 

 are schematically very much like previous repeaters as shown in Fig. 7. 

 The two directions of transmission have similar functions differing only 

 in the frequency band that is amplified and the options that are used in 

 the gain regulating circuits. 



Identical high-low pass directional filters at each end of the repeater 

 separate the directions of transmission into the high and low frequency 

 groups. Optional phase correction sections for these directional filters are 

 used along a line to improve the phase characteristics in the cut-apart 

 region. The directional filters are connected to the line through the same 

 line matching coils and secondary protection circuits used in the termi- 

 nals. Repeater input span pads are used to build out the line loss to its 



LINE 

 EQUALIZER 



REGULATOR 



HIGH GROUP 



CARRIER 

 LINE 



LINE 

 CONNECTOR 



AND 

 SECONDARY 

 PROTECTION 



OUTPUT 

 PAD 



INPUT 

 PAD 



DIRECTION 

 FILTER 



DIRECTION 

 FILTER 



INPUT 

 PAD 



OUTPUT 

 PAD 



REGULATOR 



LINE 

 CONNECTOR 



AND 

 SECONDARY 

 PROTECTION 



CARRIER 

 LINE 



LOW GROUP 



LINE 

 EQUALIZER 



Fig. 7 — PI repeater block schematic. 



