THE TYPE PI CARRIER SYSTEM 359 



by the combination of the rectified control signal voltage exceeding the 

 reference voltage of a silicon-aluminum junction diode and by the ex- 

 pansion characteristic of the variolosser. The variolosser uses a specially 

 coded set of the silicon diodes which are matched for both ac and dc 

 characteristics. Modulation products introduced by the variolosser have 

 been kept below those produced in the associated receiving demodulator. 



3.7 Signaling 



The need to transmit customer signaling information over a carrier 

 channel required the development of means of passing dialing and super- 

 vision signals toward the central office. It also required passing ringing 

 information to the remote terminal for the types of multiparty ringing 

 generally used in the Bell System, including four-party selective service, 

 eight-party semi-selective service and divided code ringing. 



These requirements were met in such a way that the carrier system 

 can be inserted into a normal voice frequency circuit and function with- 

 out requiring any change in the existing signaling equipment in 

 the central office or in the customer's telephone. The central office ter- 

 minal is activated by 20-cycle ringing signals which are reproduced at 

 the remote terminal. The remote terminal is activated by switchhook 

 signals and dial pulses which are reproduced at the central office termi- 

 nals. Thus, the two directions of signaling require completely different 

 circuits. A block schematic of the arrangement used is shown in Fig. 5. 



3.7.1 Ringing 



The customer signaling originating in Bell System central offices con- 

 sists of 20 cycles superimposed on plus or minus battery and applied 

 between either tip or ring and ground. These signals control the trans- 

 mission over the PI carrier system of three in-band frequency tones, the 

 proper combination of two of them serving to select the party to be rung 

 from the far end. The third tone (2,500 cycles), modulated at a 20-cycle 

 rate, carries the information as to whether 20-cycle ringing is present or 

 absent. In-band frequencies were chosen to encode ringing information 

 for transmission over the carrier channel because of the substantially 

 lower cost of in-band filters as compared to those required for out-of-band 

 transmission. 



The three signaling tones are generated by three transistor tone oscil- 

 lators incorporated in a PI central office terminal. One set of three oscil- 

 lators can be arranged to supply four central office channel terminals. 



The transmission of the tones is controlled by three diode-operated 



