Tfit: tVpS pi cAflHiEft SYSTEM 361 



keyers which, function independently, depending on the nature of the 

 ringing signal. The 2,500-cycle keyer responds to 20 cycles applied to 

 either the tip or ring conductors. The gating diode is back biased about 

 3 volts to prevent random noise peaks from operating it. The l,7oO-cycle 

 keyer responds to any ringing signal applied to the tip circuit. A diode 

 oppositely poled to the gating diode has a high breakdown so that any 

 reverse voltage peaks leaking through to it will not open the gate. The 

 1,150-cycle keyer responds to 20-cycle ringing voltage superimposed with 

 either plus bias voltage applied to the tip or with minus bias voltage 

 applied to the ring conductor. 



At the remote terminal the signaling tones are each selected at the 

 output of the expandor by tuned circuits, amplified by a transistor and 

 rectified to activate relays controlling the customer ringing. The 2,500- 

 cycle tone interrupted at a 20-cycle rate controls the remote ringing gen- 

 erator, the 1,750-cycle tone determines whether ringing is to be on the 

 tip or ring side of the line, and the 1,150-cycle tone whether the bias ap- 

 plied to the line is positive or negative. 



The ringing power at the remote terminal is provided by a 3,000-cycle 

 transistor oscillator which uses a 2-watt 6A transistor in the second of 

 its two stages, as shown in Fig. 6. The rectified output of the oscillator 

 is pulsed at a 20-cycle rate by a relay controlled by the 2,500-cycle in- 

 band tone and applied to the line through a low-pass filter ip reduce the 

 harmonic content of the ringing signal. Positive or negative bias is pro- 

 vided from one of two clamper diodes. In order to obtain sufficient power 

 from the ringing generator, two electrolytic capacitors are used in a volt- 

 age doubler configuration. The ringing generator draws nearly 500 mils 

 of battery current w^hich, by PI standards, is a heavy power drain. In 

 order to keep this drain to a minimum, the ringing generator is activated 

 only during the ringing period. Also the generator is connected to the 

 customer's line only during the ringing period to remove its shunting 

 effect on the talking circuit. 



3.7.2 Supervision and Dialing 



Carrier on-off signaling was chosen to transmit supervision and dialing 

 signals from the customer to the central office. This method was used 

 because of the ease of implementing it and because of savings in dc power 

 drain at the remote terminal achieved by transmitting the carrier from 

 that terminal to the central office only during the off-hook condition. 



An off -hook signal on the voice frequency extension of the remote ter- 

 minal, caused by the customer lifting his handset, is used at the remote 

 terminal to activate the transmitting amplifier and to remove a short- 



