1326 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1954 



guard channel. These voltages are then applied to rectifying circuits 

 where positive and negative dc voltages are developed and passed on 

 to the dc amplifier tubes a and b, respectively. The rf relay operates 

 first and cuts in the band elimination filter thereby preventing signaling 

 tones from entering a connected toll line and interfering with the signal- 

 ing there. However, a short spurt of tone will get through because of the 

 finite time required to operate this relay, and the r relay must therefore 

 be made slow enough so that it will not operate on this tone. This action 

 is obtained with the resistor-capacitor network (ot and Cs) in the grid 

 circuit of the associated dc amplifier. 



A short time (about 200 ms) after the r relay operates, a relay 

 (not shown) releases, which short circuits the guard network and inserts 

 enough resistance in series ^\'ith the signal network to substantially re- 

 move its tuning. The purpose of removing the guard action during the 

 idle condition is to prevent release of the receiver which would otherwise 

 be caused by occasional bursts of line noise. 



The signal network is made broad at this time for the following reason. 

 In connections to an intercepting operator, "off hook" supervision is not 

 returned to the originating end to avoid charging for the call. This means 

 that tone remains on the line to continue to hold up the receiver. At the 

 same time the intercepting operator's speech must of course be trans- 

 mitted over the line so that both speech and tone enter the receiver. 



Speech can be of a relatively high power as compared to the tone with 

 the result that the action of the limiter tends to suppress the tone and 

 could falsely release the receiver if the signal tuning were present. How- 

 ever, with broad tuning either speech or tone will hold up the receiver 

 and no trouble is encountered. 



The blocking amplifier seen in Fig. 11 has the same function as in the 

 1,600-cycle design described previously. 



Among the new features in this unit, one of the most significant is 

 the pulse-correcting circuit. This feature is a very important element 

 in the entire long distance connection since it serves to keep the length 

 of the dial pulses within specified time limits. The dial pulses on many 

 calls may have to go through a number of central offices and all their 

 associated equipment, and in each stage of the transmission path the 

 ideal 60-millisecond dial pulse may be distorted so that it becomes too 

 long or too short. 



The pulse-correction is accomplished by generating appropriate tran- 

 sient voltages, whose duration is determined by capacitor-resistor net- 

 works. These voltages are then applied to the grid of dc amplifier b in 

 Fig. 11 to perform the elongation or shortening of the pulses as required. 



