464 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1952 



A subscriber in originating a call first pre-sets the complete called line 

 number on the finger wheels of his subset. The subset has been "latched" 

 in the signaling condition by the mechanical reset on hanging up after 

 the previous call. When the subscriber then removes the handset of his 

 telephone from its cradle a line relay in the central office operates in 

 recognition of a demand for service. The line relay in turn energizes a 

 start gap of an associated cold-cathod gas tube. The gas tubes for a 

 group of lines are connected in a lockout arrangement such that only 

 one gas tube at a time can conduct in a main gap. When the tube does 

 conduct in the main gap it operates a relay wliich connects the associ- 

 ated line directly to a common originating actuator and receiver cir- 

 cuit. During the short period that one line is attached to the receiver, 

 originating service is withheld from all other lines in the same group 

 but incoming calls may be terminated to any idle line. 



The name, actuator, in this system refers to a circuit which includes 

 an amplifier for transmitting 50-cycle current to a subscriber's subset 

 over the simplex. This current is maintained at a constant amplitude 

 despite the differences between various subscriber loops and the possible 

 presence of earth potentials by the high output impedance of the ampli- 

 fier. This high output impedance is obtained by the use of 35 db of feed- 

 back from the output of the amplifier. In addition, the actuator circuit 

 also monitors its 50-cycle current flow when connected to a subscriber 

 loop as the means of maintaining supervision since no direct current is 

 permitted in the loop during the signaling period. The 50-cycle current 

 in the subscriber's set causes the complete pre-set number to be generated 

 repetitively as pulse position dialing signals which are returned to the 

 receiver circuit in the central office over the loop. The use of simplex 

 power to generate loop signals was adopted to simplify the filtering 

 problem at the receiver circuits. 



The originating receiver detects the dialing signals including the 

 occurrence of the blank interval between repetitions of a complete num- 

 ber. It decodes the signals representing the first three digits following 

 the blank interval, i.e., the called office code, and registers these digits 

 unless the check circuit indicates that another trial is necessary. The 

 action of the check circuit has been described in the Signal Receiver sec- 

 tion of this paper. The receiver ignores the signals representing the 

 called line number. Upon the successful registration of the called office 

 code the originating receiver connects to the trunk number group circuit. 



The name, number group circuit, in this system refers to a circuit 

 through which a connection may be made to the switching network 

 appearance of the control lead of any of a group of trunks or lines. In 



