AUTOMATIC SWncilIN'G SYSTEM 451 



With both sets of switchhook coiitticts now released the usual trans- 

 mitter, receiver and induction coil ai-rangement I'or transmission of 

 voice currents is connected to the tel(>phone line and all of the station 

 signaling eciuipment, including the tripping windings of the latch mag- 

 net, is disconnected from the circuit, fnterference and transmission 

 losses caused b}^ signaling eiiuijjment are thus avoided dui'ing conversa- 

 tion. When the handset is resting on its cradle between calls with both 

 sets of switchhook contacts operated, the usual ringer and ringer con- 

 denser are connected across the line for responding to incoming calls. 

 I'pon removal of the handset in answer to such an incoming call, direct 

 current is provided from the central office to trip the latched switchhook 

 contacts and thereby the set is placed immediately in the talking con- 

 dition. 



PULSE POSITION DIALING SIGNALS 



Before describing further the operation of this telephone set, it will 

 be necessary to explain briefly the dialing signals generated by it and 

 used in the system. 



From the subscriber's telephone set eight digits are transmitted for 

 a complete local area directory number and the transmission is repeated 

 as many times as necessary for the functioning of the central office 

 equipment. In order to indicate the starting point of the transmission 

 of a complete called number, a time interval of two digits duration 

 during which no signals are transmitted is provided at the beginning 

 of each transmission. Each digit interval is 0.01 seconds; therefore, a 

 time interval of 0.1 second is reriuired for the no-signal or blank period 

 and the eight digit number. 



These signals, as shown in the wave form-time diagrams of Fig. 7, 

 consist of two pulses per digit: a start pulse of 1 millisecond duration 

 and a stop pulse of 1 millisecond duration, each pulse approximately 

 a single cycle of a 1,000-cycle per second sine Avave. The time interval 

 between a start pulse and its following stop pulse is the measure of the 

 associated digit value. The start pulses are generated at intervals of 

 0.01 seconds, or 10 milliseconds, and one stop pulse is generated some 

 time during the 3.2 to 6.8 millisecond interval after each start pulse. 

 In order to provide sufficient margins to permit reliable signaling over 

 a wide variety of transmission facilities 3.2 milliseconds are allowed for 

 the decay of each pulse and the pulses themselves occupy a section of 

 the voice-frequency spectrum transmitted by practically all communi- 

 cation facilities. The possible starting times of stop pulses representing 



