1314 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1954 



is used for a short time at the beginning of each application of signaling 

 tone to help overcome line noise and attenuation variation. This power 

 increase is 14 db for the 1600-cycle system and 12 db for the 2,600-cycle 

 system. 



(c) Imitation of Signal by Speech or Plant Tones 



An in-band signaling system requires that the receiver respond to signal- 

 ing tone and at the same time be non responsive to speech formed cur- 

 rents. The principal design factors employed to achieve this feature are 

 (1) the use of "guard action," (2) the employment of as narrow a band- 

 width as practicable for the signal selective network, (3) the use of 

 volume limiting, (4) the use of the longest operate time practicable, 

 consistent with trunk signaling reciuirements, and (5) the use of the 

 highest frequency that can be handled by the voice path. 



"Guard action" is the principal means used in protecting the receiver 

 against operation on speech. It consists in the use of nearly all fre- 

 ciuencies in the voice band other than those in a narrow band centered 

 on the signaling frequency to generate a voltage which is used to oppose 

 that resulting from the signal frequency. The sum of these two voltages, 

 plotted against frequency, for a typical receiver is shown in Fig. 2. A 

 term used to specify the magnitude of the guard action is "guard-signal 

 ratio" {G/S in Fig. 2) or just "guard ratio." The amount of guard which 

 can be used is limited by signal to noise ratio because noise, like speech, 

 tends to oppose operation of the receiver. A guard ratio in the range of 

 6 to 10 db has been found to be practicable. 



Protection against signal imitation is also provided by narrowing the 

 signal frecjuency band as much as practicable, since this reduces the 

 effective operating power of voice and noise frequencies. However, the 

 extent of this narrowing is limited since the operating bandwidth must 

 be sufficient to allow for frequency variation in the signal supply, for 

 carrier shift in the transmission path, for variation in the elements of the 

 tuned circuit in the receiver and to allow for the transmission of the 

 needed side bands of the signaling pulses. 



A bandwidth of 60 to 75 cycles at the 3 db points at dialing power 

 (about — 6 dbm at zero level) has been adopted as about the minimum 

 that is practicable. Because of limiting and guard action the effective 

 bandwidth is a function of input power and in the particular designs 

 adopted approaches about 150 cycles at the just operate point. 



Volume limiting is another means used to help prevent false operation 

 on high levels of speech. The explanation of this action is illustrated in 



