IN-BAND SINGLE-FREQUENCY SIGNALING 



1319 



it would of course pcM-manciitly i)re\(Mit operation, while if it is of short 

 duration and occurred at the beginning of a signal inter\'al it would only 

 delay operation. E\'en a short delay would be harmfid to ring forwai'd or 

 dialing signals ])ut could l)e tolei'ated in disconnect or flashing signals. 



An example of how a short duration high le\'el noise affects receiver 

 operation is illustrated in Fig. G. As can be seen this particular noise 

 transient, which ha})pened to be caused by a relay in the trunk circuit, 

 occurred just prior to and overlapping the tone interval. It would se- 

 riously damage a ring forward signal. The solution to this particular 

 problem is to absorb the noise at its source, or prevent it from reaching 

 the ^'oice path. 



After the recei\^er is operated for a short time (0.2 sec or so) the guard 

 action is removed. Later on when the tone is removed it is desired that 

 the receiver release, but noise at this time will tend to prevent release. 

 The solution to this problem is a compromise in receiver sensitivity, 

 i.e., it must be sensitive enough to hold up on the weakest signaling tone 

 and yet release on the maximum noise that can be tolerated from a 

 speech point of view. Fortunately such a compromise is achieved with a 

 sensitivity that will cause the receiver to hold with the tone about 8 db 



5.0 



2.5 



-2.5- 



-5.0 L 



BEGINNING OF 

 TRANSMITTING TONE 



TRANSIENT VOLTAGE AT 



TRANSMITTING END 



(-16 TL) 



BEGINNING OF 

 RECEIVED TONE 



20 30 40 



TIME IN MILLISECONDS 



TRANSIENT VOLTAGE 



AT RECEIVING END 



(+7TL) 



Fig. 6 — Example of transient voltages generated by relay operation. 



