IX-BAXD SINGL.E-FKEQUENCY SIGNALING 1313 



In the case of two-wire lines, the signaling e(}nipment is applied to 

 the four-wire transmission paths of terminal repeaters, using a different 

 freciuency for signaling in opposite directions. Band elimination networks 

 are provided ahead of each receiving circuit to block the transmitting 

 frequenc}', which would otherwise come into the receiver via echo paths 

 and interfere with its operation. 



GENERAL DESIGN CONSIDERATIONS 



The successful use of the voice path for signaling, especially for con- 

 tinuous as contrasted to "spurt" signaling, is feasible only l)y a compro- 

 mise among a num]:)er of conflicting factors. These factors or design con- 

 siderations are (a) choice of signal freciuency, (b) signaling power and 

 receiver sensitivity, (c) imitation of signal by speech or tones, (d) inter- 

 ference to signal by other tones and noises, and (e) audibility of signaling 

 tone to operators and subscribers. 



(a) Choice of Signal Frequency 



The choice of signal freciuency is determined mainly by considerations 

 of signal imitation by speech. As will be shown later on, signal imitation 

 decreases rapidly as the signaling frequency is raised with the result that 

 the highest frequency that can be reliably handled by the transmission 

 path is used. In the case of some four-wire type lines, such as EB carrier, 

 the highest frequency that should be used is 1,600 cycles. However, the 

 use of 1,600 cycles results in an expensive signaling system and it is de- 

 sirable to have another system using a higher frequency (2,600 cycles) 

 for application to lines that can handle this frequency. These systems are 

 basically the same in principle and both are described in the present 

 article. 



For application to two-wire lines the second freciuency used is 2,000 

 cycles in the case of the older 1 ,600-cycle system and 2,400 cycles in the 

 new 2,600-cycle system. 



(6) Signaling Power and Receiver Sensitivity 



To limit cross talk into adjacent voice channels and to avoid adding 

 much signal power to the repeaters it is desirable to use the lowest prac- 

 ticable signal power consistent with a usable signal-to-noise ratio. A 

 value of —20 dbm referred to zero transmission level for the steady idle 

 tone is satisfactory for this purpose. In order to obtain an overall margin 

 of 8 db the sensitivity of the receiver is set at —28 dbm. A higher power 



