IN-BAXD SIXGLE-FREQUEXCY SIGNALING 



1321 



Iwlow normal and yet permit release in the presence of as much as 50 dba 

 of thermal noise at zero transmission level. 



SPECIFIC DESIGNS 



The two signaling systems 1,600 and 2, GOO cycles, are basically similar 

 in principle. However, as can be seen by reference to Fig. 5, a simple 

 guard type receiver having a frequency of 1,600 cycles would have too 

 many signal imitations. To ON'ercome this the guard ratio during the 

 talking condition was increased from 6 to 10 db, the minimum signal 

 interval to just cause a response was increased to 100 milliseconds during 

 the talking condition and the sensitivity was decreased to —16 dbm. 

 As a result fairly complicated timing and switching circuits are needed 

 to assure that both transmitting and receiving circuits have the right 

 condition at the right time. 



Table V gi^'es a sununary of the principal design parameters of the 

 two systems. 



DESCRIPTION OF 1,600-CYCLE DESIGN 



A front view of the 1,600-cycle main unit is shown in Fig. 7. This 

 panel is 8 inches high by 23 inches Ande and weighs about 20 lb. The 

 essential elements of the circuit are shown in Fig. 8. It connects to the 

 trunk relays over two leads e and m and into the line circuit via leads 

 labeled t, r, Ti and Ri . The transmitter, shown in the upper portion of 

 the figure, uses dc biased germanium varistors (diodes) to control the 

 application of signal current to the line, and for control functions, uses 

 four relays designated m, co, hl and rr. The functioning of the first 

 three except for tone control are described under the heading "Descrip- 

 tion of 2,600 Cycle Design" later in this article. The rr relay (not shown) 

 in conjunction with the m relay lengthens the sent pulse for the ring for- 



