682 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1952 



of the sending tube at the central office terminal, actuates the station 

 ringer, which is connected to the local loop whenever the teletypewriter 

 power s\vitch is in the OFF position. 



The circuit which terminates the TWX toll subscriber line at the 

 switchboard office is operable with all existing types of TWX cord cir- 

 cuit repeater. All the features of TWX service, including unattended 

 service, are therefore available, 



POWER DRAINS 



A channel terminal dissipates about 25 watts. Tube heaters consume 

 about half an ampere at 24 volts and the remainder of the channel term- 

 inal, exclusive of its loop-terminating portion, consumes 50 ma at 130 

 volts. The loop terminating portion dissipates 20, 30 or 62.5 ma at 80 

 volts, depending upon the type of local circuit employed. 



LINE LEVELS 



The 43A1 system is capable of working with a great variety of line 

 levels. The send level may be adjusted for any value from -|-6 dbm 

 downward. The receiving equipment operates satisfactorily with —45 

 dbm or even —50 dbm. But the levels actually used are controlled by 

 crosstalk and noise conditions in the line. 



Receiving levels are normally limited by lightning interference on 

 open wire and by noise on cable circuits. The minimum tolerable levels 

 are about —40 dbm on open wire, —45 dbm on four-wire cable circuits 

 and —35 dbm on two- wire cable. 



In Fig. 11, a comparison is made of the effects of static on the 43 Al 

 system and on the 40C amplitude-modulation system. It gives the 

 results of simultaneous tests on the 2465-cycle bands of the two systems, 

 using the static from a record made at Madison, Florida. The 43A1 

 channel could tolerate about 4 db stronger static than the 40C. 



SYSTEM LAYOUTS 



A typical circuit layout of the 43 A 1 system working in the frequency 

 band between the voice and type-C carrier on an open-wire line is shown 

 in Fig. 12. The telegraph circuits extend from 43A1 channel terminals 

 located in a central office, at the left, to 130B1 sets in subscriber stations, 

 at the right. In the central office, the send and receive paths of the chan- 

 nel terminal are combined in a hybrid coil. With the moderate degree of 

 balance provided by the network of this hybrid coil, the allowable dif- 

 ference between send and receive levels of the middle channel may be 



