544 BELL SYSTEM TECHNICAL JOURNAL 



contacts in accordance with the code for the characters being trans- 

 mitted. When the contacts are closed a current flows in the subscriber 

 line circuit for marking and when they are open this current becomes 

 zero, transmitting a spacing signal. 



On long cable pair subscriber line circuits with considerable bridged 

 capacity, the wave shape of the current received in the central office is 

 not symmetrical as regards the marking or spacing conditions, the 

 rate of building up of the marking current being much faster than its 

 rate of decay. This results in marking bias in the received signals. 

 Conversely, in subscriber line circuits containing only series inductance 

 and resistance, the received current builds up gradually to its marking 

 value and decays to zero immediately when the sending contacts are 

 opened for a space. By properly combining the inductance and 

 capacitance, it is possible to produce substantially unbiased signals 

 at the receiving end. In other words, by inserting series inductance 

 in a cable circuit, it is possible to overcome the marking bias efifect 

 mentioned above so that practically no distortion occurs in the sub- 

 scriber line. 



The marking bias may also be reduced effectively by the use of 

 series resistance in place of inductance at the subscriber station in 

 cases where it is possible to add a sufficient amount of resistance without 

 reducing the current below the desired value. The effect of series 

 resistance used in this way is to delay the building up of the current 

 when the teletypewriter sending contacts are closed after a spacing 

 signal to compensate for the delay in decay of the received current 

 after the contacts have opened. 



Both of the above methods of reducing bias are in use in the present 

 teletypewriter exchange plant. Figure 7 shows the wave shape of 

 uniformly timed marks and spaces received over a 30-mile 19-gauge 

 cable pair, illustrating the effect of the cable capacitance, and the 

 manner in which a wave shaping arrangement, consisting primarily of 

 inductance in this case, reduces the amount of marking bias in the 

 received signal by retarding the building up of current at the start of 

 each marking signal. 



Although the majority of subscriber lines are in cables, it is some- 

 times necessary to serve stations at greater distances from the teletype- 

 writer center or in situations where the use of cable pairs is not prac- 

 ticable. For these, other arrangements must be made. One method 

 of serving such stations is by means of arrangements similar to those 

 of the shorter toll circuits. Generally a telegraph repeater in an office 

 in the vicinity of the subscriber station is used, and transmission 

 between that repeater and the one in the teletypewriter center takes 

 place in the same manner as over a toll circuit of similar length. 



