170 BELL SYSTEM TECHNICAL JOURNAL 



pulses. In 3 this condition is reversed so that the net operating current 

 for the Bias relay, being the difference between the currents of 2 and 3, 

 has a rounded wave shape for both the beginnings of marks and the 

 beginnings of spaces as is indicated at 4. This is a symmetrical wave 

 about the zero axis and if there is no bias effect in the Bias relay the 

 signals will be repeated unbiased. However, if current is passed 

 through one of the auxiliary windings of the Bias relay the operating 

 point of this relay will be shifted as indicated by the dotted line and 

 the repeated signals will be biased, as shown by 5. In this case the 

 bias is 25 per cent spacing so that the repeated unit marks are 75 

 per cent of their original length. 



One of the auxiliary windings of the Bias relay is used for introducing 

 marking bias and the other for spacing bias, and the sign of the bias 

 may be changed by switching from one winding to the other. Then 

 the bias effect reverses according to the dotted line of 6, and the 

 signal is affected as is indicated by 7. By reversing the bias periodi- 

 cally under the control of a commutator arrangement an effect known 

 as "switched bias" is produced. The reversing operation occurs 60 

 times per minute and is not synchronized with the signals and accord- 

 ingly produces a fortuitous effect on some of the signals. 



The signals obtained from this device are commonly used in testing 

 the operating margins of subscriber teletypewriters. In this case 

 perfect teletypewriter signals are applied to the input of the device 

 and are distorted by a predetermined amount in passing through it. 

 These signals are then applied to the circuit extending to the sub- 

 scriber station. If the receiving teletypewriter at the station responds 

 faithfully when set at its optimum orientation point it is considered 

 to be satisfactory for service. 



'Distorted signals obtained from this device are also used to de- 

 termine the extent of the distorting effects in line circuits. For this 

 purpose the set may be connected at either the sending or the receiving 

 end of the line and a distortion-measuring set at the receiving end to 

 give an indication of the increase in distortion caused by predistorting 

 the signals. With the set used at the sending end, the results of the 

 test indicate how much distortion may be applied as from preceding 

 telegraph sections. With distortion added at the receiving end, the 

 test may be used to show the margin in the receiving apparatus 

 before failure. 



A front view of a panel-mounted telegraph signal-biasing set of the 

 relay type is shown in Fig. 20. 



