FUNDAMENTALS OF TELETYPEWRITERS 627 



by 25 per cent of one pulse length. It will be noted that under these 

 conditions each pulse will still be properly received and identified. 



Trace (c) illustrates 50 per cent marking bias in the received signal, 

 and at this point it will be noted that the No, 3 and stop pulses have 

 been so elongated that they are just on the verge of being erroneously 

 received and identified also as No. 2 and No. 5 pulses. This then is a 

 theoretical limit for proper operation with marking bias without a 

 readjustment of the receiving distributor. 



Similarly traces (d) and (e) illustrate respectively the conditions 

 when the received signals have 25 per cent and 50 per cent "spacing 

 bias," that is each marking impulse has been shortened by this per- 

 centage of one pulse length. It will be noted that 25 per cent spacing 

 bias can be easily tolerated but that with 50 per cent spacing bias the 

 No. 1 and No. 3 pulses are on the verge of failure to be recorded. This 

 then is a theoretical limit for spacing bias in the signals under this 

 adjustment. 



Traces (f) and (g) show the effect of 25 per cent marking and spacing 

 distortions respectively on the rear end of the stop or front end of the 

 start impulse, all other pulses remaining undistorted. 



Trace (h) shows the effect of distortions on the selecting impulses 

 alone. By combining this trace with traces (f) and (g) it will be seen 

 that with 25 per cent distortion of the start pulse, 25 per cent distortion 

 of the same sign is the limit for distortion on the front end of marking 

 impulses, or 25 per cent distortion of opposite sign on the rear end of 

 marking impulses. Thus for distortions other than bias, which are 

 apt to affect both start and selecting impulses in the same signal, 

 ± 25 per cent is the theoretical limit of allowable distortion. 



Traces (i) and (j) show the effects of speed inaccuracies. From 

 these it will be seen that theoretically the sending distributor could be 

 about 8.9 per cent faster or 9.5 per cent slower than the receiving 

 distributor before errors would be experienced. 



In practical machines there are, of course, inaccuracies due to 

 tolerances of manufacture, and other departures from the ideal so that 

 the above mentioned theoretical limits are not reached. However, 

 all machines used in the Bell System are required to tolerate a lengthen- 

 ing or shortening of the front end of any current impulse of at least 

 40 per cent of its length and with the same adjustment a lengthening 

 or shortening of the rear portion of any current impulse of at least 35 

 per cent with the start pulse undistorted. Since bias is nearly always 

 present to some degree in the received signals, and since as interpreted 

 by the receiving distributor it affects only the front end of the current 

 impulses as illustrated in traces (b), (c), (d) and (e) of Fig. 5, the 



