536 



BELL SYSTEM TECHNICAL JOURNAL 



An orientation device or range fuider is provided which rotates the 

 stop latcli with respect to the locking lever and thereby changes 

 the time at which selection occurs with respect to the beginning of the 

 selecting cycle. Moving the orientation range finder in effect moves 

 the solid vertical lines in Fig. 3, with respect to the signal, and with 

 perfect signals they can be moved by an amount corresponding to one 

 unit impulse. In other words the time of selection can be moved by 

 ± 50 per cent without typing errors, as shown at a in the figure. 

 (In an actual machine this range is less because of practical con- 



TO SUBSCRIBER /^^ 

 LINE 

 SELECTING 

 MAGNET 

 STOP LATCH 



'STAR T I I 



(b) "yM\'M 



(C) 



(d) 

 (e) 

 (f) 

 (9) 

 (h) 



+g5% 



;t^°/^ 





-55^, 



(1) V,"V/-'/^ 



(j ) y/ ' V. " 'J 



2 3 4 



,i:A5% 



+^ 



-2^% 



-553fe, 



fMARKING BIAS 



>2S°/o 



CHARACTER 

 SELECTED 



F 



F 



LETTERS 



F 



=55°/o 



^SPACING BIAS 



J CARRIAGE RETURN 



FORTUITOUS 

 ' DISTORTION 



'V^^^M-VM. JSEND 7.5 "/o FAST 

 lV/^mV/<\ jsEND 7.5% SLOW 



Fig. 3 — Principles of selecting'mechanism of a teletypewriter. 



siderations of design, the time of selection being variable without 

 errors over a range of about ± 40 per cent.) 



Distortion in teletypewriter signals may be "bias," which is a 

 uniform lengthening or shortening of all the marking impulses, or it 

 may be of other types which affect only certain of the signal elements."* 

 Bias is divided about equally between the ends of the impulse when 

 the signal is received from the line. However, because the selecting 

 mechanism starts rotating at the beginning of the start impulse, the 

 effect of bias is to shift all impulses forward or backward with respect 

 to this time. The result of this is that effectively there will be an 



