TRANSMISSION SYSTEM FOR TELETYPEWRITER 537 



addition to or subtraction from the front of each marking impulse, 

 with the rear of the impulses remaining unchanged. 



In an ideal machine where selection would be made instantaneously 

 the signal would be recorded correctly if it had the right condition 

 (i.e., marking if it should be marking or vice versa) at the instant of 

 selection. The particular times when the selections take place with 

 the orientation setting at the middle of the range with perfect signals 

 are shown, as mentioned before, by the vertical solid lines numbered 

 1 to 5, inclusive, in Fig. 3. Referring to cases b and d it may be seen 

 that with 25 per cent bias the correct signal is being received at the 

 point of selection and it will be interpreted correctly. However, 

 referring to cases c and e it may be seen that more than 50 per cent 

 bias will cause errors. In case c the second and fifth impulses will be 

 falsely interpreted as marking and in case e the first and third impulses 

 will be spacing instead of marking. Several examples of the effect of 

 distortion other than bias in the received signals are illustrated in 

 cases,/, g, and h of Fig. 3. 



The effect of variations in teletypewriter motor speeds on oper- 

 ating margins is illustrated in Fig. 3 by cases i and j. Case i 

 shows the result if the sending machine is faster than the receiving 

 machine. It will be noted that as the speed discrepancy becomes 

 greater the first error will be a false mark for the fifth impulse because 

 a part of the stop impulse is received on the fifth position. If perfect 

 signals are assumed, the speeds would have to be somewhat more than 

 7 per cent different to cause errors of this kind in a normal teletype- 

 writer with the range finder set in the middle of the range, but if 

 there is some signal distortion other than that from speed discrepancies, 

 such as marking bias, smaller differences in speed would be sufficient 

 to cause errors. Case j illustrates the conditions when the sending 

 distributor is slower than the receiving distributor. It will be ob- 

 served in this case that the first error as the speed discrepancy in- 

 creased would also be in the fifth impulse as the result of either the 

 fourth impulse being sufficiently prolonged to fall on the fifth selecting 

 position, or the fifth impulse being so late in starting that it is not 

 properly received on the fifth position. 



In the illustrations large speed discrepancies have been used so that 

 the shift of the signals could be shown readily on a drawing. 



General Transmission Design of TWX Network 



Telegraph circuits comprising the transmission network employed in 

 teletypewriter exchange service are laid out according to a fundamental 

 plan similar to the toll switching plan ^ used in designing the toll 



