188 BELL SYSTEM TECHNICAL JOURNAL 



expected from keyboard sending, the signals from which differ from the 

 signals previously discussed, in that the lengths of the stop signals 

 occur on a random basis, and are never shorter than .031 second, 

 depending on the typist. 



However, if signals containing bias or other distortion such as indi- 

 cated on Fig. 31 as distortion of future transition points were trans- 

 mitted over the circuit, thus decreasing the minimum interval between 

 transitions, the transient from one transition, for example, might 

 affect following transitions. A circuit having a shorter build-up time 

 could be made to introduce less distortion on transitions spaced at 

 very short intervals. The rate of build-up is a function of the area 

 under the transfer-admittance curve and from a design standpoint it 

 is necessary to provide a suitable frequency band-width to make the 

 slope of the received signals sufficient so that characteristic distortion 

 will not be excessive when closely spaced transitions are transmitted, 

 and also so that certain types of interference will not cause excessive 

 fortuitous effects. 



Where the available band width is limited the area under the admit- 

 tance curve could be increased by transmitting the permissible har- 

 monic frequencies at a greater amplitude. The transient of such a 

 circuit would continue at appreciable magnitudes for a greater length 

 of time and the minimum distortion would be increased but the general 

 circuit stability might be improved. 



The foregoing discussion of telegraph signal transmission has shown 

 that when a circuit is adjusted to transmit without distortion certain 

 selected repeated characters, the circuit can transmit on a repeated 

 basis any of the characters possible with signals of the nature repre- 

 sented by the selected characters. This is true not only for the signals 

 in which transitions are spaced at integral units of time but for signals 

 in which transitions are spaced at predetermined non-integral units of 

 time, such as start-stop teletypewriter signals. Incidental to the 

 development of the proof of the later statement, the prescribed ad- 

 mittance for transmission without distortion was evaluated. The 

 prescribed admittance for signals employing integral units between 

 transitions was also similarly determined and found to be the same as 

 that which had been determined from a somewhat different approach 

 in a previous paper. ^ 



The admittances considered have been idealized somewhat, inas- 

 much as no physical circuit will cut off completely at the higher fre- 

 quencies and, in addition, the effective transfer admittance is not only 

 a complex, and frequently nonlinear quantity, but is determined in 

 part by the characteristics of transmitting and receiving relays and 



