TRANSMISSION SYSTEM FOR TELlVrYPl'AVRITI-.R 533 



2. To transmit accurately these sudden changes in the line condition would re- 

 quire a transmission channel cai)al)le of passing an infinitely wide frefjuency hand. 



3. With a transmission channel which will pass only a limited band of fretjuencies 

 there will be alteration of the wave shape during transmission as the result of changes 

 in magnitude and phase of the various components caused by the characteristics of 

 the transmission channel, so that changes in line condition at the receiving end will 

 be gradual and in general displaced from their proper position. 



4. Theoretically all of the intelligence can be carried by transmitting waves of a 

 maximum frequency equal to that of the fundamental of the signaling speed consider- 

 ing the time interval of each signal element as a half cycle. 



5. Actually it is not economical either to transmit a very wide band of frequency 

 or to provide terminal apparatus capable of accurately recording the intelligence when 

 only a band equal in width to the frequency of the fundamental of the signaling speed 

 is transmitted. The arrangement used in practice must, therefore, be a compromise 

 between these two extremes. 



Experience has shown that in order to use economically practical 

 types of receiving apparatus it is generally necessary to have present 

 in the received signals a substantial portion of the second and third 

 harmonics of the frequency of the shortest signal element, which 

 requires in the case of 60-speed teletypewriter signals the transmission 

 of a frequency band width of somewhat more than 45 cycles. To 

 illustrate this a typical 60-speed teletypewriter signal is shown graphi- 

 cally in the upper left-hand diagram of Fig. 2. This diagram repre- 

 sents potential applied to the line for a perfect teletypewriter letter 

 "D." At the instant when the start pulse commences, as described 

 previously, the voltage applied to the line assumes its "open" value S, 

 called "spacing." This spacing condition continues for 0.022 second 

 at the end of which time the voltage suddenly assumes its "closed" 

 value M, called "marking." The marking voltage remains constant 

 through the first signaling pulse (1) in the figure. The second and 

 third elements of the teletypewriter " D " signal are spacing and during 

 these intervals the current is again of its spacing value. In the fourth 

 pulse it once more becomes marking for 0.022 second, and in the fifth 

 pulse it is again spacing. After the fifth pulse the current assumes its 

 marking value for the duration of the stop signal. 



This teletypewriter "D" signal may be further analyzed by con- 

 sidering it to be made up of sine wave components of various fre- 

 quencies and magnitudes with certain definite phase relationships. 

 It will be found theoretically to contain a number of sine waves of 

 frequencies from zero to infinity. The left-hand column of Fig. 2 

 shows a number of the more important harmonic components of the 

 "D" signal, the relative magnitudes and phase relationships being 

 as indicated. The first is the d-c. component; the second is a sine 

 wave of the same period as the over-all signal, and is referred to as the 

 first harmonic. The wave shown in part c of the figure is twice the 

 frequency of the over-all signal and is referred to as the second har- 

 monic. Following this in turn are shown the third to tenth harmonics. 



