Time Division Multiplex Systems 



By W. R. BENNETT 



Introduction , 



THE idea of transmitting and receiving independent signals over a 

 common line by means of synchronized switches at the terminals is 

 quite old and has been used in multiplex telegraphy for many years. In 

 general if N' signal channels are to be provided over one line, the switching 

 cycle includes N' equal time intervals, one of which is allotted to each 

 channel. Each channel is connected to the line throughout a part of its 

 particular time interval and is disconnected throughout the remainder of 

 the cycle. Absence of interference between the channels depends upon 

 the fact that the channels are connected to the line throughout mutually 

 exclusive time intervals. It is thus possible to avoid the use of channel 

 band filters such as are necessary in carrier systems employing frequency 

 as the basis of separation. 



Application of time division multiplex methods to telephone channels 

 has been proposed from time to time and some experiments have been 

 made. •'■''' It is fairly evident that the concept of simple on-and-off 

 switching giving alternately transmission and complete suppression for the 

 signal from a particular channel on the line is inadequate for speech waves 

 in actual telephone circuits. Imperfections in the transmission properties 

 of the line tend to distort the wave form of the successive signal components 

 and prolong the contribution of one signal into the time allotted for a differ- 

 ent channel. It is the object of this paper to present a general quantitative 

 discussion of the factors which enter into the transmission of any type of 

 signal by a system of this kind. It has been found possible to arrive at 

 definite criteria for such matters as the required switching frequency, the 

 conditions to be imposed on contact time for good crosstalk suppression 

 with economy of frequency band, and the transmission requirements which 

 must be met by the intervening circuit to hold the interference between 

 channels to tolerable values. The analysis leads directly to a physical 

 viewpoint of the whole process which, to those familiar with the carrier and 



• Patten and Minor, U. S. Patent 745,734, 1903. 

 ^Electrical World, Dec. 5, 1903. 



2 Goldschmidt, U. S. Patent 1,087,113, Feb. 17, 1914. 



* Poirson, Soc. Fr. El, .\pr. 1920. 



5 Marro, L'Onde Electrique, Oct. 1938. 



^ M. Cornilleau, Revue de Telephones, Telegraphes et T. S. P., 13 (1935), pp. 625-643. 



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