Time Division Multiplex Systems

T HE 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. 1,2,3,4,5,6 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 different 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.