A Functional Analysis Relating Delay Variation and Intersymbol Interference in Data Transmission

In this paper we will analyze and discuss one aspect of the problems associated with the transmission of digital data through an unknown linear network. The particular aspect with which we will be concerned is the effect of delay distortion on the fidelity of the transmission. Delay distortion arises generally from nonlinearity in the phase shift with frequency of the system transmission characteristic. This nonlinearity causes different frequencies of the input waveform to arrive at the receiver at different times, thereby distorting the input waveform. The problem of delay distortion is particularly acute in the voice telephone network. Since speech is relatively insensitive to phase, the switched telephone network has not been equalized for phase shift as well as it has been for attenuation. However, the need has now arisen to make use of this network for transmission of digital data at high speeds. The digital data receiver takes the waveforms it receives quite 2427 2428 T H E BELL SYSTEM T E C H N I C A L J O U R N A L , S E P T E M B E R 1903