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The restriction imposed by linear distortion on the flow of information in a communication channel is well known. In the past, the effects of this distortion have been alleviated through the use of manually adjusted equalizing or compensating networks. The adjustment of these networks is too cumbersome a process for the user of a switched communication service to perform each time a new connection is established. Therefore, in present switched networks, control of linear distortion is imposed only on the individual links. Variation between links and variation of the number of links in tandem result in channels with distributed performance. Lower distortion can be achieved by equalizing the overall connection. Recent developments have made automatic linear distortion removal (equalization) practical for synchronous data communication systems. Here an implementation is described wherein these techniques have been generalized so that automatic equalization can be provided for a communication channel independent of the signal format used in that channel. For a number of applications the speed of automatic equalization makes efficient end-to-end equalization practical in a switched network. The implementation described affords automatic minimization of the discrepancy between a specified response and the actual response of a linear transmission medium. Thus, on the one hand, it permits the automatic reduction of transmission defects such as signal dispersion and echoes, and, on the other hand, it permits the mechanized synthesis of filters with specified transfer functions. This paper reviews the general aspects of automatic equalization, describes an implementation of a general purpose automatic equalizer, discusses the theoretical performance of such an equalizer as determined from computer simulations, and lastly presents results for the equalization of real channels using the implementation described.
Lucky et al. (Wed,) studied this question.