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Abstract

A decision-feedback type equalizer comprising a forward equalizer for receiving digital pre-equalization signals and error signals as input signals, for reducing intersymbol interference caused by the lead echo of the pre-equalization signals, and for outputting forward equalization signals; a backward equalizer for reducing intersymbol interference caused by the delay echo of input signals, and for producing output in the form of backward equalization signals; an adder for adding forward equalization signals and backward equalization signals, and for outputting equalization signals; a decision device for comparing the equalization signals with reference values, and for outputting decision signals; a subtracter for subtracting the equalization signals and decision signals, and for producing and outputting error signals which are differences; an alarm emitter for detecting, from the signal states of the error signals, whether an abnormal state exists, and for emitting an alarm when an abnormal state is detected; and a selection circuit which, depending on whether an alarm emitter output alarm signal is present or absent, is used for selecting either equalization signals or decision signals, and which, when there is alarm signal input, is used for selecting equalization signals and for supplying them as input signals to the backward equalizer, such that detection of the alarm condition reduces propagation or magnification of errors caused by noise.

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