A decision feedback equalizer comprising:
- a subtractor having a first input for receiving an input digital signal and a second input for receiving a cancelling signal for cancelling an intersymbol interference component of said input digital signal;
- a decision circuit for making a binary decision on the output of said subtractor;
- an error detector connected between input and output of said decision circuit for detecting a decision error;
- a nonrecursive filter including a tapped delay line having an input end connected to the output of said decision circuit for producing a succession of nonrecursive tap signals, tap-gain multipliers for operating on said nonrecursive tap signals respectively, means for summing the outputs of said tap-gain multipliers and producing an output signal of the nonrecursive filter, and correlator means for detecting correlations between said nonrecursive tap signals and said decision error for controlling the tap gains of said tap-gain multipliers;
- a first recursive filter including a series of first and second delay-line taps, first and second tap-gain multipliers for respectively operating on signals at the first and second delay-line taps of the first recursive filter, means for summing the outputs of the first and second tap-gain multipliers and producing an output signal of the first recursive filter, third and fourth tap-gain multipliers for respectively operating on the outputs of the first and second delay-line taps of the first recursive filter, means for providing a sum of the outputs of the third and fourth tap-gain multipliers and the output of the tapped delay line of said nonrecursive filter and applying said sum to the first delay-line tap of the first recursive filter, and correlator means for detecting correlations between the inputs of said first and second delay-line taps of the first recursive filter and the decision error of said error detector for controlling the tap gains of said first and second tap-gain multipliers of the first recursive filter;
- a second recursive filter including a series of first and second delay-line taps for receiving said output signal of said first recursive filter, weighing means for weighing the output of the second delay-line tap of the second recursive filter with an attenuation coefficient of constant value, and means for detecting a difference between the output of the weighing means and the output of the first delay-line tap of the second recursive filter to produce a difference signal, a series of third and fourth delay-line taps, third and fourth tap-gain multipliers for respectively operating on the outputs of said third and fourth delay-line taps of the second recursive filter, summing means for supplying a sum of the outputs of the third and fourth tap-gain multipliers of the second recursive filter and said difference signal to an input of the third delay-line tap of the second recursive filter to produce an output signal of the second recursive filter;
- tap-gain control means for summing said attenuation coefficient of constant value with an attenuation coefficient of variable quantity to produce a positive feedback coefficient and multiplying said attenuation coefficient of constant value with said attenuation coefficient of variable quantity to produce a negative feedback coefficient, updating said attenuation coefficient of variable quantity with a correlation between said decision error and said output signal of said second recursive filter, and controlling the tap gains of said third tap-gain multipliers of said first and second recursive filters with said positive feedback coefficient and controlling the tap gains of said fourth tap-gain multipliers of said first and second recursive filters with said negative feedback coefficient;
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- an adder for supplying a sum of the output signals of said nonrecursive filter and the first recursive filter to the second input of said subtractor as said cancelling signal.