Echo canceller for bidirectional digital transmission

Claim 1

An echo canceller for a bidirectional transmission system capable of effecting a cancellation of echoes of N digital symbols transmitted on a transmission line in the last N signalling periods of a transmission, said echo canceller comprising: a shift register consisting of N cells having an input to a first cell thereof receiving a signal to be transmitted and linearly related to a coded signal transmitted on said line; a logic circuit having a first input connected to said input to said first cell and a second input connected to an output from a last cell of said shift register and having first and second outputs related to said first and second inputs by the following truth table: ______________________________________ First Input Second Input First Output Second Output ______________________________________ 0 0 1 -- 0 1 0 0 1 1 1 -- 1 0 0 1 ______________________________________ an up/down counter connected to said first and second outputs and saturable at a maximum count value of N, said counter being enabled by a signal on said first output and being controlled for up/down counting by a signal on said second output; a first sign extracting circuit connected to an output of said counter for outputting the sign of a counting performed by said counter; a second sign extracting circuit receiving a line signal from which an estimated echo signal has been subtracted; a first multiplying circuit for multiplying signs supplied at outputs of said sign extracting circuits; an adder having one input connected to said first multiplying circuit, an other input, and an output; a delay circuit connected to said output of said adder and applying to said other input of said adder the output signal thereof delayed by a signalling period, said adder adding the signals received at its inputs; a second multiplying circuit connected to said output of said adder and to said counter for multiplying an added signal supplied by said adder with an output signal from said counter to produce an output representing an echo signal to be subtracted from a signal to be received on said transmission line; and a further adder connected to said second multiplying circuit for subtracting said output representing an echo signal from said signal to be received on said transmission line.