Adjustable echo canceller
First Claim
1. An echo canceller including an echo canceller circuit adapted to be coupled between a receive path and a transmit path in a communications system, said echo canceller comprisinga digital filter responsive to a digital input signal for producing a replica signal comprising a plurality of digitized replica samples,said digital filter having an associated plurality of filter coefficients,said digital input signal comprising a plurality of digitized input samples applied to said receive path,each digitized sample of said replica signal being equal to the sum of a plurality of terms, each term being equal to the product of a respective one of said filter coefficients and of a respective factor which is a function of said digital input signal,a subtracter circuit, responsive to said replica signal and to the output signal which appears on said transmit path in response to said input signal being applied to said receive path, for subtracting said replica signal from said output signal,first processing means for first determining each of said respective factors from replica signal samples which are output by the digital filter when all the filter coefficients, except the filter coefficient by which said each factor is multiplied, are equal to zero, andsecond processing means responsive to the factors determined by said first processing means for thereafter determining said filter coefficients from a plurality of output signal samples.
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Accused Products
Abstract
An echo canceller determines the optimum value of the required digital filter coefficient from values obtained by measuring a plurality of samples of the echo signal and by measuring, for each of a predetermined number of such samples, the replica signal sample factors which, when multiplied by the filter coefficients and summed, cancel the echo signal sample. The echo signal samples as well as the factors used to determine the replica signal samples are both measured in a way that reflects the actual environment in which the echo canceller is used. To measure each of the factors of the replica signal sample corresponding to an echo signal sample, an input signal is applied to the receive path, the filter coefficient corresponding to the factor is set to a non-zero value and the other filter coefficients to zero, and the resultant replica signal sample is then measured. It is not necessary for the echo signal and replica signal measurement to be made concurrently, or from the same location.
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Citations
10 Claims
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1. An echo canceller including an echo canceller circuit adapted to be coupled between a receive path and a transmit path in a communications system, said echo canceller comprising
a digital filter responsive to a digital input signal for producing a replica signal comprising a plurality of digitized replica samples, said digital filter having an associated plurality of filter coefficients, said digital input signal comprising a plurality of digitized input samples applied to said receive path, each digitized sample of said replica signal being equal to the sum of a plurality of terms, each term being equal to the product of a respective one of said filter coefficients and of a respective factor which is a function of said digital input signal, a subtracter circuit, responsive to said replica signal and to the output signal which appears on said transmit path in response to said input signal being applied to said receive path, for subtracting said replica signal from said output signal, first processing means for first determining each of said respective factors from replica signal samples which are output by the digital filter when all the filter coefficients, except the filter coefficient by which said each factor is multiplied, are equal to zero, and second processing means responsive to the factors determined by said first processing means for thereafter determining said filter coefficients from a plurality of output signal samples.
Specification