Digital echo canceller

A digital echo canceller with a receive path between a receive input and receive output, and a send path between a send input and a send output, said echo canceller being used for cancelling an additive echo signal at the send input which has occurred in response to a receive input signal applied to the receive input, said echo canceller comprising:

• a filter combination comprising
• a first digital filter with a programmable filter coefficient memory for generating in response to the receive input signal a first replica signal which is an estimate of the echo signal, and
• a second, adaptive digital filter with a filter coefficient memory for generating in response to the receive input signal a second replica signal which is an estimate of the echo signal, the second digital filter further including means for forming an error signal that is representative of the difference between a signal applied to the send input and the second replica signal, and an adaptation processor for adaptively modifying filter coefficients in response to the receive input signal and the error signal and applying the modified filter coefficients to the filter coefficient memory of this second digital filter;
• means for forming a send output signal as the difference between the signal applied to the send input and the first replica signal;
• controllable gate means for selectively applying the modified filter coefficients to the programmable filter coefficient memory of the first digital filter;
• and
• control means for blockwise respectively determining the levels of the error signal and the send output signal, and for generating in response to the levels thus found a control signal for the gate means which depends in a predetermined manner on differences between said levels;
• characterized in that:
• in the said filter combination the second digital filter is a frequency-domain block-adaptive filter having a block length of N' components and having, for each signal block m, a number of N' frequency-domain filter coefficients W(p;m) with p=0,1,2, . . . ,N'-1, and the first digital filter is a time-domain programmable digital filter having a number of N time-domain filter coefficients w(i;m) with i=0,1,2, . . . ,N-1, with N' exceeding N;
• and
• the echo canceller further includes transforming means, cascaded with the gate means, for forming the N time-domain filter coefficients w(i;m) required in the first digital filter as components of an N'-point Discrete Orthogonal Transform of a block of N' frequency-domain filter coefficients W(p;m).