Adaptive echo cancellation method and device for implementing said method

In a telephone system for providing a communication path between a first and a second telephone and having a pair of unidirectional conductors for carrying digital telephone signals in opposite directions between the first and second telephones, first and second bidirectional conductors and first and second hybrid transformers for respectively connecting the said first and second telephones to opposite ends of the said pair of unidirectional conductors to provide a complete signal path between the said first and second telephones, at least one transversal filter having an input connected to one of the said unidirectional conductors and having a plurality of coefficients taps for generating a replica of the echo signal present in the said other unidirectional conductor, and means responsive to the filter output and the signal present on the said other unidirectional conductor for subtracting the replica generated in the filter from the signal present on the said other unidirectional conductor to cancel the echo present in the said other unidirectional conductor, a method for setting the coefficients of the transversal filter comprising the steps:

• a) setting a coefficient threshold (Thrshld) to a predefined value;
• b) setting initial relative sampling time limits for defining initial coefficient boundaries;
• c) setting initial filter coefficients to predefined values;
• d) subtracting the echo replica provided by the transversal filter from the signal present on the said other unidirectional conductor and deriving therefrom an updated error signal e(n);
• e) deriving a new set of coefficients using the updated error signal e(n) and replacing the filter coefficients with the new set of coefficients;
• f) repeating steps d) and e) for a predetermined number of periods T of the digital telephone signal present on the said one unidirectional conductor;
• g) after the said predetermined number of periods T redefining the coefficient boundaries based on the current coefficient threshold value and dropping those filter coefficients outside of said threshold to define an updated coefficient set;
• and
• h) computing a new coefficient threshold value for subsequent use in step g) based on said updated coefficient set and repeating steps d) through h).