Echo cancelling system for multiplex telephone circuits

An echo cancelling system, for use with multiplex telephone channels comprising:

• first pseudo echo generating means for generating pseudo echo signals according to a first adaptive echo-path model having a short convergence time;
• a plurality of second pseudo echo generating means equal in number to a number of multiplexed telephone channels for generating pseudo echo signals each according to a respective one of a plurality of second adaptive echo-path models equal in number to the number of multiplexed telephone channels and each of which is shorter than the first adaptive echo-path model and has a longer convergence time than the first adaptive echo-path model;
• subtractor means for subtracting the respective pseudo echo signals developed by said second pseudo echo generating means from respective echo signals applied to the respective multiplexed telephone channels to develop residual echo signals applied to the respective multiplexed telephone channels and equal to the differeneces between the respective echo signals and pseudo echo signals;
• detection means for detecting when a residual echo signal exceeds a certain threshold value and for generating a control signal when the residual echo signal exceeds the threshold value;
• and
• control means responsive to the control signal for switching said first pseudo echo generating means in substitution for the corresponding one of said second pseudo echo generating means corresponding to the detected residual echo signal for generating pseudo echo signals according to the first adaptive echo-path model until the residual echo signal is less than the threshold value and the thereafter the transferring the first adaptive echo-path model to said corresponding second pseudo echo generating means and switching said corresponding second pseudo echo generating means in substitution for said first pseudo echo generating means to maintain the residual echo signal below the threshold value.