Echo canceller

In a communications system in which digital information such as PCM data signals having a given symbol rate are conveyed over a two wire circuit, with said two wire circuit coupled to an input of a line hybrid, with said hybrid providing at one output a transmission channel port for transmitting data to said two wire line and at another output a receiving channel port for receiving data representing data symbols from said two wire line, said system undesirably including due to imperfection in said hybrid unwanted echo signals due to undesired coupling between said transmitting and receiving channels, the combination therewith of apparatus for cancelling said unwanted echo signals, comprising:

• analog to digital converter means having an input coupled to said receiving channel port for providing at an output a digital signal for each received data symbol and having a control input to provide said digital signal indicative of said received data symbol at a specific timing interval,
• a digital filter having a transform selected to provide a filtered response at an output according to (1-Z.sup.-1) and having an input coupled to the output of said analog to digital converter,
• a subtractor circuit having a first input coupled to the output of said digital filter,
• a sample and hold circuit means having an input coupled to said transmission port of said hybrid and having an output,
• a digital adaptive echo simulator having an input coupled to the output of said sample and hold circuit with an output coupled to a second input of said subtractor circuit, said echo simulator incuding a transversel filter having variable coefficient capability and which filter includes plurality of coefficient generators for generating filter coefficients at given delays, with said delay of each generator controlled by an associated multiplexer for each coefficient generator,
• an adaptive descision feedback equalizer means having an input coupled to the output of said subtractor circuit for providing at an output a timing signal indicative of the timing of the received data symbols as applied to said subtractor and as modified by said adaptive echo simulator said equalizer including means for continuously estimating the impulse response at said receiving channel port based on said timing signal,
• a timing extraction controller having an input coupled to said feedback equalizer and responsive to said timing signal for providing at an output a control signal which output is coupled to said control input of said analog to digital converter, whereby said analog to digital converter is forced to provide said digital output signal according to said control signal from said equalizer to thereby cause said output of said subtractor to provide a version of said received signal from which said unwanted signals have been substantially removed.