Listener echo cancellation

DETAILED DESCRIPTION The listener echo cancellation means of the present invention employs a finite impulse response (FIR) filter with adjustable coefficients to approximate the listener echo in a telephone line.
The cancellation means then subtracts the approximated echo from the line signal.
By implementing a phase correction loop, the circuit compensates for phase changes in the listener echo brought on during additional round trips through the telephone line.
Cancelling the echo permits the slicer to make better decisions, about which constellation point is actually being represented.
The first block of the listener echo cancellation means is an adder used to subtract the approximated echo signal from the line signal.
The output of the adder is received by a slicer (quantizer) which makes an estimate of what the correct signal should be.
This "correct" signal is then inputted into a bulk delay equivalent to a telephone line round trip delay.
This delayed signal is then passed through an adaptive FIR filter.
This filter then outputs the approximated echo to the first adder block where the approximated echo is used to cancel the echo in the line signal.
In addition, the "correct" signal from the slicer is subtracted from the output of the adder, thereby producing an error signal.
This, error signal is then multiplied by the conjugate of the approximated echo signal; the imaginary part of this product is then passed through a low pass loop filter to obtain the phase difference.
This phase difference value is then used for two purposes.
The phase difference is provided to a cosine/sine table and used to phase shift the approximated echo, and also, the negative phase difference is provided to a cosine/sine table and used to phase shift the error signal.
The phase shifted error signal is used in a fuction that adjusts the coefficients of the adaptive FIR filter.