ECHO CANCELLER WITH VARIABLE THRESHOLD

• US 3,789,165 A
• Filed: 04/24/1972
• Issued: 01/29/1974
• Est. Priority Date: 04/24/1972
• Status: Expired due to Term

• Current Assignee: Henri George Suyderhoud, Michael Onufry Jr, Samuel Joseph Campanella
• Original Assignee: Henri George Suyderhoud, Michael Onufry Jr, Samuel Joseph Campanella
• Inventors: Onufry, Michael Jr., Campanella, Samuel Joseph, Suyderhoud, Henri George
• Primary Examiner(s): Claffy, Kathleen H.
• Assistant Examiner(s): Faber, Alan
• Application Number:
  US05/246,785
• Time in Patent Office: 645 Days
• Field of Search: 179/170.2
• US Class Current: 379/406.11
• CPC Class Codes: H04B 3/23 using a replica of transmitted signal in the time domain, e.g. echo cancellers

Abstract

An echo canceller having a digital transversal filter with adjustable gain coefficients and an adaptive control loop for achieving minimum echo and similar speed of convergence for loud and soft talkers. Minimum echo is attained by subtracting a synthesized echo from the real echo, the synthesized echo being formed in a digital transversal filter by multiplying a stored replica of the impulse response times the incoming signal. The stored replica is updated using the steepest descent technique by adjusting each of the stages of the replica memory a given amount. Adjustment is made when the echo error and the sampled incoming signal are above respective threshold levels. The threshold level for the sampled signal is made dependent upon the average of the samples over a period of time to provide substantially the same speed of convergence for loud and soft talkers.

Claims

1. In an echo canceller of the type having a transversal filter means for performing convolution of an input signal on a receive line and a replica of the impulse response of an echo path to generate an approximation of an echo signal for subtraction from a real echo signal on a send line, and an adaptive control loop responsive to the residual echo resulting from said substraction and to stored samples of said input signal for incrementally varying said replica to reduce said residual echo, said transversal filter comprising means to store a plurality of samples of said input signal and for replacing the oldest sample with each new sample, said adaptive control loop comprising means for varying elements of said replica only in response to said residual echo and said individual stored samples being greater than threshold levels, the improvement comprising, variable threshold means in said adaptive control loop responsive to said stored samples for comparing each stored sample to a threshold level proportional to the average value of the amplitudes of said stored samples.

2. An echo canceller as claimed in claim 1 wherein said variable threshold means comprises, arithmetic means responsive to the amplitude of said stored samples for generating a value proportional to the average of the amplitude of said stored samples during each sample interval, and comparison means for comparing each said sample to said generated value during each sample period and for providing an output indicative of the sign of each said sample which exceeds said generated value.

3. An echo canceller for reducing echoes on the send side of a four wire system caused by signals received on the receive side of said four wire system, said echo canceller comprising, a. means for periodically sampling the signals on said receive side, b. sample storage means for storing the latest N of said samples, c. replica storage means adapted to store a replica of the impulse response of an echo path, d. means for multiplying and summing, during each sample period, the contents of said sample storage means and said replica storage means to compute a replica of an echo, e. subtraction means for subtracting said echo replica from an echo on said send line to form a residual echo, f. first threshold means responsive to said residual echo for providing an indication of the sign of said residual echo if said residual echo exceeds a first threshold level, g. second threshold means responsive to the samples in said sample storage means for providing an indication of the sign of each of said samples which exceeds a value proportional to the average amplitude of said stored samples, and h. replica storage correction means responsive to said first and second threshold means for altering said stored replica.

4. An echo canceller as claimed in claim 3, wherein said second threshold means comprises arithmetic means responsive to the amplitude of said stored samples for generating a value propOrtional to the average of the amplitudes of said stored samples during each sample interval, and comparison means for comparing each said sample to said generated value during each sample period and for providing an output indicative of the sign of each said sample which exceeds said generated value.