Improvements in or relating to Echo Cancelling Apparatus for Communication Systems

1,185,000. Echo suppressors. WESTERN ELECTRIC CO. Inc. 4 Dec., 1968 [8 Dec., 1967], No. 57510/68. Heading H4R. In an echo cancelling apparatus in which an adaptive transversal filter 17 synthesises an echo signal from the signal on the incoming pair 13 of the four wire line and subtracts the synthesized echo signal from the signals in the outgoing pair 12, the multipliers of the transversal filter being continuously adjusted in accordance with the result of a cross correlation of the outgoing signal with the signal present at the taps of the delay line of the transversal filter fed with the incoming signal, the residual echo signal, which is still present in the outgoing signal after the synthesized echo signal has been subtracted, is eliminated from the outgoing line signal by a filter 31, which has a high attenuation at each end of the pass band of the system. The residual echo is due to the lack of identity of the synthesized echo signal with the actual echo signal, due to the finite length of the delay line in the transversal filter. The filter 31 may be switched out by closing contacts 32 when a speech detector 33 detects no incoming signal on line 13.