Echo Suppressor Design in Telepone Communications

III a companion paper by J. W. Emling and D. Mitchell, 1 it was shown that the problems due to telephone echoes increase as the propagation time between speakers increases. As a result of the recent interest in satellite transmission, efforts have been made by several groups within and outside of Bell Telephone Laboratories to build echo suppressors particularly suited to the long delays encountered in such communications. In the first part of this paper, an elementary echo suppressor is described so that the reader may become familiar with some of the terminology used in this field and problems associated with the use of echo suppressors. Then, additions and modifications made to the simple echo suppressor to improve its performance are discussed. Finally, some of the more recent suppressor designs are illustrated. The echo suppressors to be described in the later sections of this paper are the same echo suppressors used in the subjective tests reported in a companion paper by Riesz and Klemmer. I I . AN E L E M E N T A R Y ECHO S U P P R E S S O R