Echo phenomena in telephone systems employing speech compression

Echoes on subscriber voice circuits have only been a problem when international satellite links are involved. This is due to a reflection at the far end combined with a round trip delay of approximately 500 ms. To provide an acceptable service, the local PTT will provide echo cancelling facilities on these international routes. These facilities are not required for domestic routes since the worst case round trip propagation delay is approximately 15 ms. However, with the introduction of speech compression technology, long processing delays are encountered. These long delays can produce unacceptable echoes and thus echo cancellation must be an integral part of such a system. The development, at Alcatel Altech Telecoms of Trunkex, a point to point telephone system that utilises speech compression, brought to the forefront the problem of echoes and showed that the appropriate specifications and standards were inadequate. Investigation of the problem produced a solution that was implemented almost entirely in software.