A "high" pass filter is employed to shape the low frequency input characteristic to a bidirectional repeater including echo cancelers. This frequency shaping eliminates excessive low frequency signal components from being supplied to the echo cancelers. Consequently, the number of taps, i.e., amplitude...http://www.google.de/patents/US4628157?utm_source=gb-gplus-sharePatent US4628157 - Bidirectional adaptive voice frequency repeater
