Performance of an Adaptive Echo Canceller Operating in a Noisy, Linear, Time-Invariant Environment

The echo problem in the telephone network is caused by the interaction of the following three factors: (i) The impedance mismatches that exist at hybrid junctions cause reflections of incident electrical waves, (u) The existence of a bi-directional transmission medium permits the reflected signal to reach the talker as echo. (Hi) Time delay due to the finite propagation time of a signal makes the echo annoying. Historically the problem has been alleviated by increasing trunk loss, balancing hybrids, applying four-wire circuits where practical, and providing echo suppressors.1 Echo suppressors are used when the echo delay exceed about 45 ms. An echo suppressor is a voice-operated device which switches a large loss in the echo path, as shown in Fig. la. This loss blocks the echo effectively but also tends to block speech from the near-end customer 785 -786 T H E BELL SYSTEM TECHNICAL JOURNAL, MARCH 1971 ·ECHO PATH X ( t ) S P E E C H FROM FAR-END CUSTOMER THIS ECHO SUPPRESSOR PROTECTS FAR-END CUSTOMER SIMILAR CONFIGURATION ON FAR-END I N P U T TO DOUBLETALKING DETECTOR TO RESTORE DUPLEX OPERATION HYBRID NEAR-END CUSTOMER