Echo Cancellation
Saeed V. Vaseghi · 1996
Echo is the repetition of a wave due to reflection from points where the characteristics of the material through which the wave propagates changes. Acoustic echoes are due to reflection of the sound waves from walls, floors, ceilings, windows and other objects. Telephone line echoes result from impedance mismatch at the telephone exchange hybrids where the subscriber's two-wire line is connected to a four-wire line. Echoes can also result from a feedback path set up between the speaker and the microphone in a teleconference or hearing aid system. The perceptual effects of an echo depends on the time delay between the incident and the reflected waves, the strength of the reflected waves and the number of paths through which the waves are reflected. Acoustic echo is usually reflected from a multitude of different surfaces and travels through different paths. If the time delay is not too long then the acoustic echo may be perceived as a soft reverberation, and it may even add to the artistic quality of the sound. Concert halls and church halls with desirable reverberation characteristics can enhance the quality of a musical performance. Telephone line echoes, and acoustic feedback echoes in teleconference and hearing aid systems, are undesirable and annoying and can be quite disruptive. In this chapter we study the methods of removing line echoes from telephone and data telecommunication systems, and the acoustic feedback echoes from microphone-loudspeaker systems.