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ECHO CANCELLATION 14.1 14.2 14.3 14.4 14.5 14.6 14.7 Introduction: Acoustic and Hybrid Echoes Telephone Line Hybrid Echo Hybrid Echo Suppression Adaptive Echo Cancellation Acoustic Echo Sub-band Acoustic Echo Cancellation Summary cho is the repetition of a waveform due to reflection from points where the characteristics of the medium through which the wave propagates changes. Echo is usefully employed in sonar and radar for detection and exploration purposes. In telecommunication, echo can degrade the quality of service, and echo cancellation is an important part of communication systems. The development of echo reduction began in the late 1950s, and continues today as new. | Advanced Digital Signal Processing and Noise Reduction Second Edition. Saeed V. Vaseghi Copyright 2000 John Wiley Sons Ltd ISBNs 0-471-62692-9 Hardback 0-470-84162-1 Electronic 14 ECHO CANCELLATION 14.1 Introduction Acoustic and Hybrid Echoes 14.2 Telephone Line Hybrid Echo 14.3 Hybrid Echo Suppression 14.4 Adaptive Echo Cancellation 14.5 Acoustic Echo 14.6 Sub-band Acoustic Echo Cancellation 14.7 Summary Echo is the repetition of a waveform due to reflection from points where the characteristics of the medium through which the wave propagates changes. Echo is usefully employed in sonar and radar for detection and exploration purposes. In telecommunication echo can degrade the quality of service and echo cancellation is an important part of communication systems. The development of echo reduction began in the late 1950s and continues today as new integrated landline and wireless cellular networks put additional requirement on the performance of echo cancellers. There are two types of echo in communication systems acoustic echo and telephone line hybrid echo. Acoustic echo results from a feedback path set up between the speaker and the microphone in a mobile phone hands-free phone teleconference or hearing aid system. Acoustic echo may be reflected from a multitude of different surfaces such as walls ceilings and floors and travels through different paths. Telephone line echoes result from an impedance mismatch at telephone exchange hybrids where the subscriber s 2-wire line is connected to a 4-wire line. The perceptual effects of an echo depend on the time delay between the incident and reflected waves the strength of the reflected waves and the number of paths through which the waves are reflected. Telephone line echoes and acoustic feedback echoes in teleconference and hearing aid systems are undesirable and annoying and can be disruptive. In this chapter we study some methods for removing line echo from telephone and data telecommunication systems and acoustic .
