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ECHO CANCELLATION

INTRODUCTIONINTRODUCTION

Echo cancellation is the process of removing echo from a voice communication in order to improve the voice call quality.

Echo cancellation is often needed because speech

compression techniques and packet processing delays generate echo

Echo cancellation not only improves quality but it also reduces bandwidth consumption because of its silence suppression technique

What is echoWhat is echo

Sound waves which have been reflected to a listener with sufficient magnitude and time delay so as to be perceived separately from those communicated directly to the listener

TYPES OF ECHOTYPES OF ECHO

Acoustic Echo

Hybrid Echo

ACOUSTIC ECHOACOUSTIC ECHO

 This mainly depend upon quality and type of the equipment

Acoustic Echo is caused by direct and indirect feedback from speaker to microphone

HYBRID ECHOHYBRID ECHO

The most prevalent cause of echo in PSTN is impedance1 mismatches within the hybrid where 4-wire phone circuits are converted to 2-wire circuits

The presence of echo occurs whenever the replicated signal delay exceeds 10 msecs, and becomes apparent to the speaker as reflected voice when the delay exceeds as little as 16 msec

The electrically generated echo occurs when the incoming energy from the far end speaker is reflected back toward the speaker as a slightly altered and delayed replication due to impedance mismatch in the hybrid (Figure )

ECHO CANCELLATION ECHO CANCELLATION PROCESSPROCESS

In order to combat the echo phenomenon, an echo canceller is employed

Today's echo cancellers use sophisticated algorithms running on high speed Digital Signal Processors (DSPs) to combat the echo

HISTORY OF ECHO CANCELLATIONHISTORY OF ECHO CANCELLATION

The late 1950 marked he birth of the echo control in the telecommunication industry with the development of first echo suppression devices

. Echo cancellation theory was early in the 1960 by AT&T bell labs, followed by the introduction of first echo cancellation system in the late 1960 by COMSAT

More sophisticated digital interface, multi channel echo canceller systems were also developed to address new echo problems associated with long distance digital telephony systems IN 1990

CONTROLLING ACOUSTIC CONTROLLING ACOUSTIC ECHOECHO

DELAYDELAY

Delay in telephony context is the time during which voice signals travel across the network .

The greater the round trip delay and impedance mismatch, the worse the potential echo.

PRINCIPLE OF ECHO PRINCIPLE OF ECHO CANCELLERCANCELLER

There are actually two opportunities for undesired echo on a voice band data connection — the near-end hybrid and one or more far-end hybrids. One difficulty with the far-end echo that we will have to address is the possible frequency offset that it experiences, just as with the far-end data signal. The digital subscriber loop application is easier than the voice band data canceller in this respect, since there is no far-end echo mechanism. The echo cancellation method of full-duplex transmission is illustrated in Fig.

Fig. A Echo cancellation method of full-duplex data transmission

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Fig. A Echo cancellation method of full-duplex data transmission

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Fig. b The principle and notation of an echo canceller.RECEIVER

Fig. c A transversal filter echo canceller.

ADAPTIVE FILTERINGADAPTIVE FILTERING. An echo canceller must cancel the entire tail. each hybrid circuit is

slightly different, each echo tail is different as well. Many factors determine the echo path. It is even possible for an echo tail to change while a circuit is active. This could happen when a second telephone extension is taken off-hook in parallel with the first one. Due to these variations in echo tails, it is necessary for an echo canceller to adapt to the tail continuously. Adaptive Filtering is employed within echo cancellers to this end. The adaptive filters should converge quickly, but not so quickly that they might diverge under some conditions. This is especially important when a circuit is first established. The

amount of time it takes the echo canceller to adapt to an echo path is referred to as the "convergence time".

 

BASEBAND CHANNELBASEBAND CHANNEL

h(t) = g(t) f(t),

 Fig. A. Two configurations for a base band channel echo canceller. Cancellation using a. the sampledtransmitted data waveform and b. the transmitted data symbols.

PASS BAND CHANNELPASS BAND CHANNEL

The pass band echo canceller is considerably different from the base band channel case

There are two obvious differences:• The canceller input is complex-valued.• The transmitter modulator is included in the transmit path, so that the echo path is time varying.

ECHO PATH MODELECHO PATH MODELFig. a. The transmitter and echo path for a pass band channel echo canceller. a. The transmitter, echo path, and a phase splitter at the receiver input. b. An

equivalent model for the path from transmitted data symbols to received analytic signal consisting of a base band echo channel followed by modulator.

c. An alternative model consisting of a modulator followed by pass band echo channel.

where the equivalent base band complex-valued response is

VARIATION OF ECHO CANCELLERVARIATION OF ECHO CANCELLER

   

 

ineEchoCanceller- Short Tail (Fig. 4)Figure 4. Line Echo Canceller

Network Echo Canceller- Long Tail (Fig. 5)Figure 5. Network Echo Canceller

FAILING TO ACHIEVE ECHO FAILING TO ACHIEVE ECHO CANCELLATIONCANCELLATION

Nonlinear distortions in hardware

Input and Output signal requirements

Incorrect code synchronization

CODEC SYNCHRONIZATION VIA ADAPTIVE INTERPOLATORS

Requirements summaryRequirements summary

The non-linear distortions in the hardware’s must be sufficiently low The microphone and loudspeaker placement and

echo insulation should be done carefully The signal delays in the software’s be fixed Codec synchronization must be done carefully The operating system, device drivers and the rest

of software running in the system must meet at the real time requirements of the echo canceller

CONCLUSIONCONCLUSION

Wireless phones are increasingly being regarded as essential communication tools, dramatically impacting how people approach day to day personal and business communications. As new network infra structures are implemented and competition between wireless carriers increases, digital wireless subscribers are becoming ever more critical of the service and voice quality yhey receive from network providers. A key technology to provide near wireline voice quality across a wireless carrier’s network is Echocancellation

REFERENCESREFERENCES   S.Vaseghi, Advanced digital signal processing and

noise   S.Haykin, Adaptive filter theory

Reduction              www.iec.org              www.lectrosonics.com              www.ti.com              www.octasic.com              www.claritytechnic.comwww.spirit.com

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