Anas- Echo Cancellation

SEMINAR SUBMITTED BY ANAS.A.S ROLL.NO.5

ECHO CANCELLATION

INTRODUCTION  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 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 ECHO  Acoustic  Hybrid

Echo

Echo

ACOUSTIC 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 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 PROCESS  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 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 ECHO

DELAY

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 CANCELLER There are actually two opportunities for undesired echo on a voice band data connection — the nearend 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 fullduplex data transmission . Fig. A Echo cancellation method of full-duplex data transmission .

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

Fig. c A transversal filter echo canceller.

ADAPTIVE 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 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 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 MODEL Fig. 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 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 CANCELLATION Nonlinear Input

distortions in hardware

and Output signal requirements

Incorrect

code synchronization

 CODEC

SYNCHRONIZATION VIA ADAPTIVE INTERPOLATORS

Requirements 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

CONCLUSION 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

REFERENCES 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.com  www.spirit.com 

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