ANAS- ECHO CANCELLATION
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Transcript of ANAS- ECHO CANCELLATION
SEMINAR SUBMITTED BY
ANAS.A.S
ROLL.NO.5
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
.
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 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
THANK YOU