Echo Cancellation Ian Hung

7/30/2019 Echo Cancellation Ian Hung

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echocancellationIan Hung

2B Computer EngineeringUniversity of Waterloo

August 17th, 2001


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Presentation

Overview

Squelching Echo in a Digital Cellular Network

A. Background and Significance

B. Echo Basics

C. Process of Echo Cancellation

D. Echo Control

E. Evaluating Audio Quality

F. Future Trends

This presentation will explore the different methods of handlingecho inherent within a digital cellular network.


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A. Background and Significance


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A. Background andSignificance

Wireless phones have gained enormous popularityand are becoming essential communication toolsthat impact our lives from day-to-day personaltasks to how business is performed.

The key is to maintain customerloyalty by improving network quality thusgenerating additional revenue.

Speech quality is thebenchmarkatwhich the quality ofthe network isassessed.

For this reason, a considerable amount of research in the removal of inherent hybrid and acoustic echo with the aimofimproving perceived voice quality has been conducted.


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B. Echo Basics


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B. EchoBasics

Hybrid EchoAcoustic EchoOther Echo Sources

Communications Network General Overview

Cell Site

MobileSwitching

Center(MSC)

LocalTelephoneExchange

LongDistanceToll Office



4 Wire

4 Wire

4 Wire

4 Wire 4 Wire

2 Wire

Due to the cost of cabling, two-wire circuits are used to connectthe telephone to the localtelephone exchange rather thanfour-wire trunks.


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Hybrid

Device

2 Wire Port

B. EchoBasics

Hybrid Echo

4 Wire Receive Port

4 Wire Send Port

Echo


Hybrid transformersare used to interfacethis two-wire circuit

with a four-wire

station.

This conversion does not workperfectly and results in electricalenergy leaking back to the talkerin the form of echo.

When there is little round-tripdelay (28ms), it creates apositive sense that the call is liveby adding sidetone.


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Intrusive echo results when the total network delay exceeds 36ms.

The amount of echo depends on how well the hybrid matches both circuits.

This is measured as echo return loss (ERL).

High ERL value = weak signal being reflected.

B. EchoBasics



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Acoustic Echo

Audio reflected from window

Audio reflected from dash

Audio reflected from door

This form of echo originates fromhandsets and is produced bypoor isolation between themicrophone and speaker.

Multipath echo is transmittedback to the distant end andheard as echo.

B. EchoBasics


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B. EchoBasics

Background noise is generated when the mobile is operated in hands-free mode.

This noise is passed through the vocoder causing further distortion in speech.

Delay in either speech compression or transmission equipment further induce echo and degrade voice quality.

especially true when the call is processed through mixed network infrastructures including copper wire, fiber optic lines,microwave connections, international gateways, and satellite transmission


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HybridEcho Canceller

Send toFar End

Line-Side

Receive fromFar End

Longhaul

Endpath

Drop-Side

S-in S-out

R-inR-out

ERLE

Echo Canceller Implementation in the Digital Wireless Network


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Hybrid

S-in S-out

R-inR-out

Echo Canceller Schematic Diagram

ConvolutionProcessor

H-RegisterX-Register

Subtractor

Double-TalkDetector

NLP

EchoEstimate

ErrorSignal

ResidualEcho

Contents within either registersare multiplied together, aprocess called convolution,producing the echo estimate thatis fed to the Subtractor.

The endpath ismapped

Hybrid's impulseresponse must belearned- known asadaptation


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Hybrid

S-in S-out

R-inR-out




Subtractor

Double-TalkDetector

NLP

EchoEstimate

ErrorSignal

ResidualEcho

Subtractor eliminates the echoestimate from the convolutionprocessor

H-register is adjustedaccordingly, refining the estimateof the impulse response, in aprocess called convergence


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Hybrid

S-in S-out

R-inR-out




Subtractor

Double-TalkDetector

NLP

EchoEstimate

ErrorSignal

ResidualEcho

Error signal from the Subtractoris ignored causing the contentswithin the H-register tofreeze

Double-talkoccurs whenboth near andfar-end callersare speaking atthe same time.

Near-end speech during double-talk can distort the error signaland confuse the adaptationprocess.


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Hybrid

S-in S-out

R-inR-out




Subtractor

Double-TalkDetector

NLP

EchoEstimate

ErrorSignal

ResidualEcho

NLP: Reduces the residual echoto inaudible levels based on theadaptive suppression threshold

Signals above the threshold areallowed to pass where as signals

belowthethresholdareremoved

Nonlinear echoes:

clipped speechsignals

speech compression

poor qualityspeakerphones

pulse codemodulation (PCM)

challengeconvolutionprocessor todevelop an accurateecho estimate.


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D. Echo Control


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D. Echo Control

Handling Hybrid EchoHandling Acoustic EchoHandling Other Complex Echo

D. Echo Control

Hybrid

Echo Canceller Implementation in the Digital Wireless Network

Cell SiteMobile

SwitchingCenter(MSC)

Public SwitchedTelephone Network

(PSTN)

4 Wire

4 Wire

2 Wire 4 Wire

EchoCanceller

When a call is made from the mobile user to thePSTN, the caller will experience echo-freespeech, while the receiver may experiencepoor speech quality.

Therefore, the echo canceller must work

both ways to accommodate users fromeither ends.


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D. Echo Control


D. Echo Control

Echo Canceller Implementation in the Mobile Handset

Echo

EchoCanceller

To MSC(no Echo)

From MSC

Microphone (Speech w/Echo)

Loudspeaker

During conversation, the audiopicture dynamicallychanges,which causes the canceller toadapt continually.

EC should be capable of receiving 270 msof delay. Any less and the remaining

echo would be beyond the ability for thecanceller to remove them.

Acoustic echo return loss enhancement(AERLE), the amount of attenuationapplied to unfavorable echo, has a value

of65 dB as the minimum requirementwith the non-linear processor enabled.


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D. Echo Control


D. Echo Control

Back-to-Back Implementation in the Digital Wireless Network

Due to strongpricepressures,most handsets do not control echovery well.

In some cases, such handsets are

known to produce a terminalcompilingloss of24dB.

Performance is dependent upon both network-side EC integration, and mobile-side handset design

EchoCancelled

Voice

Sample

Voice

Sample

EchoCancelled

Hybrid

2 Wire 4 Wire


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E. Evaluating Audio Quality


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D. Echo ControlE. Evaluating Audio Quality

A. Total Echo Removal1. No echo at the beginning of the call2. No Echo throughout the call - call is stable without bursts or distortion3. Residual Echo (low-level) is absent throughout the call

B. Double-Talk Clarity1. No echo or scratching noises throughout the call2. Speech is clearly audible during double-talk3. No clipping at the beginning and end of speech*Occasionally, echo cancellers may be fooled due to high background noise. If double-talk detection

is handled incorrectly, near-end speech, perceived to be echo, may be attenuated.C. Background Audio Transparency

On contrary, background audio transparency is the ability for an echo canceller to allow backgroundnoise to be heard.

Poor DSP = audio fading in and out

Better DSP = consistent background audio.

Noise injection to fill-in for background audio is occasionally used.

Replacing these intelligible background sounds with random noise can be irritating.


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F. Future Trends


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D. Echo ControlF. Future Trends

In the pursuit for greater call clarity, echo cancellers have been thecenter of recent development, and have now become highlysophisticated transmission equipment at the heart of highly complexnetworks.

Effective implementation of echo cancellation across the network holdsthe key to improving call quality and ultimately, strengthen customer

satisfaction, boost revenues, and reduce subscriber churn.

Echo Cancellation Ian Hung

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