Echo%20Cancellation

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echo
cancellation
Ian Hung 2B Computer Engineering University of
Waterloo August 17th, 2001
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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
Presentation Overview
This presentation will explore the different
methods of handling echo inherent within a
digital cellular network.
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A. Background and Significance
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Speech quality is the benchmark at which the
quality of the network is assessed.
Wireless phones have gained enormous popularity
and are becoming essential communication tools
that impact our lives from day-to-day personal
tasks to how business is performed.
The key is to maintain customer loyalty by
improving network quality thus generating
additional revenue.
For this reason, a considerable amount of
research in the removal of inherent hybrid and
acoustic echo with the aim of improving perceived
voice quality has been conducted.
A. Background and Significance
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B. Echo Basics
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Cell Site
Mobile Switching Center (MSC)
4 Wire
4 Wire
Local Telephone Exchange
2 Wire
4 Wire
Long Distance Toll Office
Long Distance Toll Office
Long Distance Toll Office
4 Wire
4 Wire
Communications Network General Overview
Due to the cost of cabling, two-wire circuits are
used to connect the telephone to the local
telephone exchange rather than four-wire trunks.

• Hybrid Echo
• Acoustic Echo
• Other Echo Sources

B. Echo Basics
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Hybrid transformers are used to interface this
two-wire circuit with a four-wire station.
4 Wire Send Port
This conversion does not work perfectly and
results in electrical energy leaking back to the
talker in the form of echo.
Hybrid Device
2 Wire Port
Echo
4 Wire Receive Port
When there is little round-trip delay (28ms), it
creates a positive sense that the call is live by
adding sidetone.
Hybrid Echo

• Hybrid Echo
• Acoustic Echo
• Other Echo Sources

B. Echo Basics
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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.

• Hybrid Echo
• Acoustic Echo
• Other Echo Sources

B. Echo Basics
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Audio reflected from window
Audio reflected from dash
Audio reflected from door
Multipath echo is transmitted back to the distant
end and heard as echo.
This form of echo originates from handsets and is
produced by poor isolation between the microphone
and speaker.
Acoustic Echo

• Hybrid Echo
• Acoustic Echo
• Other Echo Sources

B. Echo Basics
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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

• Hybrid Echo
• Acoustic Echo
• Other Echo Sources

B. Echo Basics
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C. Process of Echo Cancellation
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ERLE
Send to Far End
Echo Canceller
S-in
S-out
Hybrid
Line-Side
Longhaul
Drop-Side
Receive from Far End
R-in
R-out
Endpath
Echo Canceller Implementation in the Digital
Wireless Network
C. Process of Echo Cancellation
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Hybrid's impulse response must be learned- known
as adaptation
Residual Echo
S-in
S-out
Subtractor
NLP
Echo Estimate
Error Signal
Convolution Processor H-Register X-Register
Hybrid
Double-Talk Detector
The endpath is mapped
R-out
R-in
Contents within either registers are multiplied
together, a process called convolution, producing
the echo estimate that is fed to the Subtractor.
Echo Canceller Schematic Diagram
C. Process of Echo Cancellation
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Subtractor eliminates the echo estimate from the
convolution processor
Residual Echo
S-in
S-out
Subtractor
NLP
Echo Estimate
Error Signal
Convolution Processor H-Register X-Register
Hybrid
Double-Talk Detector
H-register is adjusted accordingly, refining the
estimate of the impulse response, in a process
called convergence
R-out
R-in
Echo Canceller Schematic Diagram
C. Process of Echo Cancellation
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Double-talk occurs when both near and far-end
callers are speaking at the same time.
Residual Echo
S-in
S-out
Subtractor
NLP
Near-end speech during double-talk can distort
the error signal and confuse the adaptation
process.
Echo Estimate
Error Signal
Error signal from the Subtractor is ignored
causing the contents within the H-register to
freeze
Convolution Processor H-Register X-Register
Hybrid
Double-Talk Detector
R-out
R-in
Echo Canceller Schematic Diagram
C. Process of Echo Cancellation
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Nonlinear echoes clipped speech
signals speech compression poor quality
speakerphones pulse code modulation (PCM)
challenge convolution processor to develop an
accurate echo estimate.
Residual Echo
S-in
S-out
Subtractor
NLP
Echo Estimate
Error Signal
NLP Reduces the residual echo to inaudible
levels based on the adaptive suppression threshold
Convolution Processor H-Register X-Register
Hybrid
Double-Talk Detector
R-out
R-in
Signals above the threshold are allowed to pass
where as signals below the threshold are removed
Echo Canceller Schematic Diagram
C. Process of Echo Cancellation
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D. Echo Control
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Cell Site
Mobile Switching Center (MSC)
4 Wire
Echo Canceller
4 Wire
Hybrid
Public Switched Telephone Network (PSTN)
2 Wire
4 Wire
When a call is made from the mobile user to the
PSTN, the caller will experience echo-free
speech, while the receiver may experience poor
speech quality. Therefore, the echo canceller
must work both ways to accommodate users from
either ends.
Echo Canceller Implementation in the Digital
Wireless Network

• Handling Hybrid Echo
• Handling Acoustic Echo
• Handling Other Complex Echo

D. Echo Control
D. Echo Control
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During conversation, the audio picture
dynamically changes, which causes the canceller
to adapt continually.
From MSC
Loudspeaker
Echo Canceller
Echo
EC should be capable of receiving 270 ms of
delay. Any less and the remaining echo would be
beyond the ability for the canceller to remove
them.
To MSC (no Echo)
Microphone (Speech w/Echo)
Acoustic echo return loss enhancement (AERLE),
the amount of attenuation applied to unfavorable
echo, has a value of 65 dB as the minimum
requirement with the non-linear processor enabled.
Echo Canceller Implementation in the Mobile
Handset

• Handling Hybrid Echo
• Handling Acoustic Echo
• Handling Other Complex Echo

D. Echo Control
D. Echo Control
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Performance is dependent upon both network-side
EC integration, and mobile-side handset design
Due to strong price pressures, most handsets do
not control echo very well. In some cases,
such handsets are known to produce a terminal
compiling loss of 24 dB.
Echo Cancelled Voice Sample
Voice Sample Echo Cancelled
Hybrid
2 Wire
4 Wire
Back-to-Back Implementation in the Digital
Wireless Network

• Handling Hybrid Echo
• Handling Acoustic Echo
• Handling Other Complex Echo

D. Echo Control
D. Echo Control
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E. Evaluating Audio Quality
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A. Total Echo Removal   1. No echo at the
beginning of the call 2. No Echo throughout the
call - call is stable without bursts or
distortion 3. Residual Echo (low-level) is
absent throughout the call   B. Double-Talk
Clarity   1. No echo or scratching noises
throughout the call 2. Speech is clearly
audible during double-talk 3. 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 background noise 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.
D. Echo Control
E. Evaluating Audio Quality
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F. Future Trends
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In the pursuit for greater call clarity, echo
cancellers have been the center of recent
development, and have now become highly
sophisticated transmission equipment at the heart
of highly complex networks. Effective
implementation of echo cancellation across the
network holds the key to improving call quality
and ultimately, strengthen customer satisfaction,
boost revenues, and reduce subscriber churn.
D. Echo Control
F. Future Trends