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echocancellation
Ian Hung2B Computer Engineering
University of WaterlooAugust 17th, 2001
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 handling echo inherent within a digital cellular network.
A. Background and Significance
A. Background and Significance
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.
Speech quality is the benchmark at which the quality of the network is assessed.
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.
B. Echo Basics
B. Echo Basics
•Hybrid Echo•Acoustic Echo•Other Echo Sources
Communications Network General Overview
Cell Site
Mobile
Switching
Center
(MSC)
Local
Telephone
Exchange
Long
Distance
Toll Office
Long
Distance
Toll Office
Long
Distance
Toll 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 connect the telephone to the local telephone exchange rather than four-wire trunks.
Hybrid
Device
2 Wire Port
B. Echo Basics
Hybrid Echo
4 Wire Receive Port
4 Wire Send Port
Echo
•Hybrid Echo•Acoustic Echo•Other Echo Sources
Hybrid transformers are used to interface
this two-wire circuit with a four-
wire station. This conversion does not work perfectly and results in electrical energy leaking back to the talker in the form of echo.
When there is little round-trip delay (28ms), it creates a positive sense that the call is live by adding sidetone.
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. Echo Basics
•Hybrid Echo•Acoustic Echo•Other Echo Sources
•Hybrid Echo•Acoustic Echo•Other Echo Sources
Acoustic Echo
Audio reflected from window
Audio reflected from dash
Audio reflected from door
This form of echo originates from handsets and is produced by poor isolation between the microphone and speaker.
Multipath echo is transmitted back to the distant end and heard as echo.
B. Echo Basics
•Hybrid Echo•Acoustic Echo•Other Echo Sources
B. Echo Basics
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
C. Process of Echo Cancellation
C. Process of Echo Cancellation
HybridEcho Canceller
Send to Far End
Line-Side
Receive from Far End
Longhaul
Endpath
Drop-Side
S-in S-out
R-inR-out
ERLE
Echo Canceller Implementation in the Digital Wireless Network
C. Process of Echo Cancellation
Hybrid
S-in S-out
R-inR-out
Echo Canceller – Schematic Diagram
Convolution
Processor
H-Register
X-Register
Subtractor
Double-Talk
Detector
NLP
Echo Estimate
Error Signal
Residual Echo
Contents within either registers are multiplied together, a process called convolution, producing the echo estimate that is fed to the Subtractor.
The endpath is mapped
Hybrid's impulse response must be learned- known as adaptation
C. Process of Echo Cancellation
Hybrid
S-in S-out
R-inR-out
Echo Canceller – Schematic Diagram
Convolution
Processor
H-Register
X-Register
Subtractor
Double-Talk
Detector
NLP
Echo Estimate
Error Signal
Residual Echo
Subtractor eliminates the echo estimate from the convolution processor
H-register is adjusted accordingly, refining the estimate of the impulse response, in a process called convergence
C. Process of Echo Cancellation
Hybrid
S-in S-out
R-inR-out
Echo Canceller – Schematic Diagram
Convolution
Processor
H-Register
X-Register
Subtractor
Double-Talk
Detector
NLP
Echo Estimate
Error Signal
Residual Echo
Error signal from the Subtractor is ignored causing the contents within the H-register to freeze
Double-talk occurs when both near and far-end callers are speaking at the same time.
Near-end speech during double-talk can distort the error signal and confuse the adaptation process.
C. Process of Echo Cancellation
Hybrid
S-in S-out
R-inR-out
Echo Canceller – Schematic Diagram
Convolution
Processor
H-Register
X-Register
Subtractor
Double-Talk
Detector
NLP
Echo Estimate
Error Signal
Residual Echo
NLP: Reduces the residual echo to inaudible levels based on the adaptive suppression threshold
Signals above the threshold are allowed to pass where as signals below the threshold are removed
Nonlinear echoes:
clipped speech signals
speech compression
poor quality speakerphones
pulse code modulation (PCM)
…challenge convolution processor to develop an accurate echo estimate.
D. Echo Control
D. Echo Control
•Handling Hybrid Echo•Handling Acoustic Echo•Handling Other Complex Echo
D. Echo Control
Hybrid
Echo Canceller Implementation in the Digital Wireless Network
Cell Site
Mobile
Switching
Center
(MSC)
Public Switched
Telephone Network
(PSTN)
4 Wire
4 Wire
2 Wire 4 Wire
Echo
Canceller
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.
D. Echo Control
•Handling Hybrid Echo•Handling Acoustic Echo•Handling Other Complex Echo
D. Echo Control
Echo Canceller Implementation in the Mobile Handset
Echo
Echo
Canceller
To MSC (no Echo)
From MSC
Microphone (Speech w/Echo)
Loudspeaker
During conversation, the audio picture dynamically changes, which causes the canceller to adapt continually.
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.
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.
D. Echo Control
•Handling Hybrid Echo•Handling Acoustic Echo•Handling Other Complex Echo
D. Echo Control
Back-to-Back Implementation in the Digital Wireless Network
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.
Performance is dependent upon both network-side EC integration, and mobile-side handset design
Echo
Cancelled
Voice
Sample
Voice
Sample
Echo
Cancelled
Hybrid
2 Wire 4 Wire
E. Evaluating Audio Quality
D. Echo ControlE. Evaluating Audio Quality
A. Total Echo Removal
1. 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 Clarity
1. 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 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.
F. Future Trends
D. Echo ControlF. Future Trends
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.
