This document discusses echo cancellation technology used in mobile calls. It begins with an introduction and then describes the different types of echo that can occur, including acoustic echo from microphones picking up speaker audio, and hybrid echo caused by two-wire telephone connections. Echo is caused by delays in digital networks and can degrade call quality. Echo cancellation works by using adaptive filters to model the echo and compare incoming audio to cancel out echo signals, removing over 80-90% of echo. Further residual echo is eliminated using nonlinear processing. Complex algorithms are also used to control acoustic echo by computing speech models and subtracting predicted echo from microphone signals.

1. ECHO CANCELLATION
The Nature Of Echo And How Echo Cancellation Is Helping In Making
Mobile Call Meet Acceptable Quality Standers
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2. TOPICS
• Introduction
• Types Of Echo
• Causes Of Echo
• The Combined Problem On Digital Cellular Networks
• The Process Of Echo Cancellation
• Controlling Acoustic Echo
• Room Of Improvement In The Handset
• Future Of Echo Cancellation Technology
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3. INTRODUCTION
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INTRODUCTION

5. ECHO TYPES
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• Acoustic echo
o originates in a local audio loop back that occurs when a microphone(s), pick up audio signals from a
speaker(s), and sends it back to an originating participant.
o Acoustic echo can be intensified when sensitive microphone(s) are used, as well as when the
microphone and/or speaker volume is turned up to a high level, and also when the microphone and
speaker(s) are positioned so that the microphone is close to one or more of the speakers.
o This echo is aggravated by reflective acoustic echo reflected by walls and/or objects.

6. ECHO TYPES
• Hybrid Echo
o The vast majority of the public telephone system local loop wiring is done using two-wire connections
whereby the same pair of wires carries voice signals in both directions. In the telephone company's
central office, a two-to-four wire conversion is done using a hybrid circuit.
o This electrically generated echo is created as voice signals reflected back to the speaker from four wire
Circuit
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7. CAUSES OF ECHO
• Acoustic echo apart, background noise is generated through the network when analog and digital are
operated in hand free mode .
• Digital Processing Delay and speech compression techniques further contribute to echo generation and
degraded voice quality in wireless network.
• one-way Delay is Greater than 16ms
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9. COMBINED PROBLEMS ON DIGITAL CELLULAR
NETWORK
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10. THE PROCESS OF ECHO CANCELLATION
• 1st Step
The echo canceler employs a digital adaptive filter to set up a model of voice signal and echo passing
through the echo canceller
As voice path passes back through cancellation system , the echo canceler compares the signal and the
model to cancel exiting echo dynamically.
Removing more than 80-90% of echo signals.
• 2nd Step
Utilize a non-linear processor to eliminate the remaining residual echo by attenuating the signal below the
noise floor .
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11. CONTROLLING ACOUSTIC ECHO
• Complex algorithm are used to compute speech models
• Involves generating the sum from reflected echo of original speech
• Subtracting this from any signal microphone picks up.
• The Results is pure speech of person talking
Echo Prediction
Types of Handset and Mobile Model
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12. THANKS FOR ATTENTION
AHMED SHELBAYA
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