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Visit us at:

www.octasic.com & www.octware.net

Your Way.Your Way.Communicate.Communicate.

Achieving Superior VoIP Sound Quality with Software Echo Cancellation

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Which EchoCancellation

solution are you using?

Who is affected by poor voice quality?

You

Your Customers

Your Company’s

Image

Everyone!

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Can you Talk the Talk?

• Definition of Echo

• Network Delays

• Network Scenarios

• Echo Cancellation Requirements & Features

• Echo Cancellation Solutions

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Echo

• What is it?– Echo is the repetition of a sound caused by the

reflection of sound waves.

• Types of Echo– Electrical / Line

– Caused by impedance mismatch in hybrid in analog local loop

– Acoustic– Caused by sound emitted from a speaker and captured

by the microphone after a delay.

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Hybrid-caused Electrical Echo

• Cause: mismatched impedances in Hybrid (H)

• Noticeable and distracting when delay > 20ms.

– PSTN network: on long distance calls > 3000km (~1800miles)

– On all VoP calls

Person A Person B

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Handset Cross-Talk

• Mechanical or electromagnetic problem– Occurs with cell phones

• Many variables with wireless equipment – Many makers & models– Angle at which it’s held

Network & Delay

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Voice over Packet Delay Calculations

Cause of Delay Time (ms) Details

Packetization 10 – 20 10-20 ms packets

Processing 1 – 2

Network Queuing 2 – 45 • Transmission time negligible

• VPN N.A. quoted by MCI

PDV Buffering 5 – 30

Processing 2 – 20 • Resource Queuing

Total round trip delay

20 – 117 ms

In Packet Networks, delay is always > 20ms, so echo is always noticeable and distracting.

Network Scenarios

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Echo Canceller in Network

Far end Local end

Pointed this way

Local end Far end

Pointed this way

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EC in Packet Network

IPNetwork

Trunking GW

TDMNetwork

DLC

H

Trunking GW

TDMNetwork

DLC

H

EC

EC

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EC in Enterprise Network

IPNetwork

IP PBX GW

TDMCarrier

Network

DLC

H

EC

(Acoustic)

EC

(Electrical)

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EC in IAD / VoIP Home Gateway

AccessPlatform

IPNetwork

IAD

Inverse GW

TDMNetwork

DLC

H

TDMNetwork

DLC

HEC

EC

IAD

MAC VoIP SLICECEthernet POTS

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QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

EC in IAD / VoIP Home Gateway

AccessPlatform

IPNetwork

WiFiHGW

IP Phone

TDMNetwork

DLC

H

TDMNetwork

DLC

HEC

EC

IAD

MAC VoIP SLICECEthernet POTS

Echo Cancellation (EC)

Requirements & Features

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Inside an Echo Canceller

• Composed of 2 Parts

1) Adaptive Filter (AF) • Removes linear echo

2) Non-Linear Processor (NLP)• Removes residual, non-linear echo

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Adaptive Filter’s (AF) Functionality

1) Samples Rin and Sin

2) Builds mathematical model of echo path

3) Simulates echo and feeds it to subtractor

4) Subtractor removes bulk of echo (linear portion) on return path (Sin)

5) Voice then enters NLP

Rin

Sin

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Echo Cancellation Algorithms

• Least Square (LS)– Used in Block processing– More robust than LMS

• Especially in presence of double-talk and background noise

• Least Mean Square (LMS)– Recursive Algorithm– Most common algorithm– Requires a lot of adjustments

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Convergence

AF Algorithms for EC Compete in 2 Types of Races

1) Sprints for Fast Convergence– Start of calls– Re-converging after change in echo path

2) Marathons for Continual Improvements – Despite noise in echo signal– Through silence & double-talk

AF’s ChallengeTo simultaneously achieve2 conflicting characteristics:

Fast Convergence & High Stability

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Divergence

Adaptive Filter Problem • Cause:

– No suitable solution for line model found with mathematical algorithm

• Result:– Can corrupt signal– Can even add echo!

Good ECs are tuned to avoid divergence

in nearly all line conditions

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Non-Linear Processor’s Functionality

The NLP:

1) Evaluates residual echo

2) Removes all signals below a certain threshold

3) Replaces those signals with simulated background noise

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NLP’s Design Characteristics

• Frequency-aware• Able to handle different situations

– Changes in frequency of BGN– Start and end of speech

Acoustic Echo Cancellation

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Speakerphones and Double-talk

• Acoustic echo common with speakerphones

• Some speakerphone makers simplified EC’s task– Walkie-talkie style design

• Only one person’s voice transmitted at a time.

– Result• Unnatural & unpleasant conversation

• Solution: Allow & handle double-talk– With High quality EC solution

• Adaptive filter key for Double-Talk

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Acoustic Echo Cancellation

• Adaptive Filter– Limited amount of linear echo– Changing signal

• Strong NLP required– High quality background noise matching– Long tail coverage– Operate well despite varying environment & echo point

• With user’s movement

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Mic & Speaker Separation

• Most Acoustic Echo is inside the box– Best isolation reduces echo– The lower the echo level the better the sound quality

• A few things to watch– Separate enclosure for Mic & Speaker– Excellent Mic to chassis isolation

Device

Local Talk

Echo

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Acoustic Echo Saturation

Normal Signal Saturated SignalNo linear

relationship

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Signal Saturation

• Speaker Output– Amplifier is powerful enough (linear part of curve)– DAC is linear– Speaker is not overdriven

• Microphone Input– Analog Circuit is linear

• No frequency cut-offs– Microphone enclosure

• Designed to avoid membrane saturation– A/D range is properly adjusted

Device

Local Talk

Echo

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Vibration on Casing

• Casing vibration is non-linear– This cannot be removed by EC– Can only be addressed through Noise Reduction

• Causes:– The speaker is not attached properly

• Vibrates against the casing and creates noise

– The speaker is not isolated from the chassis• Makes another part of the device vibrate

Echo Cancellation Solutions

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Software Echo Cancellation

The Need• Long processing delays cause echo in all IP communications• EC is essential to restoring voice quality

Essential Features• Transparent algorithm• High quality background noise handling• Excellent double-talk handling• Fast convergence

Benefits• Delivers carrier-grade voice quality• Eliminates support calls from your customers• Plug & Play• Cost-effective

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Software Echo Cancellation

Mobile & IP Telephony

• Hand-helds• DECT Phones• Speakerphones• IP Phones• Soft phones

Acoustic Echo Cancellation (AEC)Line Echo Cancellation (LEC)

EC Solutions for all Endpoints

® Asterisk is a trademark of Digium, Inc.

IP PBX & Asterisk® Market

Integrated Access Devices

(IADs)

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EC & Voice Quality Solutions

Software Solution: SoftEcho

• 1-8 channels• For consumer equipment

– Asterisk® based IP PBX– Softphones– IP & speaker phones

• CPU usage: 5% per channel*• 8 ch. high quality EC: 40%

CPU usage

Hardware Solution:OCT6100

• 8-672 channels• For enterprise & carrier

equipment• Available on interface cards

from many vendors

• Max. power: 2W at 672 channels

Deployed worldwide in wireless, wireline & VoP networks

Digium

For 8+ channels

Thank you!

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Visit us at:

www.octasic.com & www.octware.net

Your Way.Your Way.Communicate.Communicate.