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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
4
Which EchoCancellation
solution are you using?
Who is affected by poor voice quality?
You
Your Customers
Your Company’s
Image
Everyone!
5
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!
37
Visit us at:
www.octasic.com & www.octware.net
Your Way.Your Way.Communicate.Communicate.